Publications

2026

1. Development of a Mixed-Precision Discontinuous Galerkin Framework for Compressible Flow Calculations

   B. K. W. Yeo, and M. Ihme

   AIAA SCITECH 2026 Forum, 2026

   Abs DOI

   Discontinuous Galerkin (DG) methods combine features of finite element (FE) and finite volume (FV) solvers to solve partial differential equations in an element-local framework. This offers inherent advantages for massively-parallel computing architectures due to the high arithmetic intensity and lower memory requirements of element-local schemes. This paper presents a DG framework built using JAX, which targets several hardware platforms using the XLA compiler. The framework achieves an arbitrarily high order of accuracy and correctly simulates canonical fluid dynamics test cases using single-precision, but the associated convergence rate degrades very quickly using single-precision. Concurrently, single- and double-precision compute time is compared across a variety of hardware architecture including graphics processing units (GPUs), tensor processing units (TPUs), and both x86_64 and ARM computers. It is shown that single-precision produces as much as 2x speedup even on traditional architectures. This motivates the development of a mixed-precision framework that combines the speedup from using single-precision with the improved accuracy from double-precision representations. To this end, this paper demonstrates that selectively increasing the precision of certain operations in a DG solver can lower the machine-precision error floor. Some observations on the contributions of the precision of different operations in a DG solver to its overall accuracy and convergence rate are also presented.

2. Two-component dynamics in supercritical CO₂ from inelastic X-ray scattering

   A. Majumdar, P. Sun, M. Singleton, L. Paolasini, A. Bosak, A. Q. R. Baron, J. Hastings, and M. Ihme

   Scientific Reports, 2026

   Abs DOI Open Access

   Supercritical fluids are characterized by unique thermodynamic properties. One of these properties is the existence of two-component dynamics that is associated with distinct low-frequency and high-frequency vibrational responses of the fluid. However, the origin of this behavior remains unknown. By combining inelastic X-ray scattering and molecular dynamics simulations, we show that this behavior can be connected to density heterogeneities arising from molecular clusters. Analyses of measurements and molecular trajectories suggest that the two-component dynamics emerges due to distinct momentum fluctuations of clustered and unbound molecules. This connection between clusters and two-component dynamics highlights the importance of molecular-structural heterogeneities in supercritical fluids, colloids, and condensed-matter systems.

3. Dynamics of iodine geminate recombination in supercritical xenon solvent: Caging effect

   M. Mirakhory, A. Majumdar, M. Ihme, and A. C. T. Duin

   The Journal of Chemical Physics, 2026

   Abs DOI

   Understanding the dynamics of chemical reactions in solutions is vital, as their rates and kinetics are significantly affected by the solvent environment. Supercritical solvents offer extensive applications in chemical reactions by enabling the manipulation of the solution environment. In this study, we investigate the geminate recombination of iodine in a supercritical xenon solvent by using ReaxFF-based molecular dynamics simulations. Our findings reveal that the highest iodine recombination rate occurs near supercritical conditions, while lower-pressure conditions lead to reduced collision rates and unstable recombination, and higher-pressure conditions hinder iodine diffusion, resulting in a lower recombination rate. Our analysis shows that the xenon local density at the time of recombination is at least 2.5 times higher than the global density, confirming the presence of xenon clusters surrounding the Iodine atoms. This observation is further supported by coordination number analysis, which confirms an elevated xenon local density during recombination. In addition, the correlation between the total energy of xenon atoms within a cluster and recombined iodine atoms underscores the kinetic energy transfer process, validating the occurrence of geminate recombination. The excess kinetic energy from the recombining iodine atoms is transferred to the surrounding xenon atoms. Our examination of geminate recombination demonstrates that iodine atoms confined within xenon clusters-whether through manual insertion of atoms or the fast dissociation of an iodine molecule within xenon clusters-are more likely to recombine as primary geminate recombination. However, extending the iodine molecule dissociation time allows iodine atoms to diffuse out of the cluster, and the recombination to shift toward secondary geminate recombination.

4. Site-specific discrepancy analyses and quantification of 3D printed SiC structures via x-ray computed tomography

   E. Cakmak, D. Richardson, S. Chaturvedi, G. Vignat, M. Ihme, and T. Koyanagi

   SSRN Electronic Journal, 2026

   DOI Open Access
5. Third-body stabilization of supercritical CO2 in CO oxidation: development and application of a ReaxFF force field for the CO/O/CO2 system

   E. H. Chowdhury, M. Aryanpour, Y. K. Shin, B. Ramos-Alvarado, M. Ihme, and A. C. Duin

   Fuel, 2026

   DOI
6. Confinement-induced shift of thermodynamic response and energetic heterogeneities in supercritical fluids

   J. Fan, and M. Ihme

   Energy, 2026

   DOI
7. Nanoscale ultrafast lattice modulation with a free-electron laser

   H. Li, N. Wang, L. Zhang, S. Song, Y. Sun, M.-L. Ng, T. Sato, D. Hanlon, S. Dahal, M. D. Balcazar, V. Esposito, S. She, C. C. Ornelas-Skarin, J. Vila-Comamala, C. David, N. Berndt, P. R. Miedaner, Z. Zhang, M. Ihme, M. Trigo, K. A. Nelson, J. B. Hastings, A. A. Maznev, L. Foglia, S. W. Teitelbaum, D. A. Reis, and D. Zhu

   Nature Physics, 2026

   DOI Open Access
8. Non-premixed ammonia combustion in porous media to promote thermal cracking of NH3: a low-emission burner concept

   D. Kretzler, R. Puri, B. Stelzner, T. Zirwes, G. Vignat, F. P. Hagen, O. T. Stein, M. Ihme, and D. Trimis

   SSRN Electronic Journal, 2026

   DOI Open Access
9. A general probabilistic approach for the analysis of cycle-to-cycle variations in internal combustion engines

   L. Engelmann, M. Ihme, A. Kempf, and B. Böhm

   Combustion and Flame, 2026

   DOI

2025

1. A physics-informed machine learning approach for predicting dynamic behavior of reacting flows with application to hydrogen jet flames

   P. Sharma, W. T. Chung, and M. Ihme

   Combustion and Flame, 2025

   DOI Open Access
2. Burn parameters affect PAH emissions at conditions relevant for prescribed fires

   K. Töpperwien, G. Vignat, A. J. Feinberg, C. Daube, M. W. Alton, E. C. Fortner, M. R. Canagaratna, M. F. Kling, M. Johnson, K. C. Nadeau, S. C. Herndon, J. T. Jayne, and M. Ihme

   Atmospheric Pollution Research, 2025

   DOI
3. Data assimilation of state-indirect observations for constraining detonation wave dynamics

   J. J. Hansen, D. Brouzet, and M. Ihme

   Proceedings of the Combustion Institute, 2025

   DOI
4. Iodine recombination in xenon solvent: Clusters in the gas to liquid-like state transition

   M. Mirakhory, A. Majumdar, M. Ihme, and A. C. T. Duin

   The Journal of Chemical Physics, 2025

   Abs DOI

   Supercritical fluids (SCFs) have attracted significant attention as solvents for chemical reactions due to their unique properties, such as high diffusivity, low viscosity, and tunable solvation properties. These properties profoundly influence reaction kinetics and are often attributed to the formation of molecular clusters within SCFs. To study the effect of supercritical solvent on chemical reactivity and dynamics of reactions, one needs to understand the dynamics of clusters in supercritical fluid. Extensive experiments on the photodissociation and recombination of iodine in supercritical fluids served as a model system for understanding these effects. Experimental studies have been complemented by theoretical and computational investigations, which mostly employ Monte Carlo or empirical molecular dynamics simulations. However, computational studies using non-reactive force fields and ab initio approaches present challenges in capturing reactive processes at larger scales within supercritical fluids. In this work, we developed the ReaxFF parameters by training against quantum mechanics data. ReaxFF reactive force field based molecular dynamics simulations were performed, studying the dynamics of a xenon solvent and cage effect at different thermodynamic conditions for the iodine recombination reaction. We show that the conditions near the critical point are the optimal conditions to study the cage effect. We show that the average lifetime of xenon clusters ranging between 5 and 11 ps is comparable to iodine geminate recombination. Our simulation results of iodine recombination in xenon solvent demonstrate the higher probability of iodine molecule formation in the presence of xenon clusters. Finally, we show that the supercritical condition exhibits the highest recombination rate for iodine atoms.

5. Contributions of Acute Wildfire Smoke Exposure to Excess Mortality from the 2025 Los Angeles Fires

   K. Töpperwien, J. X. Yu, and M. Ihme

   Environmental Science & Technology Letters, 2025

   Abs DOI

   In January 2025, a series of severe wildfires in Los Angeles County burned over 50000 acres, destroyed over 16000 structures, and caused 30 fatalities. These fires, driven by intense Santa Ana winds, rank among the most destructive and deadliest fires in California’s history. Beyond the destruction and loss of lives, these fires also contributed to unhealthy levels of pollutants, such as fine particulate matter (PM 2.5 ), which cause increased respiratory and cardiopulmonary morbidity, yet this smoke-specific mortality burden is not routinely documented in official reports of wildfire damage and losses. By employing high-resolution air-quality forecast data, we quantify the excess mortality from smoke exposure during a wildfire episode in Los Angeles County in 2025. We found unhealthy air quality in the most populous census tracts immediately after the fire outbreak that persisted for several days. We estimated a total of 14 unaccounted excess deaths from acute wildfire smoke exposure, which is equivalent to 47% of direct fire fatalities. Our study shows that excess mortality from smoke exposure, even for a few days, causes increased mortality, which requires consideration for a comprehensive assessment of wildfires.

6. A self-consistent analysis of cluster morphology in supercritical carbon dioxide from Small Angle X-ray Scattering

   P. Muhunthan, A. Majumdar, K. Younes, G. Vignat, H. Li, I. Rajković, T. Weiß, D. Sokaras, and M. Ihme

   Chemical Physics Letters, 2025

   DOI
7. Supercritical Ethanol–CO₂ Mixtures Exhibit Microscopic Immiscibility: A Combined Study Using X-ray Scattering and Molecular Dynamics Simulations

   J. Fan, T. Yoon, G. Vignat, H. Li, K. Younes, A. Majumdar, P. Muhunthan, D. Sokaras, T. Weiß, I. Rajković, and M. Ihme

   The Journal of Physical Chemistry Letters, 2025

   Abs DOI

   Supercritical mixtures of ethanol (EtOH) and carbon dioxide (CO 2 ) are classified as type-I mixtures, with complete macroscopic miscibility. However, differences in molecular polarity and interactions suggest a distinct phase behavior at the microscopic level. Here, we combine small angle X-ray scattering experiments and molecular dynamics (MD) simulations to investigate the microscopic structure of EtOH–CO 2 mixtures under supercritical conditions. The structure factor exhibits nonlinear composition-dependent behavior, revealing pronounced local density fluctuations. The complementary MD simulations, using optimized force field parameters, provide atomistic insight, showing that EtOH forms self-associated, hydrogen-bonded aggregates, while CO 2 remains more uniformly distributed. Cluster analysis identifies a preferential EtOH-rich composition exceeding the bulk average, governed by a balance between energetic and entropic competition. These results demonstrate that, contrary to macroscopic expectations, the mixture exhibits significant microscopic heterogeneity and immiscibility, which may influence solubility, reactivity, transport properties, and thermodynamic response functions. These findings challenge the conventional views of type-I fluids and emphasize the necessity of revising mixture states and considering molecular polarity.

8. Examining fire spread dynamics in canyon terrain through physics-based modeling: Mechanisms of fire line rotation and non-local fire behavior

   K. Töpperwien, M. Wang, Y.-F. Chen, C. Gazen, J. Anderson, and M. Ihme

   Proceedings of the Combustion Institute, 2025

   DOI
9. Deep reinforcement learning for adaptive control of thermoacoustic instabilities in a lean-premixed methane/hydrogen/air combustor

   B. Akoush, G. Vignat, R. Finley, W. T. Chung, and M. Ihme

   Combustion and Flame, 2025

   DOI
10. Nanoscale Ultrafast Lattice Modulation with Hard X-ray Free Electron Laser

    H. Li, N. Wang, L. Zhang, S. Song, Y. Sun, M.-L. Ng, T. Sato, D. F. Hanlon, S. Dahal, M. Balcazar, V. Esposito, S. She, C. Ornelas-Skarin, J. Vila‐Comamala, C. Dávid, N. Berndt, P. R. Miedaner, Z. Zhang, M. Ihme, M. Trigo, K. A. Nelson, J. Hastings, A. A. Maznev, L. Foglia, S. W. Teitelbaum, D. A. Reis, and D. Zhu

    ArXiv.org, 2025

    Abs DOI Open Access

    Understanding and controlling microscopic dynamics across spatial and temporal scales has driven major progress in science and technology over the past several decades. While ultrafast laser-based techniques have enabled probing nanoscale dynamics at their intrinsic temporal scales down to femto- and attoseconds, the long wavelengths of optical lasers have prevented the interrogation and manipulation of such dynamics with nanoscale spatial specificity. With advances in hard X-ray free electron lasers (FELs), significant progress has been made developing X-ray transient grating (XTG) spectroscopy, aiming at the coherent control of elementary excitations with nanoscale X-ray standing waves. So far, XTGs have been probed only at optical wavelengths, thus intrinsically limiting the achievable periodicities to several hundreds of nm. By achieving sub-femtosecond synchronization of two hard X-ray pulses at a controlled crossing angle, we demonstrate the generation of an XTG with spatial periods of 10 nm. The XTG excitation drives a thermal grating that drives coherent monochromatic longitudinal acoustic phonons in the cubic perovskite, SrTiO3 (STO). With a third X-ray pulse with the same photon energy, time-and-momentum resolved measurement of the XTG-induced scattering intensity modulation provides evidence of ballistic thermal transport at nanometer scale in STO. These results highlight the great potential of XTG for studying high-wave-vector excitations and nanoscale transport in condensed matter, and establish XTG as a powerful platform for the coherent control and study of nanoscale dynamics.

11. A staged deep learning approach to spatial refinement in 3D temporal atmospheric transport

    M. G. Fernández-Godino, W. T. Chung, A. Gowardhan, M. Ihme, Q. Kong, D. D. Lucas, and S. C. Myers

    Artificial Intelligence in Geosciences, 2025

    Abs DOI Open Access

    High-resolution spatiotemporal simulations effectively capture the complexities of atmospheric plume dispersion in complex terrain. However, their high computational cost makes them impractical for applications requiring rapid responses or iterative processes, such as optimization, uncertainty quantification, or inverse modeling. To address this challenge, this work introduces the Dual-Stage Temporal Three-dimensional UNet Super-resolution (DST3D-UNet-SR) model, a highly efficient deep learning model for plume dispersion predictions. DST3D-UNet-SR is composed of two sequential modules: the temporal module (TM), which predicts the transient evolution of a plume in complex terrain from low-resolution temporal data, and the spatial refinement module (SRM), which subsequently enhances the spatial resolution of the TM predictions. We train DST3D-UNet-SR using a comprehensive dataset derived from high-resolution large eddy simulations (LES) of plume transport. We propose the DST3D-UNet-SR model to significantly accelerate LES of three-dimensional (3D) plume dispersion by three orders of magnitude. Additionally, the model demonstrates the ability to dynamically adapt to evolving conditions through the incorporation of new observational data, substantially improving prediction accuracy in high-concentration regions near the source.

12. Effect of Synthetic Aviation Fuels on the Stochastic Ignition of Fuel Droplets on Hot Surfaces

    G. Vignat, Y. Ma, J. Z. Ho, Y.-H. Cho, N. Hashimoto, T. Dinelli, T. Yoon, C. A. Fisher, and M. Ihme

    AIAA SCITECH 2025 Forum, 2025

    Abs DOI

    When a flammable liquid is put in contact with a very hot surface, thermal ignition of fuel vapors can occur. In the event of a leak, this process, called hot surface ignition, can lead to fires in aircrafts, spacecrafts, vehicles, and machinery. In the aerospace industry, design practices and certification processes must ensure that this fire hazard is mitigated. In the present work, we performed experiments to assess whether sustainable aviation fuels have a different hot surface ignition behavior compared to petroleum-derived jet fuel. In a canonical configuration, 2.5mm fuel droplets were released onto a high temperature optically accessible surface. After a 150mm fall representative of a typical aircraft engine compartment, the droplets broke up upon impact and ignition occured if the surface temperature was sufficiently high. The temperature of ignition was quantified and we found that all investigated fuels had a similar or higher hot surface ignition temperature to jet-A in this configuration. The temperature of ignition spanned a 70K range for the 4 fuels investigated. High speed shadowgraphy revealed the effect of fuel surface tension on the droplet break-up process, and high speed chemiluminescence imaging revealed the multi-step ignition process that the droplet underwent.

13. Shock-Induced Vanishing Dynamics of Water-Droplet Interface

    B. Krisna, N. Ly, and M. Ihme

    AIAA SCITECH 2025 Forum, 2025

    Abs DOI

    Novel combustion systems in aerospace propulsion systems as scramjets and rotating detonation engines utilize higher pressure conditions to achieve optimal thermodynamic efficiency. As a result, the combustion chamber pressure may affect the interface stability of water, one of the main combustion products. Although the immiscibility between the liquid water droplet and the ambiance guarantees the formation of the subcritical interface, its dynamics and impact is typically neglected in many simulations. These are even more prominent with interfacial dynamics due to transcritical effects that arise due to strong shocks inside the combustion chamber, which is typical in modern detonation engines. To elucidate this, we consider a shock of M=1.36 for a water/nitrogen system, where the post-shock liquid- and vapor-phase conditions (T_1=600 K and T_2=730 K) are fully transcritical with respect to the critical mixing point. In this work, we employ the Regularized Interface Method (RIM), since it is able to resolve both subcritical interfacial dynamics along with supercritical mixing dynamics. Vanishing and spontaneous emergence of the interface is observed, indicating the need of resolving the interfacial dynamics and effects.

14. Interface-Capturing Simulation of High-Pressure LOX/GH2 Mixing Layer

    N. Ly, and M. Ihme

    AIAA SCITECH 2025 Forum, 2025

    Abs DOI

    The mixing dynamics of injected propellants is a key factor in determining the ignition performance and combustion-instability response of liquid rocket engines. A key source of uncertainty in the prediction of LOX/GH2 mixing dynamics is the immiscibility of the injected propellants at supercritical pressure conditions, which induces liquid/vapor phase separation and surface-tension effects. However, modeling of LOX/GH2 mixing layers typically employs the Diffuse Interface Method (DIM), where interfacial dynamics are neglected. Recently, the Regularized Interface Method (RIM) was developed to consistently model the interfacial flows of immiscible mixtures at high pressure. Thus, the objective of this paper is to provide quantification of the impact of resolving interfacial effects on modeling of LOX/GH2 injection dynamics in both cryogenic temperature and transcritical conditions, simulating cold-start and stable operation, respectively. To accomplish this, we analyze simulation results of a benchmark high-pressure LOX/GH2 mixing layer configuration using RIM and DIM formulations at these respective conditions. Results of the study will quantify the need for interface-capturing simulations of propellant injection, even at supercritical pressure, as well as shed light on the complex transition dynamics from subcritical interfacial flows to supercritical mixing as the surface temperature is heated beyond the mixture’s critical point.

15. Observations and analysis of near-critical fuel injection

    Z. Alexander, T. Krause, N. Ly, U. Hegde, M. Hicks, and M. Ihme

    The Journal of Supercritical Fluids, 2025

    DOI
16. Multi-modal effects on indirect noise induced by turbulent entropy fields

    D. Brouzet, B. Krisna, and M. Ihme

    Journal of Fluid Mechanics, 2025

    Abs DOI Open Access

    Planar entropy waves are commonly assumed for predicting indirect combustion noise. However, the non-planar and turbulent nature of flows found in most practical combustors challenges this assumption. In the present paper, we examine the indirect noise generated by non-planar and turbulent entropy fields through subsonic nozzles. Firstly, we introduce a new transfer function framework that accounts for the contribution of non-planar Fourier modes of the entropy field to the indirect noise spectra. When applied to a turbulent flow field, this method demonstrates a significant improvement in spectral predictions compared with a conventional approach that only considers the planar mode. Secondly, simulations show that non-planar Fourier modes become significant above a threshold frequency f_thresh , found in the mid- to high-frequency range. This contribution of non-planar modes is explained by two-dimensional shear effects that distort the entropy waves. A scaling relation that uses residence times along streamlines is developed for f_thresh , showing good agreement with simulation results. Finally, we show that the indirect noise from non-planar entropy modes found in aviation combustors can be significant at frequencies below 1 kHz, which might be relevant in situations of thermo-acoustic instabilities coupled to indirect noise.

17. Machine learning and data-driven approaches

    M. Ihme, J. Z. Ho, and W. T. Chung

    In Elsevier eBooks, 2025

    DOI
18. Physics-Based Model for Predicting Jet Plumes in Supersonic Retropropulsion

    D. Wu, K. Töpperwien, S. Wada, and M. Ihme

    Journal of Spacecraft and Rockets, 2025

    Abs DOI

    Supersonic retropropulsion (SRP) is an enabling technology for human missions to Mars and other planetary bodies, as existing parachute systems cannot provide sufficient deceleration for human-scale payloads. SRP is the use of rockets to decelerate a spacecraft by exhausting in the opposite direction of its descent. We present a physical model that predicts the boundary of a high-thrust and underexpanded single-jet plume in blunt penetration mode at zero angle of attack, which is then used in a control-volume analysis to predict the deceleration force caused by the impingement of the freestream flow on the spacecraft heatshield. The jet plume model is based on the axisymmetric method of characteristics and accounts for the post-bow-shock pressure and Mach disk location on the jet plume boundary. This model considers effects of stagnation pressure in the recirculation region for which we add a pressure deficit term in the force computation. Results demonstrate improved accuracy in predicting jet plume boundaries and force, under various operating conditions compared to existing analytical methods. Our method offers an improved low-order physics-based approach that aids in performing rapid analyses of SRP quantities of interest, thus adding to the engineering methods for designing entry, descent, and landing architectures.

2024

1. Analysis of weak secondary waves in a rotating detonation engine using large-eddy simulation and wavenumber-domain filtering

   G. Vignat, D. Brouzet, M. Bonanni, and M. Ihme

   Combustion and Flame, 2024

   DOI Open Access
2. Experimental demonstration of a two-stage porous media burner for low-emission ammonia combustion

   G. Vignat, T. Zirwes, É. Boigné, and M. Ihme

   Proceedings of the Combustion Institute, 2024

   DOI
3. Artificial intelligence as a catalyst for combustion science and engineering

   M. Ihme, and W. T. Chung

   Proceedings of the Combustion Institute, 2024

   DOI Open Access
4. Analysis of residence time distribution in a cavity-stabilized scramjet combustor

   M. Bonanni, A. Norris, and M. Ihme

   Proceedings of the Combustion Institute, 2024

   DOI
5. Heterogeneous Cluster Energetics and Nonlinear Thermodynamic Response in Supercritical Fluids

   J. Fan, N. Ly, and M. Ihme

   Physical Review Letters, 2024

   Abs DOI Open Access

   Microstructural heterogeneities arising from molecular clusters directly affect the nonlinear thermodynamic properties of supercritical fluids. We present a physical model to elucidate the relation between energy exchange and heterogeneous cluster dynamics during the transition from liquidlike to gaslike conditions. By analyzing molecular-dynamics data and employing physical principles, the model considers contributions from three key processes, namely, changing cluster density, cluster separation, and transfer of molecules between clusters. We show that the proposed model is consistent with the energetics at subcritical conditions and can be used to explain the nonlinear behavior of thermodynamic response functions, including the peak in the isobaric heat capacity.

6. A high-fidelity ensemble simulation framework for interrogating wildland-fire behaviour and benchmarking machine learning models

   M. Wang, M. Ihme, C. Gazen, Y. Chen, and J. R. Anderson

   International Journal of Wildland Fire, 2024

   Abs DOI Open Access

   Background Wildfire research uses ensemble methods to analyse fire behaviours and assess uncertainties. Nonetheless, current research methods are either confined to simple models or complex simulations with limitations. Modern computing tools could allow for efficient, high-fidelity ensemble simulations. Aims This study proposes a high-fidelity ensemble wildfire simulation framework for studying wildfire behaviour, assessing fire risks, analysing uncertainties, and training machine learning (ML) models. Methods We present a simulation framework that integrates the Swirl-Fire large-eddy simulation tool for wildfire predictions with the Vizier optimisation platform for automated run-time management of ensemble simulations and large-scale batch processing. All simulations are executed on tensor-processing units to enhance computational efficiency. Key results A dataset of 117 simulations is created, each with 1.35 billion mesh points. The simulations are compared to existing experimental data and show good agreement in terms of fire rate of spread. Analysis is performed for fire acceleration, mean rate of spread, and fireline intensity. Conclusions Strong coupling between wind speed and slope is observed for fire-spread rate and intermittency. A critical Froude number that delineates fires from plume-dominated to wind-dominated is identified and confirmed with literature observations. Implications The ensemble simulation framework is efficient in facilitating large-scale parametric wildfire studies.

7. Direct observation of ultrafast cluster dynamics in supercritical carbon dioxide using X-ray Photon Correlation Spectroscopy

   A. Majumdar, H. Li, P. Muhunthan, A. Späh, S. Song, Y. Sun, M. Chollet, D. Sokaras, D. Zhu, and M. Ihme

   Nature Communications, 2024

   Abs DOI Open Access

   Supercritical fluids exhibit distinct thermodynamic and transport properties, making them of particular interest for a wide range of scientific and engineering applications. These anomalous properties emerge from structural heterogeneities due to the formation of molecular clusters at conditions above the critical point. While the static behavior of these clusters and their effects on the thermodynamic response functions have been recognized, the relation between the ultrafast cluster dynamics and transport properties remains elusive. By measuring the intermediate scattering function in carbon dioxide at conditions near the critical point with X-ray photon correlation spectroscopy, we directly capture the cross-over dynamics between 4 and 13 picoseconds, revealing the transition between ballistic and diffusive motion. Complementary analysis using large-scale molecular dynamics simulations reveals that this behavior arises from collisions between unbound molecules and clusters. This study provides direct evidence of the ultrafast momentum exchange between clusters, which has significant impact on transport properties, solvation processes, and reaction kinetics in supercritical fluids. A transition from ballistic to diffusive motion within 10 ps is observed in supercritical carbon dioxide with X-ray photon correlation spectroscopy. Collisions of unbound molecules with clusters are responsible for the ultrafast momentum exchange.

8. Mixtures Recomposition by Neural Nets: A Multidisciplinary Overview

   A. Nicolle, S. Deng, M. Ihme, N. Kuzhagaliyeva, E. Al Ibrahim, and A. Farooq

   Journal of Chemical Information and Modeling, 2024

   Abs DOI

   Artificial Neural Networks (ANNs) are transforming how we understand chemical mixtures, providing an expressive view of the chemical space and multiscale processes. Their hybridization with physical knowledge can bridge the gap between predictivity and understanding of the underlying processes. This overview explores recent progress in ANNs, particularly their potential in the ’recomposition’ of chemical mixtures. Graph-based representations reveal patterns among mixture components, and deep learning models excel in capturing complexity and symmetries when compared to traditional Quantitative Structure-Property Relationship models. Key components, such as Hamiltonian networks and convolution operations, play a central role in representing multiscale mixtures. The integration of ANNs with Chemical Reaction Networks and Physics-Informed Neural Networks for inverse chemical kinetic problems is also examined. The combination of sensors with ANNs shows promise in optical and biomimetic applications. A common ground is identified in the context of statistical physics, where ANN-based methods iteratively adapt their models by blending their initial states with training data. The concept of mixture recomposition unveils a reciprocal inspiration between ANNs and reactive mixtures, highlighting learning behaviors influenced by the training environment.

9. Small-scale turbulent characteristics in transcritical wall-bounded flows

   F. Li, W. Zhang, B. Bai, and M. Ihme

   Journal of Fluid Mechanics, 2024

   Abs DOI Open Access

   Considerable efforts have been devoted to the understanding of the small-scale characteristics in turbulent flows. While the universality of small-scale quantities has been established for incompressible flows, their extension to high-pressure transcritical flows remains an open area of research. To address this question, we investigate the real-fluid thermodynamic effects on small-scale velocity statistics of high-pressure transcritical wall-bounded turbulence. We show that in the locally isotropic region for transcritical flows, low-order moments of small-scale statistics collapse for all cases and Kolmogorov’s (1941) theory holds. However, real-fluid thermodynamic effects introduce deviations in the tails of the probability density function for the velocity derivative and, consequently, high-order moments of velocity gradients and dissipation rate in transcritical flows cannot collapse in the locally isotropic region. Analysis of the intermittency shows that the low-order structure functions in transcritical flows follow extended self-similarity, while the dependence of the intermittency factor on real-fluid effects is observed for high-order structure functions. The real-fluid effects on intermittency are explained by turbulent structures related to rare events. Additionally, the dissipation rate moments for transcritical flows follow a universal scaling with Reynolds number, and the scaling exponents are different from those of incompressible flows. These results extend the small-scale universality in incompressible flows (Schumacher et al. , Proc. Natl Acad. Sci. USA , vol. 111, 2014, pp. 10961–10965) to realistic flows with significant changes in thermodynamic properties, and provide a physical underpinning of the scaling laws of small-scale statistics at transcritical conditions.

10. A regularized-interface method as a unified formulation for simulations of high-pressure multiphase flows

    N. Ly, and M. Ihme

    Journal of Computational Physics, 2024

    DOI Open Access
11. A high-order diffused-interface approach for two-phase compressible flow simulations using a discontinuous Galerkin framework

    N. Tonicello, and M. Ihme

    Journal of Computational Physics, 2024

    DOI
12. Approximately pressure-equilibrium-preserving scheme for fully conservative simulations of compressible multi-species and real-fluid interfacial flows

    H. Terashima, N. Ly, and M. Ihme

    Journal of Computational Physics, 2024

    DOI
13. Examining diesel-spray assisted ignition of ammonia under reactivity-controlled conditions using large-eddy simulations

    P. Sharma, D. Brouzet, W. T. Chung, and M. Ihme

    Proceedings of the Combustion Institute, 2024

    DOI Open Access
14. Experiment and modeling of stochastic ignition and combustion of fuel droplets impacting a hot surface

    N. Ly, Y. Ma, G. Vignat, N. Hashimoto, and M. Ihme

    Proceedings of the Combustion Institute, 2024

    DOI
15. Mechanisms and models of the turbulent boundary layers at transcritical conditions

    F. Li, W. Zhang, and M. Ihme

    Physical Review Fluids, 2024

    Abs DOI Open Access

    To provide theoretical support for the development of wall models for simulating transcritical flows, we investigate structural properties in the turbulent boundary layers of transcritical flows. Primary innovations include: i) Substantiating the inner-outer interactions in turbulent boundary layers with strong variations in thermodynamic properties; ii) Extending the traditionally incompressible attached eddy model to transcritical flows and provide new scaling for the mixing length model and the wall-attached eddy model by considering inner-outer interactions. This mixing length model can be used in the nonequilibrium wall model for wall-modeled large eddy simulations (WMLES) methodology.

16. A versatile pressure-cell design for studying ultrafast molecular-dynamics in supercritical fluids using coherent multi-pulse x-ray scattering

    P. Muhunthan, H. Li, G. Vignat, E. R. Toro, K. Younes, Y. Sun, D. Sokaras, T. Weiß, I. Rajković, T. Osaka, I. Inoue, S. Song, T. Sato, D. Zhu, J. L. Fulton, and M. Ihme

    Review of Scientific Instruments, 2024

    Abs DOI Open Access

    Supercritical fluids (SCFs) can be found in a variety of environmental and industrial processes. They exhibit an anomalous thermodynamic behavior, which originates from their fluctuating heterogeneous micro-structure. Characterizing the dynamics of these fluids at high temperature and high pressure with nanometer spatial and picosecond temporal resolution has been very challenging. The advent of hard x-ray free electron lasers has enabled the development of novel multi-pulse ultrafast x-ray scattering techniques, such as x-ray photon correlation spectroscopy (XPCS) and x-ray pump x-ray probe (XPXP). These techniques offer new opportunities for resolving the ultrafast microscopic behavior in SCFs at unprecedented spatiotemporal resolution, unraveling the dynamics of their micro-structure. However, harnessing these capabilities requires a bespoke high-pressure and high-temperature sample system that is optimized to maximize signal intensity and address instrument-specific challenges, such as drift in beamline components, x-ray scattering background, and multi-x-ray-beam overlap. We present a pressure cell compatible with a wide range of SCFs with built-in optical access for XPCS and XPXP and discuss critical aspects of the pressure cell design, with a particular focus on the design optimization for XPCS.

17. A Normal-Score Ensemble Kalman Filter for 1D Shock Waves

    J. J. Hansen, D. Brouzet, and M. Ihme

    AIAA SCITECH 2024 Forum, 2024

    Abs DOI

    A normal-score ensemble Kalman filter (NS-EnKF) formulation was developed and implemented to evaluate performance in assimilating observations into non-linearly evolving, non-Gaussian distributed systems. The NS-EnKF was benchmarked against the stochastic ensemble Kalman filter (EnKF) without localization on the Lorenz-96 system. Each filter was run for 100 stochastic trials with 10, 25, and 100 ensemble members. Compared to the EnKF, assimilation of 10 and 25 ensemble members with the NS-EnKF generated predictions which were more stable and had lower state prediction errors. When 100 ensemble members were used, the EnKF had lower prediction error on average while the NS-EnKF continued to offer greater solution stability. Filters were also compared using a numerical shock tube configuration with uncertainty in initial pressures and temperatures. Several trials were run to assimilate sparse pressure observations with varying measurement error into pressure, temperature, and velocity fields. While the relative performance of the EnKF and NS-EnKF varied with the assumed measurement error, both filters improved predictions of shock location as well as pressure, temperature and velocity fields. Results indicate that the NS-EnKF can provide superior prediction accuracy to the EnKF in cases where non-Gaussian distributed states are prevalent, but that further study is required to determine the value of the NS-EnKF in improving state prediction for shock-related applications.

18. Stable supercritical interfaces do not exist without surface tension

    N. Ly, and M. Ihme

    Nature Communications, 2024

    Abs DOI Open Access

    Recently, Longmire et al. 1 studied the thermal diffusion process of (i) the steady-state configuration of a 1D heat-conduction problem with fixed Dirichlet boundary conditions and (ii) the unsteady configuration of supercritical liquid-like droplets placed in a supercritical gas-like environment. From theoretical analyses of the 1D boundary-value problem (BVP), they proposed a necessary condition on the fluid’s thermo-transport properties for the emergence of an inflection point in the steady-state solution, which they then interpreted as a stable supercritical interfacial structure that can be exhibited by pure fluids even in the absence of surface tension. Longmire et al. 1 then further corroborated their claim of stable supercritical interfacial dynamics, limited to single-species systems, using results from their supercritical droplet diffusion simulations, most importantly the observed initial density-gradient sharpening dynamics. Here, we offer an alternative explanation of their analyses and observations that does not imply a stable supercritical pure-fluid interface in the absence of capillary forces.

19. Coupling of detonation structure and upstream inhomogeneities in a rotating detonation engine

    M. Bonanni, D. Brouzet, G. Vignat, and M. Ihme

    Proceedings of the Combustion Institute, 2024

    DOI
20. On the importance of species immiscibility in mixing-layer dynamics at supercritical pressures

    N. Ly, and M. Ihme

    International Journal of Multiphase Flow, 2024

    DOI
21. Augmenting filtered flame front displacement models for LES using machine learning with a posteriori simulations

    J. Z. Ho, M. Talei, D. Brouzet, W. T. Chung, P. Sharma, and M. Ihme

    Proceedings of the Combustion Institute, 2024

    Abs DOI Open Access

    The Flame Surface Density (FSD) model is an affordable combustion model that has been widely used in simulating turbulent premixed flames . In Large Eddy Simulations (LES) with FSD, the combined effect of reaction and diffusion is governed by the Filtered Flame Front Displacement (FFFD) term. While the existing modelling approaches for this term are computationally cost-effective, their predictions still show inconsistencies in certain cases. This study aims to address these inconsistencies by generating Machine Learning (ML) models for the FFFD and FSD terms using the DNS data of a turbulent premixed jet flame. With this approach, the relevance of certain input parameters as well as certain modelling assumptions used for the FFFD term are assessed. Overall, it is found that the resolved curvature term is the most important input parameter to consider and that the resolved progress variable should also be considered in the models. It is shown that the ML models perform significantly better than legacy, algebraic formulations using a priori testing. To further assess the performance of ML, one of the ML models is employed in a a posteriori LES and compared against simulations with the algebraic model. The ML simulation is stable and yields encouraging improvements on key physical parameters regarding the flame length and the FFFD distribution. Novelty and Significance Statement: This research is of importance because it answers fundamental and practical questions related to the use of combustion modelling approaches, specifically the Flame Surface Density (FSD) and the Filtered Flame Front Displacement (FFFD) models, by means of Machine Learning (ML) algorithms. From a fundamental aspect, we show that two features which are typically not considered as inputs in combustion models, i.e., the progress variable and the resolved curvature, are key to consider for improved predictions of the model, more so than features which are typically used in FSD modelling, i.e., u Δ ′ , Δ , and | ∇ c ¯ | . From a practical standpoint, we demonstrate a framework to use the developed ML combustion model a posteriori in a LES, without any stability issues. Overall, these findings are key to guide further traditional and ML improvement efforts on combustion models.

22. Predictions of instantaneous temperature fields in jet-in-hot-coflow flames using a multi-scale U-Net model

    J. A. Kildare, W. T. Chung, M. J. Evans, Z. F. Tian, P. R. Medwell, and M. Ihme

    Proceedings of the Combustion Institute, 2024

    Abs DOI Open Access

    A multi-scale U-Net machine learning (ML) model is developed to assess its validity as a surrogate for non-intrusive flame temperature measurement in jet-in-hot-coflow (JHC) flames. Inputs to the model are simultaneous hydroxyl (OH) and formaldehyde (CH2O) planar laser-induced fluorescence (PLIF) measurements, with target temperature fields derived from Rayleigh scattering measurements. Coflow oxygen (O2) concentration, jet Reynolds number, coflow temperature, and fuel inputs were considered in the dataset, resulting in 33 unique flame conditions, and ∼17,000 training images. The ML model reconstructs the instantaneous temperature images to within an absolute error of 7 ± 10% of the measured values for the initial natural gas/ethylene model. Transfer learning was employed to accelerate training across conditions of different fuels, with mean absolute reconstruction error of 7 ± 9% for models of ethanol and dimethyl ether. A fourth model including data from all four fuels was trained using transfer learning, with 6 ± 9% error. Multi-layer perceptron (MLP) classification models were applied to the deepest layers of the four-fuel model to identify whether physical and chemical flame features were being encoded. Experimentally verifiable features were found to be encoded within the latent space of the model, demonstrating that it is not solely an image regression tool, but is capable of predicting flame features including the fuel type, initial coflow temperature, downstream axial location, and coflow O2 concentration. These findings indicate that the model is not only well suited for temperature predictions, but can provide scientific insight as well.

23. An enrichment wall modeling framework for spectral element methods

    S. Brill, P. Pal, M. Ameen, C. Xu, and M. Ihme

    Physics of Fluids, 2024

    Abs DOI Open Access

    In the present work, a first-of-its-kind enrichment wall-model is developed within the spectral element method (SEM) framework for large-eddy simulations (LES) of wall-bounded turbulent flows. The method augments the polynomial solution in the wall-adjacent elements with an analytical law-of-the-wall enrichment function representing the mean velocity near the wall. In the solution representation, this enrichment function captures the large gradients in the boundary layer, which allows the polynomial modes to represent the turbulent fluctuations. The enriched solution is able to resolve the shear stress at the wall without any modification to the no-slip wall boundary conditions, which allows for greater accuracy in the near-wall region compared to traditional methods. The enrichment wall modeling approach is implemented in a high-order SEM computational fluid dynamics solver, Nek5000, and its performance is assessed in turbulent channel flow wall-modeled LES for a range of Reynolds numbers. It is demonstrated that the enrichment wall-model improves solution accuracy on under-resolved near-wall grids as compared to traditional shear stress wall-models.

24. Ensemble predictions of laser ignition with a hybrid stochastic physics-embedded deep-learning framework

    W. T. Chung, C. Laurent, D. Passiatore, and M. Ihme

    Proceedings of the Combustion Institute, 2024

    DOI Open Access
25. Uncertainty quantification in coupled wildfire–atmosphere simulations at scale

    P. Schwerdtner, F. Law, M. Wang, C. Gazen, Y. Chen, M. Ihme, and B. Peherstorfer

    PNAS Nexus, 2024

    Abs DOI Open Access

    Uncertainties in wildfire simulations pose a major challenge for making decisions about fire management, mitigation, and evacuations. However, ensemble calculations to quantify uncertainties are prohibitively expensive with high-fidelity models that are needed to capture today’s ever-more intense and severe wildfires. This work shows that surrogate models trained on related data enable scaling multifidelity uncertainty quantification to high-fidelity wildfire simulations of unprecedented scale with billions of degrees of freedom. The key insight is that correlation is all that matters while bias is irrelevant for speeding up uncertainty quantification when surrogate models are combined with high-fidelity models in multifidelity approaches. This allows the surrogate models to be trained on abundantly available or cheaply generated related data samples that can be strongly biased as long as they are correlated to predictions of high-fidelity simulations. Numerical results with scenarios of the Tubbs 2017 wildfire demonstrate that surrogate models trained on related data make multifidelity uncertainty quantification in large-scale wildfire simulations practical by reducing the training time by several orders of magnitude from 3 months to under 3 h and predicting the burned area at least twice as accurately compared with using high-fidelity simulations alone for a fixed computational budget. More generally, the results suggest that leveraging related data can greatly extend the scope of surrogate modeling, potentially benefiting other fields that require uncertainty quantification in computationally expensive high-fidelity simulations.

26. FireBench: A High-fidelity Ensemble Simulation Framework for Exploring Wildfire Behavior and Data-driven Modeling

    Q. Wang, M. Ihme, C. Gazen, Y. Chen, and J. R. Anderson

    arXiv (Cornell University), 2024

    Abs DOI Open Access

    Background. Wildfire research uses ensemble methods to analyze fire behaviors and assess uncertainties. Nonetheless, current research methods are either confined to simple models or complex simulations with limits. Modern computing tools could allow for efficient, high-fidelity ensemble simulations. Aims. This study proposes a high-fidelity ensemble wildfire simulation framework for studying wildfire behavior, ML tasks, fire-risk assessment, and uncertainty analysis. Methods. In this research, we present a simulation framework that integrates the Swirl-Fire large-eddy simulation tool for wildfire predictions with the Vizier optimization platform for automated run-time management of ensemble simulations and large-scale batch processing. All simulations are executed on tensor-processing units to enhance computational efficiency. Key results. A dataset of 117 simulations is created, each with 1.35 billion mesh points. The simulations are compared to existing experimental data and show good agreement in terms of fire rate of spread. Computations are done for fire acceleration, mean rate of spread, and fireline intensity. Conclusions. Strong coupling between these 2 parameters are observed for the fire spread and intermittency. A critical Froude number that delineates fires from plume-driven to convection-driven is identified and confirmed with literature observations. Implications. The ensemble simulation framework is efficient in facilitating parametric wildfire studies.

27. A Spectral Element Enrichment Wall-Model

    S. Brill, P. Pal, M. Ameen, C. Xu, and M. Ihme

    arXiv (Cornell University), 2024

    Abs DOI Open Access

    We develop an enrichment wall-model within the spectral element method (SEM) framework for large-eddy simulations of wall-bounded flows. The method augments the polynomial solution in the wall-adjacent elements with an analytical law-of-the-wall enrichment function representing the mean velocity near the wall. In the solution representation, this enrichment function captures the large gradients in the boundary layer, which allows the polynomial modes to represent the turbulent fluctuations. The enriched solution is able to resolve the shear stress at the wall without any modification to the no-slip wall boundary conditions. The performance of the SEM enrichment wall-modeling approach is assessed in turbulent channel flow LES for a range of Reynolds numbers. The results show that the proposed method improves solution accuracy on under-resolved near-wall grids as compared to the traditional shear stress wall-models.

28. A Regularized-Interface Method for Unified Simulations of High-Pressure Multiphase Flows

    N. Ly, and M. Ihme

    SSRN Electronic Journal, 2024

    DOI Open Access
29. Generalisation of the Spectral Difference scheme for the diffused-interface five equation model

    N. Tonicello, G. Lodato, and M. Ihme

    arXiv (Cornell University), 2024

    Abs DOI Open Access

    The present work focuses on the generalisation of the Spectral Difference (SD) scheme to the reduced Baer-Nunziato system known as five-equation model for the simulation of two immiscible compressible fluids. This five equation model is considered with the additional Allen-Cahn regularisation to avoid both over-diffusion and over-thinning of the phase field representing the interface. Finally, in order to preserve contact discontinuities, in the reconstruction step of the spectral difference scheme, a change of variables from conservative to primitive is used. This approach is shown to be beneficial in avoiding pressure oscillations at material interfaces. An extensive series of numerical tests are proposed to assess accuracy and robustness of the present method. Both kinematic (Rider-Kothe vortex) and two-phase flow problems (Rayleigh-Taylor instability, shock-droplet interaction, Taylor-Green vortex) are considered.

30. Effect of Burn Parameters on PAH Emissions at Conditions Relevant for Prescribed Fires

    K. Töpperwien, G. Vignat, A. J. Feinberg, C. Daube, M. W. Alton, E. C. Fortner, M. R. Canagaratna, M. F. Kling, M. Johnson, K. C. Nadeau, S. C. Herndon, J. T. Jayne, and M. Ihme

    arXiv (Cornell University), 2024

    Abs DOI Open Access

    Wildfire smoke is a health hazard as it contains a mixture of carcinogenic volatile compounds and fine particulate matter. In particular, exposure to polycyclic aromatic hydrocarbons (PAHs) is a major concern, since these compounds have been recognized as important contributors to the overall carcinogenic risk of smoke exposure. In this work, gas and particle-phase PAH emissions from the combustion of Eastern White Pine (pinus strobus) were quantified using time-of-flight mass spectrometry over a range of burn conditions representative of wildfires and prescribed fires. These experiments allow for controlling conditions of fuel moisture, heat flux, and oxygen concentration to understand their impact on PAH emissions. We find that optimal conditions for fuel moisture content of 20 - 30%, heat load onto the sample of 60 - 70 kW/m^2, and oxygen concentrations of the burn environment of 5 - 15% can reduce the emissions of the heavy molar weight PAHs by up to 77%. Our analysis shows that the relative carcinogenic risk can be reduced by more than 50% under optimal conditions, offering a way for reducing emission exposure from forest treatment activities.

31. Low Order Model for Generating Supersonic Cavity Combustion Ignition Probability Maps

    R. Chen, M. Bonanni, and M. Ihme

    AIAA AVIATION FORUM AND ASCEND 2024, 2024

    Abs DOI

    Ignition probability maps are useful for characterizing ignition mechanisms in supersonic combustion engines. However, evaluating the ignition probability in 3D scramjet geometries using high-fidelity ensemble simulations is computationally expensive. This work offers a cost-effective alternative by using low-order models to approximate results obtained from 3D large-eddy simulations (LES), with a focus on the RC19 scramjet geometry located at the Air Force Research Laboratory (AFRL). To accomplish this, a quasi-3D RC19 mesh geometry that assumes spanwise uniformity is used to generate a 2D probabilistic ignition map using LES. Validated through comparison with experimental and computational ignition maps of the RC19 scramjet cavity, this map is used as a baseline of comparison for low-order model approaches. Statistical quantities from an inert simulation of the quasi-3D scramjet cavity are then sampled to perform stochastic 0D perfectly stirred reactor (PSR) simulations and produce a low-order ignition map. Next, the 0D simulation is modified to incorporate the advection and mixing properties present in the flow, improving the prediction. The ignition maps for the quasi-3D geometry generated from LES and 0D PSR models are compared to assess the performance of the proposed modeling approaches. The 0D PSR and advecting 0D PSR maps produced mean absolute errors of 22.8% and 12.7%, respectively, with a 99.7% reduction in computational cost. Qualitatively, the advecting 0D PSR map produced a profile similar to the baseline LES map in shape and region of highest ignition probability with exception to the left wall region due to the effects of flame kernel growth. Overall, the low-order models serve well as a preliminary assessment of ignition probability by finding ignition locations where higher fidelity simulations can then be focused.

32. Turbulence in disguise: Reactive flows in porous media mimic turbulent behavior

    É. Boigné, S. Sobhani, J. C. Ferguson, and M. Ihme

    Physical Review Fluids, 2024

    Abs DOI

    Chemically reacting flows through porous media are widespread in biological, environmental, and engineering applications. Yet, understanding these flows remains an outstanding challenging. This letter reveals that hydrodynamic dispersion affects reaction fronts in ways analogous to free turbulence. Our findings thus point to a regime diagram that elucidates the pore-scale coupling between fundamental processes, offering valuable theoretical insights into these complex flows.

33. A Self-Consistent Analysis of Cluster Morphology in Supercritical Carbon Dioxide Using Small Angle X-Ray Scattering

    P. Muhunthan, A. Majumdar, K. Younes, G. Vignat, H. Li, I. Rajković, T. Weiß, D. Sokaras, and M. Ihme

    SSRN Electronic Journal, 2024

    DOI Open Access
34. Video: Large-eddy simulation of the AFRL rotating detonation rocket engine

    M. Bonanni, G. Vignat, D. Brouzet, and M. Ihme

    77th Annual Meeting of the APS Division of Fluid Dynamics - Gallery of Fluid Motion, 2024

    DOI Open Access
35. COMPOSTIONAL INHOMOGENEITIES AS A SOURCE OF INDIRECT COMBUSTION NOISE: PHYSICAL MECHANISMS AND FUEL EFFECTS

    M. Ihme, J. R. O’Brien, and J. Kim

    24th International Conference on Sound and Vibration 2017, London Calling, 2024

    DOI

2023

1. A Review of Physics-Informed Machine Learning in Fluid Mechanics

   P. Sharma, W. T. Chung, B. Akoush, and M. Ihme

   Energies, 2023

   Abs DOI Open Access

   Physics-informed machine-learning (PIML) enables the integration of domain knowledge with machine learning (ML) algorithms, which results in higher data efficiency and more stable predictions. This provides opportunities for augmenting—and even replacing—high-fidelity numerical simulations of complex turbulent flows, which are often expensive due to the requirement of high temporal and spatial resolution. In this review, we (i) provide an introduction and historical perspective of ML methods, in particular neural networks (NN), (ii) examine existing PIML applications to fluid mechanics problems, especially in complex high Reynolds number flows, (iii) demonstrate the utility of PIML techniques through a case study, and (iv) discuss the challenges and opportunities of developing PIML for fluid mechanics.

2. Experimental and numerical investigation of flame stabilization and pollutant formation in matrix stabilized ammonia-hydrogen combustion

   G. Vignat, T. Zirwes, E. R. Toro, K. Younes, É. Boigné, P. Muhunthan, L. V. Simitz, D. Trimis, and M. Ihme

   Combustion and Flame, 2023

   DOI Open Access
3. Supercritical fluids behave as complex networks

   F. Simeski, and M. Ihme

   Nature Communications, 2023

   Abs DOI Open Access

   Supercritical fluids play a key role in environmental, geological, and celestial processes, and are of great importance to many scientific and engineering applications. They exhibit strong variations in thermodynamic response functions, which has been hypothesized to stem from the microstructural behavior. However, a direct connection between thermodynamic conditions and the microstructural behavior, as described by molecular clusters, remains an outstanding issue. By utilizing a first-principles-based criterion and self-similarity analysis, we identify energetically localized molecular clusters whose size distribution and connectivity exhibit self-similarity in the extended supercritical phase space. We show that the structural response of these clusters follows a complex network behavior whose dynamics arises from the energetics of isotropic molecular interactions. Furthermore, we demonstrate that a hidden variable network model can accurately describe the structural and dynamical response of supercritical fluids. These results highlight the need for constitutive models and provide a basis to relate the fluid microstructure to thermodynamic response functions.

4. Recurrent Convolutional Deep Neural Networks for Modeling Time-Resolved Wildfire Spread Behavior

   J. Burge, M. Bonanni, R. L. Hu, and M. Ihme

   Fire Technology, 2023

   DOI Open Access
5. Integrated experimental and computational analysis of porous media combustion by combining gas-phase synchrotron μ CT, IR-imaging, and pore-resolved simulations

   É. Boigné, T. Zirwes, D. Y. Parkinson, G. Vignat, P. Muhunthan, H. Barnard, A. A. MacDowell, and M. Ihme

   Combustion and Flame, 2023

   DOI
6. Improving volume-averaged simulations of matrix-stabilized combustion through direct X-ray µCT characterization: Application to NH3/H2-air combustion

   T. Zirwes, G. Vignat, E. R. Toro, É. Boigné, K. Younes, D. Trimis, and M. Ihme

   Combustion and Flame, 2023

   Abs DOI Open Access

   Porous media combustion (PMC) relies on internal heat recirculation in an open-cell ceramic foam matrix to enhance the flame speed of fuels with poor combustion properties. Volume-averaged simulations are often used to study the combustion performance and pollutant emissions of such systems. However, due to the varying complexity of matrix geometries found in practical burners, as well as the wide range of closure models for the constitutive relations of the solid phase, contradicting statements about the predictive accuracy of these volume-averaged models can be found in the literature. In this work, we propose an open-source modeling framework for accurate volume-averaged PMC simulations by using first-principles methods to determine effective properties used in closure models. This framework relies on adequately characterizing the topology of the solid matrix, using commonly available X-ray computed microtomography. With this approach, significant improvements in accuracy are reported compared to empirical models from the literature. The framework based on first-principle evaluations of constitutive relations is compared against experimental measurements conducted on an interface-stabilized burner operated with premixed NH3/H2-air. The model shows good agreement for exhaust gas composition and stability limits. The proposed simulation framework performs significantly better than state-of-the-art techniques that employ commonly used empirical correlations for effective matrix properties. We present a new open-source simulation framework for improved characterization of porous media combustion. By utilizing µCT techniques, accurate effective matrix properties can be determined from first-principle simulations. These effective properties are used in closure models for 1D volume-averaged reacting flow simulations using appropriate sub-models for heat recirculation. This modeling framework is able to reliably predict stability limits while conventional closure models yield erroneous trends. Assessment of the resulting modeling framework is performed using experiments with exhaust gas characterization performed on a NH3/H2-air porous media burner.

7. Analysis of direct and indirect noise in a next-generation aviation gas turbine combustor

   D. Brouzet, B. Krisna, D. C. McCormick, C. A. Reimann, J. Mendoza, and M. Ihme

   Combustion and Flame, 2023

   DOI Open Access
8. Cost-constrained adaptive simulations of transient spray combustion in a gas turbine combustor

   D. Mohaddes, D. Brouzet, and M. Ihme

   Combustion and Flame, 2023

   DOI Open Access
9. Analysis of real-fluid thermodynamic effects on turbulent statistics in transcritical channel flows

   F. Li, J. Guo, B. Bai, and M. Ihme

   Physical Review Fluids, 2023

   Abs DOI Open Access

   High-pressure transcritical turbulence with strong density fluctuations are encountered in many technical applications. Understanding of the turbulent energy transport modulated by real-fluid thermodynamic effects at transcritical conditions remains an open issue for developing reliable computational models. We clarify the effects of real-fluid thermodynamics through budget analyses of turbulent kinetic energy and dissipation rate from DNS of transcritical channel flows. From this physical underpinning, new scaling laws for turbulent length scales and turbulent kinetic energy budgets are proposed, thereby contributing to improvements of wall-models for LES and RANS simulations.

10. Machine Learning for Combustion Chemistry

    T. Echekki, A. Farooq, M. Ihme, and S. M. Sarathy

    In Lecture notes in energy, 2023

    Abs DOI Open Access

    Abstract Machine learning provides a set of new tools for the analysis, reduction and acceleration of combustion chemistry. The implementation of such tools is not new. However, with the emerging techniques of deep learning, renewed interest in implementing machine learning is fast growing. In this chapter, we illustrate applications of machine learning in understanding chemistry, learning reaction rates and reaction mechanisms and in accelerating chemistry integration.

11. Analysis of ducted fuel injection at high-pressure transcritical conditions using large-eddy simulations

    J. Guo, D. Brouzet, W. T. Chung, and M. Ihme

    International Journal of Engine Research, 2023

    Abs DOI Open Access

    Ducted fuel injection (DFI) is a proposed fuel injection concept for achieving substantial reductions in emissions. In this concept, the fuel is injected through a coannular duct, resulting in increased fuel-air mixing and minimized formation of soot and other unwanted combustion products. Apart from comprehensive experimental investigations on DFI, so far computational studies have been limited to single-point Reynolds-averaged Navier Stokes simulations. Therefore, the objective of this work is to complement these studies by performing large-eddy simulations using a diffuse-interface method to examine the physical mechanisms and combustion processes of DFI, specifically focusing on the mixing process and the effect of fuel-ducting on combustion and pollutant emissions. To this end, finite-rate chemistry simulations are performed of the DFI configuration corresponding to the Engine Combustion Network Spray A injector at transcritical conditions (n-dodecane fuel, 60 bar pressure and 1000 K temperature chamber conditions). A two-equation soot model is employed for the qualitative analysis of soot emissions. Direct comparisons of averaged and instantaneous flow field results with the Spray A configuration are performed to assess the effect of DFI on the first- and second-stage ignition and soot formation. Compared to the free-spray condition, the results show that the DFI case exhibits a combination of (i) increased mass flow rate and entrained air, (ii) larger pressure drop magnitude and flow velocity, and (iii) a closer-to-stoichiometric mixture composition (both globally and locally), each of which is conjectured to contribute toward reduced soot production.

12. A high-resolution large-eddy simulation framework for wildland fire predictions using TensorFlow

    M. Wang, M. Ihme, R. Linn, Y. Chen, V. Yang, F. Sha, C. B. Clements, J. S. McDanold, and J. Anderson

    International Journal of Wildland Fire, 2023

    Abs DOI Open Access

    Background Wildfires are becoming more severe, so we need improved tools to predict them over a wide range of conditions and scales. One approach towards this goal entails the use of coupled fire/atmosphere modelling tools. Although significant progress has been made in advancing their physical fidelity, existing tools have not taken full advantage of emerging programming paradigms and computing architectures to enable high-resolution wildfire simulations. Aims The aim of this study was to present a new framework that enables landscape-scale wildfire simulations with physical representation of combustion at an affordable cost. Methods We developed a coupled fire/atmosphere simulation framework using TensorFlow, which enables efficient and scalable computations on Tensor Processing Units. Key results Simulation results for a prescribed fire were compared with experimental data. Predicted fire behavior and statistical analysis for fire spread rate, scar area, and intermittency showed overall reasonable agreement. Scalability analysis was performed, showing close to linear scaling. Conclusions While mesh refinement was shown to have less impact on global quantities, such as fire scar area and spread rate, it benefits predictions of intermittent fire behavior, buoyancy-driven dynamics, and small-scale turbulent motion. Implications This new simulation framework is efficient in capturing both global quantities and unsteady dynamics of wildfires at high spatial resolutions.

13. Assessing requirements for modeling radiation in diffusion flames using an analytical, non-local model

    G. Fraga, B. Wu, M. Ihme, and X. Zhao

    Combustion and Flame, 2023

    Abs DOI Open Access

    An analytical approach for the calculation of radiative self-absorption in flames is presented, with the objective of providing an effective and computationally inexpensive tool for selecting radiation modeling methods for combustion applications. This one-dimensional non-local model approximates the flame structure as an infinitely long cylinder where all properties vary only along the radial direction . Flamelet solutions are then mapped to the radial direction and the radiative source terms, in particular the absorption, are computed analytically. The model is validated for a laminar jet flame and a turbulent pool fire, with results for the net radiative loss and the flame self-absorption showing good agreement to those of multidimensional, coupled simulations. To illustrate its applicability, the model is employed to study the characteristics of radiation in high-pressure combustion, in fire suppression and in hydrogen combustion. For the pressurized case, increased significance of radiation self-absorption is observed compared to an atmospheric flame, and accounting for the non-gray nature of radiation is found to be critical. In the fire suppression case, comparisons of the S-curves show that the choice of radiation model modifies the lower extinction limit, which becomes more important with reduced oxygen content. Lastly, for the hydrogen flame, the Planck-mean absorption coefficient is found to be more uniformly distributed in the mixture fraction space, and the optical thickness is smaller than a hydrocarbon flame at comparable conditions, indicating that radiation modeling requirements for that flame are likely less stringent. In general, the findings regarding radiation characteristics made on the basis of the non-local model are consistent with observations reported in the literature, but without the need for complex coupled combustion-radiation calculations. Therefore, the model provides a viable approach for selecting appropriate radiation models in combustion simulations.

14. Reaction nanoscopy of ion emission from sub‐wavelength propanediol droplets

    P. Rosenberger, R. Dagar, W. Zhang, A. Majumdar, M. Neuhaus, M. Ihme, B. Bergues, and M. F. Kling

    Nanophotonics, 2023

    Abs DOI Open Access

    Droplets provide unique opportunities for the investigation of laser-induced surface chemistry. Chemical reactions on the surface of charged droplets are ubiquitous in nature and can provide critical insight into more efficient processes for industrial chemical production. Here, we demonstrate the application of the reaction nanoscopy technique to strong-field ionized nanodroplets of propanediol (PDO). The technique’s sensitivity to the near-field around the droplet allows for the in-situ characterization of the average droplet size and charge. The use of ultrashort laser pulses enables control of the amount of surface charge by the laser intensity. Moreover, we demonstrate the surface chemical sensitivity of reaction nanoscopy by comparing droplets of the isomers 1,2-PDO and 1,3-PDO in their ion emission and fragmentation channels. Referencing the ion yields to gas-phase data, we find an enhanced production of methyl cations from droplets of the 1,2-PDO isomer. Density functional theory simulations support that this enhancement is due to the alignment of 1,2-PDO molecules on the surface. The results pave the way towards spatio-temporal observations of charge dynamics and surface reactions on droplets.

15. Turbulence in Focus: Benchmarking Scaling Behavior of 3D Volumetric Super-Resolution with BLASTNet 2.0 Data

    B. Akoush, D. Brouzet, J. Chen, W. T. Chung, J. Guo, M. Ihme, K. S. Jung, A. Poludnenko, B. Savard, P. Sharma, M. Talei, and A. Tamkin

    Advances in Neural Information Processing Systems 36, 2023

    DOI
16. Corrosive Influence of Carbon Dioxide on Crack Initiation in Quartz: Comparison With Liquid Water and Vacuum Environments

    F. Simeski, and M. Ihme

    Journal of Geophysical Research Solid Earth, 2023

    Abs DOI Open Access

    Abstract The stimulation of crack growth in quartz and siliceous materials by injecting carbon dioxide (CO 2 ) represents a key technology in long‐term carbon storage and in the development of natural gas wells. While this technology is widely used, the molecular impact of CO 2 interactions on the solid matrix is only incompletely understood. In this work, we employ reactive molecular dynamics simulations to study how the CO 2 fluid environment affects the mechanical properties of pre‐cracked single‐crystal quartz. The thermodynamic conditions of interest are those relevant to subsurface reservoirs. We report how structural properties of quartz—bond length distribution and crack tip shape—evolve upon introduction of a fluid. These properties are directly related to macroscopic quantities of the global stress–strain curves, thus reaffirming the inherent coupling across multiple scales for fluid–solid interactions in the subsurface. We find that CO 2 reduces the fracture toughness of quartz by 12.1% compared to that of quartz in vacuum, thereby promoting crack growth and enhancing fluid transport in the subsurface.

17. Evaluation of Electron Tomography Capabilities for Shale Imaging

    L. Frouté, É. Boigné, I. Jolivet, E. Chaput, P. Creux, M. Ihme, and A. R. Kovscek

    Microscopy and Microanalysis, 2023

    Abs DOI Open Access

    Despite the advantageous resolution of electron tomography (ET), reconstruction of three-dimensional (3D) images from multiple two-dimensional (2D) projections presents several challenges, including small signal-to-noise ratios, and a limited projection range. This study evaluates the capabilities of ET for thin sections of shale, a complex nanoporous medium. A numerical phantom with 1.24 nm pixel size is constructed based on the tomographic reconstruction of a Barnett shale. A dataset of 2D projection images is numerically generated from the 3D phantom and studied over a range of conditions. First, common reconstruction techniques are used to reconstruct the shale structure. The reconstruction uncertainty is quantified by comparing overall values of storage and transport metrics, as well as the misclassification of pore voxels compared to the phantom. We then select the most robust reconstruction technique and we vary the acquisition conditions to quantify the effect of artifacts. We find a strong agreement for large pores over the different acquisition workflows, while a wider variability exists for nanometer-scale features. The limited projection range and reconstruction are identified as the main experimental bottlenecks, thereby suggesting that sample thinning, advanced holders, and advanced reconstruction algorithms offer opportunities for improvement.

18. Accelerating Large‐Eddy Simulations of Clouds With Tensor Processing Units

    S. Chammas, M. Wang, T. Schneider, M. Ihme, Y.-F. Chen, and J. C. Anderson

    Journal of Advances in Modeling Earth Systems, 2023

    Abs DOI Open Access

    Abstract Clouds, especially low clouds, are crucial for regulating Earth’s energy balance and mediating the response of the climate system to changes in greenhouse gas concentrations. Despite their importance for climate, they remain relatively poorly understood and are inaccurately represented in climate models. A principal reason is that the high computational expense of simulating them with large‐eddy simulations (LES) has inhibited broad and systematic numerical experimentation and the generation of large data sets for training parametrization schemes for climate models. Here we demonstrate LES of low clouds on tensor processing units (TPUs), application‐specific integrated circuits that were originally developed for machine learning applications. We show that TPUs in conjunction with tailored software implementations can be used to simulate computationally challenging stratocumulus clouds in conditions observed during the Dynamics and Chemistry of Marine Stratocumulus (DYCOMS) field study. The TPU‐based LES code successfully reproduces clouds during DYCOMS and opens up the large computational resources available on TPUs to cloud simulations. The code enables unprecedented weak and strong scaling of LES, making it possible, for example, to simulate stratocumulus with 10× speedup over real‐time evolution in domains with a 34.7 km × 53.8 km horizontal cross section. The results open up new avenues for computational experiments and for substantially enlarging the sample of LES available to train parameterizations of low clouds.

19. Local Rearrangement in Adsorption Layers of Nanoconfined Ethane

    F. Simeski, J. Wu, S. Hu, T. T. Tsotsis, K. Jessen, and M. Ihme

    The Journal of Physical Chemistry C, 2023

    Abs DOI Open Access

    Fluids under nanoconfinement control many environmental and engineering processes. In particular, nanoconfined ethane commonly occurs in hydrocarbon production and gas separation applications. While previous research has measured the pore critical properties and self-diffusivity of nanoconfined ethane, the relationship between molecular organization and phase behavior in nanoporous media persists as a knowledge gap. Through experimental adsorption measurements and molecular dynamics simulations, we report the impact of pore wall proximity, pore size, and temperature on the structure of nanoconfined ethane. We offer evidence for an energetically favorable rearrangement of ethane molecules in the adsorption layer that may contribute to more accurate gas-in-place estimates and more efficient design of gas separation membranes.

20. On the noise analysis of a N+3 next-generation combustor through LES

    D. Brouzet, M. Ihme, D. C. McCormick, C. A. Reimann, and J. Mendoza

    AIAA AVIATION 2023 Forum, 2023

    Abs DOI

    View Video Presentation: https://doi.org/10.2514/6.2023-3939.vid A next-generation axially-staged combustor operating at take-off conditions is analyzed through large eddy simulations (LES) with the objective to analyze scaling laws for combustion noise modeling. Comparisons to experimental results are reported, including spray, chemiluminescence, and acoustic measurements. Given the high computational cost of LES, semi-empirical models are necessary to estimate combustion noise in aviation engines, especially in the preliminary design stages. Legacy models have proven valuable for many applications, but the databases underpinning these models are outdated, narrow in scope, and with limited geometric variations. To address these limitations, we tackle the low-order modeling aspect of this next-generation combustor by assessing scaling laws for the peak frequency and spectral shape of direct combustion noise. Existing combustion spectra models represent well the direct noise acoustic source term, as long as the frequency of peak emission is accurate. It is shown that the latter can be estimated with the model from Rajaram & Lieuwen (2009), at least when turbulent premixed flames are considered.

21. The Local Electronic Structure of Supercritical CO₂from X-ray Raman Spectroscopy and Atomistic-Scale Modeling

    P. Muhunthan, O. A. P. Mellone, T. Kröll, D. Sokaras, and M. Ihme

    The Journal of Physical Chemistry Letters, 2023

    Abs DOI Open Access

    Supercritical CO 2 is encountered in several technical and natural systems related to biology, geophysics, and engineering. While the structure of gaseous CO 2 has been studied extensively, the properties of supercritical CO 2, particularly close to the critical point, are not well-known. In this work, we combine X-ray Raman spectroscopy, molecular dynamics simulations, and first-principles density functional theory (DFT) calculations to characterize the local electronic structure of supercritical CO 2 at conditions around the critical point. The X-ray Raman oxygen K-edge spectra manifest systematic trends associated with the phase change of CO 2 and the intermolecular distance. Extensive first-principles DFT calculations rationalize these observations on the basis of the 4sσ Rydberg state hybridization. X-ray Raman spectroscopy is found to be a sensitive tool for characterizing electronic properties of CO 2 under challenging experimental conditions and is demonstrated to be a unique probe for studying the electronic structure of supercritical fluids.

22. Autonomous screening of complex phase spaces using Bayesian optimization for SAXS measurements

    K. Younes, M. Poli, P. Muhunthan, I. Rajković, S. Ermon, T. Weiß, and M. Ihme

    Nuclear Instruments and Methods in Physics Research Section A Accelerators Spectrometers Detectors and Associated Equipment, 2023

    DOI Open Access
23. Adaptive Modeling of Supersonic Combustion in a Cavity-Stabilized Scramjet

    M. Bonanni, and M. Ihme

    AIAA SCITECH 2023 Forum, 2023

    Abs DOI

    View Video Presentation: https://doi.org/10.2514/6.2023-1466.vid Large-eddy simulations of scramjet engines with accurate combustion modeling is generally prohibitively expensive because of the complex combustion processes occurring within these systems. Under these conditions, traditional tabulated chemistry methods perform poorly, leading to a reliance on finite-rate chemistry. The Pareto-efficient combustion (PEC) framework enables the dynamic assignment of different combustion models to specific regions of the computational domain, based on user-specified accuracy of quantities of interest and cost requirements. This enables the simulation to maintain fidelity where necessary and otherwise reduce cost where possible. To date, this framework has only been applied to low-Mach combustion problems. This work extends this PEC framework to supersonic combustion via application to steady-state combustion in the RC19 scramjet combustor rig at Wright-Patterson Air Force Base. The effectiveness of the framework is demonstrated via a comparison against experimental data to evaluate simulation accuracy.

24. A high-order diffused-interface approach for fully compressible two-phase flow simulations using a Discontinuous Galerkin method

    N. Tonicello, and M. Ihme

    AIAA SCITECH 2023 Forum, 2023

    Abs DOI

    View Video Presentation: https://doi.org/10.2514/6.2023-2154.vid A diffused-interface approach based on Allen-Cahn phase-field equation is developed within a high-order Discontinuous Galerkin framework. The interface capturing technique is based on the balance between an explicit diffusion contribution and a sharpening term in the phase-field equation. The latter term involves the computation of the normal vectors to the interface. Due to the steep profile of the interface, the computation of the normal vectors can be particularly delicate in such conditions. To this end, a non-linear preconditioning strategy is proposed in this work where an additional smooth level-set function is advected by the flow and used for the evaluation of the normal vectors. It is shown that for appropriate choices of numerical fluxes and parameters of the model, the phase-field remains bounded without any need for explicit clipping or mass redistribution. The proposed diffused-interface technique is then implemented within a five equation model for fully compressible two-phase flows. In order to preserve isolated interfaces, a quasi-conservative discretisation of the phase-field equation is employed. A series of numerical experiments are performed to assess the accuracy and robustness of the developed methodology, including inviscid two-phase flows.

2022

1. Combustion machine learning: Principles, progress and prospects

   M. Ihme, W. T. Chung, and A. Mishra

   Progress in Energy and Combustion Science, 2022

   Abs DOI Open Access

   Progress in combustion science and engineering has led to the generation of large amounts of data from large-scale simulations, high-resolution experiments, and sensors. This corpus of data offers enormous opportunities for extracting new knowledge and insights—if harnessed effectively. Machine learning (ML) techniques have demonstrated remarkable success in data analytics, thus offering a new paradigm for data-intense analyses and scientific investigations through combustion machine learning (CombML). While data-driven methods are utilized in various combustion areas, recent advances in algorithmic developments, the accessibility of open-source software libraries, the availability of computational resources, and the abundance of data have together rendered ML techniques ubiquitous in scientific analysis and engineering. This article examines ML techniques for applications in combustion science and engineering. Starting with a review of sources of data, data-driven techniques, and concepts, we examine supervised, unsupervised, and semi-supervised ML methods. Various combustion examples are considered to illustrate and to evaluate these methods. Next, we review past and recent applications of ML approaches to problems in combustion, spanning fundamental combustion investigations, propulsion and energy-conversion systems, and fire and explosion hazards. Challenges unique to CombML are discussed and further opportunities are identified, focusing on interpretability, uncertainty quantification, robustness, consistency, creation and curation of benchmark data, and the augmentation of ML methods with prior combustion-domain knowledge.

2. Next Day Wildfire Spread: A Machine Learning Dataset to Predict Wildfire Spreading From Remote-Sensing Data

   F. Huot, R. L. Hu, N. Goyal, T. Sankar, M. Ihme, and Y. Chen

   IEEE Transactions on Geoscience and Remote Sensing, 2022

   Abs DOI Open Access

   Predicting wildfire spread is critical for land management and disaster preparedness. To this end, we present ’Next Day Wildfire Spread,’ a curated, large-scale, multivariate data set of historical wildfires aggregating nearly a decade of remote-sensing data across the United States. In contrast to existing fire data sets based on Earth observation satellites, our data set combines 2D fire data with multiple explanatory variables (e.g., topography, vegetation, weather, drought index, population density) aligned over 2D regions, providing a feature-rich data set for machine learning. To demonstrate the usefulness of this data set, we implement a neural network that takes advantage of the spatial information of this data to predict wildfire spread. We compare the performance of the neural network with other machine learning models: logistic regression and random forest. This data set can be used as a benchmark for developing wildfire propagation models based on remote sensing data for a lead time of one day.

3. Chemical and Reactive Transport Processes Associated with Hydraulic Fracturing of Unconventional Oil/Gas Shales

   A. D. Jew, J. L. Druhan, M. Ihme, A. R. Kovscek, I. Battiato, J. Kaszuba, J. Bargar, and G. E. Brown

   Chemical Reviews, 2022

   Abs DOI Open Access

   Hydraulic fracturing of unconventional oil/gas shales has changed the energy landscape of the U.S. Recovery of hydrocarbons from tight, hydraulically fractured shales is a highly inefficient process, with estimated recoveries of <25% for natural gas and <5% for oil. This review focuses on the complex chemical interactions of additives in hydraulic fracturing fluid (HFF) with minerals and organic matter in oil/gas shales. These interactions are intended to increase hydrocarbon recovery by increasing porosities and permeabilities of tight shales. However, fluid-shale interactions result in the dissolution of shale minerals and the release and transport of chemical components. They also result in mineral precipitation in the shale matrix, which can reduce permeability, porosity, and hydrocarbon recovery. Competition between mineral dissolution and mineral precipitation processes influences the amounts of oil and gas recovered. We review the temporal/spatial origins and distribution of unconventional oil/gas shales from mudstones and shales, followed by discussion of their global and U.S. distributions and compositional differences from different U.S. sedimentary basins. We discuss the major types of chemical additives in HFF with their intended purposes, including drilling muds. Fracture distribution, porosity, permeability, and the identity and molecular-level speciation of minerals and organic matter in oil/gas shales throughout the hydraulic fracturing process are discussed. Also discussed are analysis methods used in characterizing oil/gas shales before and after hydraulic fracturing, including permeametry and porosimetry measurements, X-ray diffraction/Rietveld refinement, X-ray computed tomography, scanning/transmission electron microscopy, and laboratory- and synchrotron-based imaging/spectroscopic methods. Reactive transport and spatial scaling are discussed in some detail in order to relate fundamental molecular-scale processes to fluid transport. Our review concludes with a discussion of potential environmental impacts of hydraulic fracturing and important knowledge gaps that must be bridged to achieve improved mechanistic understanding of fluid transport in oil/gas shales.

4. Combustion of lean ammonia-hydrogen fuel blends in a porous media burner

   G. Vignat, B. Akoush, E. R. Toro, É. Boigné, and M. Ihme

   Proceedings of the Combustion Institute, 2022

   DOI
5. Structure of the thermal boundary layer in turbulent channel flows at transcritical conditions

   J. Guo, X. I. A. Yang, and M. Ihme

   Journal of Fluid Mechanics, 2022

   Abs DOI Open Access

   Enhanced fluctuations, steep gradients, and intensified heat transfer are characteristics of wall-bounded turbulence at transcritical conditions. Although such conditions are prevalent in numerous technical applications, the structure of the thermal boundary layer under realistic density gradients and heating conditions remains poorly understood. Specifically, statistical descriptions of the temperature field in such flows are provided inconsistently using existing models. To address this issue, direct numerical simulations are performed by considering fully developed transcritical turbulent channel flow at pressure and temperature conditions that cause density changes of a factor of up to O(20) between the hot and cold walls. As a consequence of the proximity of the Widom line to the hot wall, significant asymmetries are observed when comparing regions near the cold wall and near the hot wall. Previous transformations that attempt to collapse the near-wall mean temperature profiles among different cases to a single curve are examined. By addressing model deficiencies of these transformations, a formulation for an improved mean temperature transformation is proposed, with appropriate considerations for real fluid effects that involve strong variations in thermodynamic quantities. Our proposed transformation is shown to perform well in collapsing the slope of the logarithmic region to a single universal value with reduced uncertainty. Coupled with a predictive framework to estimate the non-universal shift parameter of the logarithmic region using a priori information, our transformation provides an analytic profile to model the near-wall mean temperature. These results thus provide a framework to guide the development of models for wall-bounded transcritical turbulence.

6. BLASTNet: A call for community-involved big data in combustion machine learning

   W. T. Chung, K. S. Jung, J. H. Chen, and M. Ihme

   Applications in Energy and Combustion Science, 2022

   Abs DOI Open Access

   Many state-of-the-art machine learning (ML) fields rely on large datasets and massive deep learning models (with O(109) trainable parameters) to predict target variables accurately without overfitting. Within combustion, a wealth of data exists in the form of high-fidelity simulation data and detailed measurements that have been accumulating since the past decade. Yet, this data remains distributed and can be difficult to access. In this work, we present a realistic and feasible framework which combines (i) community involvement, (ii) public data repositories, and (iii) lossy compression algorithms for enabling broad access to high-fidelity data via a network-of-datasets approach. This Bearable Large Accessible Scientific Training Network-of-Datasets (BLASTNet) is consolidated on a community-hosted web-platform (at https://blastnet.github.io/), and is targeted towards improving accessibility to diverse scientific data for deep learning algorithms. For datasets that exceed the storage limitations in public ML repositories, we propose employing lossy compression algorithms on high-fidelity data, at the cost of introducing controllable amounts of error to the data. This framework leverages the well-known robustness of modern deep learning methods to noisy data, which we demonstrate is also applicable in combustion by training deep learning models on lossy direct numerical simulation (DNS) data in two completely different ML problems — one in combustion regime classification and the other in filtered reaction rate regression. Our results show that combustion DNS data can be compressed by at least 10-fold without affecting deep learning models, and that the resulting lossy errors can even improve their training. We thus call on the research community to help contribute to opening a bearable pathway towards accessible big data in combustion.

7. Interaction of preferential evaporation and low-temperature chemistry in multicomponent counterflow spray flames

   M. Bonanni, and M. Ihme

   Proceedings of the Combustion Institute, 2022

   DOI
8. Regimes of evaporation and mixing behaviors of nanodroplets at transcritical conditions

   N. Ly, A. Majumdar, and M. Ihme

   Fuel, 2022

   DOI Open Access
9. The dynamics of non-premixed flames subjected to a transverse acoustic mode

   D. Brouzet, S. You, M. A. Plascencia, M. Roa, and M. Ihme

   Combustion and Flame, 2022

   DOI
10. On the hot surface ignition of a wall-stagnating spray flame

    D. Mohaddes, and M. Ihme

    Combustion and Flame, 2022

    DOI Open Access
11. Simulations of Dusty Flows over Full-Scale Capsule During Martian Entry

    E. J. Ching, N. Singh, and M. Ihme

    Journal of Spacecraft and Rockets, 2022

    Abs DOI

    Suspended dust particles can have adverse effects on heat shields during Mars atmospheric entry. For instance, they can increase both surface heating and surface recession. Current studies on this subject often make many simplifications, focus on a specific physical process, or include flow conditions different from those on Mars. To address this, we perform computational fluid dynamics simulations of hypersonic dusty flows over a full-scale entry capsule based on a realistic flight trajectory. The simulations are performed with an Euler–Lagrange methodology in which the carrier gas solution is obtained with a discontinuous Galerkin method. We employ a recently developed physics-based drag correlation that obtains better agreement with experimental data than popular existing correlations. To improve understanding of the relevant physical processes, which are still not very well understood, the effects of various factors, such as particle size distribution, angle of attack, and two-way coupling, on dust-induced heating augmentation and erosion are assessed.

12. Computing Thermodynamic Properties of Fluids Augmented by Nanoconfinement: Application to Pressurized Methane

    N. Singh, F. Simeski, and M. Ihme

    The Journal of Physical Chemistry B, 2022

    Abs DOI Open Access

    Nanoconfined fluids exhibit remarkably different thermodynamic behavior compared to the bulk phase. These confinement effects render predictions of thermodynamic quantities of nanoconfined fluids challenging. In particular, confinement creates a spatially varying density profile near the wall that is primarily responsible for adsorption and capillary condensation behavior. Significant fluctuations in thermodynamic quantities, inherent in such nanoscale systems, coupled to strong fluid-wall interactions give rise to this near-wall density profile. Empirical models have been proposed to explain and model these effects, yet no first-principles based formulation has been developed. We present a statistical mechanics framework that embeds such a coupling to describe the effect of the fluid-wall interaction in amplifying the near-wall density behavior for compressible gases at elevated pressures such as pressurized methane in confinement. We show that the proposed theory predicts accurately the adsorbed layer thickness as obtained with small-angle neutron scattering measurements. Furthermore, the predictions of density under confinement from the proposed theory are shown to be in excellent agreement with available experimental and atomistic simulations data for a range of temperatures for nanoconfined methane. While the framework is presented for evaluating the near-wall density, owing to its rigorous foundation in statistical mechanics, the proposed theory can also be generalized for predicting phase-transition and nonequilibrium transport of nanoconfined fluids.

13. Jet-entrainment sampling: A new method for extracting particles from flames

    H. A. Michelsen, É. Boigné, P. E. Schrader, O. Johansson, M. F. Campbell, R. Bambha, and M. Ihme

    Proceedings of the Combustion Institute, 2022

    DOI Open Access
14. Deep Learning for High-Resolution Wildfire Modeling

    M. A. Finney, J. Forthofer, X. Liu, J. Burge, M. Ihme, F. Sha, Y. Chen, J. Hickey, and J. Anderson

    In Imprensa da Universidade de Coimbra eBooks, 2022

    Abs DOI Open Access

    We demonstrate the use of a deep learning (DL) approach for representing the behavior of a high-resolution physics-based wildland fire spread model. The ultimate objective is being able to efficiently use the DL model for intensive simulations of large fires while retaining fidelity to the fine-scale physical processes. We begin with a fire model that reduces the spatial domain of the fire spread problem to one dimension (1D). The 1D model explicitly resolves cm-scale fuel variations, heat transfer and heating/drying dynamics of individual fuel particles and burning behavior of the bed. We then ran the fire model for 78,125 factorial combinations of fuel, weather, and topographic conditions as training data for the DL algorithm. The results of the DL analysis show overall agreement of 96% of the variation in fire behavior as represented by steady state rate of spread, flame length and flame zone depth. Exceptions to the DL regression indicate areas where more work is required in refining the resolution in training cases and use of advanced methods of embedding the fire model inside the DL algorithm loop.

15. Turbulence-induced bias in time-averaged laser absorption tomography of correlated concentration and temperature fields with a first-order correction

    C. Wei, N. Perakis, D. I. Pineda, F. N. Egolfopoulos, M. Ihme, and R. M. Spearrin

    Combustion and Flame, 2022

    DOI Open Access
16. Destabilization of binary mixing layer in supercritical conditions

    N. Ly, and M. Ihme

    Journal of Fluid Mechanics, 2022

    Abs DOI Open Access

    Compressible mixing layer instabilities are of importance to a wide range of environmental and industrial applications. Past studies have focused on either ideal-gas or real-fluid thermodynamic regimes of single-species mixing layers. However, mixing layers of binary mixtures at supercritical conditions, commonly encountered in fuel injection systems, introduce additional complexities due to the added compositional degree-of-freedom. Moreover, the effect of strong variations in thermodynamic response functions across the Widom line on the binary mixing layer stability remains poorly understood. Thus, the objective of this study is to examine the coupling between the hydrodynamic instability and the real-fluid thermodynamics across the Widom line and its effects on the overall binary mixing layer dynamics. To this end, we develop a linear stability analysis of the full binary-species compressible transport equations coupled with the PC-SAFT equation of state. Analysis shows the existence of a novel instability mechanism that arises from juxtapositioning of the Widom-line transition and the hydrodynamic inflection point. This novel thermodynamically induced instability mechanism has the net effect of destabilizing the binary mixing layer at lowering supercritical conditions towards the critical pressure point. This is in contrast to previous stability analyses of supercritical single-species mixing layers, where increasing pressure destabilizes the flow due to its effect on reducing the density stratification. The discovered thermodynamically induced instability mechanism of binary mixing flows highlights the need for an extension of classical instability criteria to incorporate the effect of strong variations in the thermodynamic response functions across the Widom line on mixing layer instability.

17. Dynamics and structure of detonations in stratified product-gas diluted mixtures

    D. Brouzet, G. Vignat, and M. Ihme

    Proceedings of the Combustion Institute, 2022

    DOI
18. Towards Data-Informed Motion Artifact Reduction in Quantitative CT Using Piecewise Linear Interpolation

    É. Boigné, D. Y. Parkinson, and M. Ihme

    IEEE Transactions on Computational Imaging, 2022

    Abs DOI

    We propose to use piecewise linear interpolation (PLI) in time to reduce motion artifacts in transmission computed tomography (CT). PLI is motivated by the natural occurrence of piecewise-linear evolution of voxel values during object motion. The method is specifically examined in the context of high-accuracy quantitative measurements that are compromised by small motions, and particularly sub-pixel motion. Compared to existing methods, the proposed approach offers three advantages: (i) the flexibility in the interpolation parameters provides a framework for joint optimization and data-informed dynamic CT, (ii) both continuous motion and sudden changes in voxel values can be represented while preserving the continuity of the interpolated solution, and (iii) the compactness of the interpolation functions reduces the increase in algorithmic cost. Total variation regularization is used with a second-order accurate discretization, and the resulting formulation is solved with the Chambolle-Pock proximal algorithm. The applicability of the method in practical cases is demonstrated using synchrotron data, with an algorithmic cost of two to four times that of equivalent static reconstruction algorithms.

19. Quail: A lightweight open-source discontinuous Galerkin code in Python for teaching and prototyping

    E. J. Ching, B. Bornhoft, A. Lasemi, and M. Ihme

    SoftwareX, 2022

    Abs DOI Open Access

    In this paper, we present Quail, a lightweight discontinuous Galerkin solver written in Python. The aim of this code is to serve not only as a teaching tool for newcomers to the rapidly growing field, but also as a prototyping platform for testing algorithms, physical models, and other features in the discontinuous Galerkin framework. Code readability, modularity, and ease of use are emphasized. Currently, Quail solves first- and second-order partial differential equations on 1D and 2D unstructured meshes. A variety of time stepping schemes, quadrature rules, basis types, equation sets, and other features are included. The structure and capabilities of the code, as well as representative examples involving propagation of a 2D isentropic vortex and a 2D Riemann problem with a gravity source term, will be discussed.

20. Computation of hypersonic viscous flows with the thermally perfect gas model using a discontinuous Galerkin method

    E. J. Ching, K. Bensassi, Y. Lv, and M. Ihme

    International Journal for Numerical Methods in Fluids, 2022

    Abs DOI Open Access

    Abstract This article is concerned with the numerical simulation of high‐speed thermally perfect flows using high‐order discontinuous Galerkin (DG) methods. In these methods, the polynomial approximation of the solution is susceptible to spurious oscillations, especially in high‐gradient regions. The introduction of variable thermodynamic properties, important for hypersonic flows, can exacerbate this issue. Given the general lack of satisfactory high‐order DG simulations of these types of flows, our primary objective is to demonstrate the success of the formulation proposed here in computing flows in this regime. Secondary goals are to (a) highlight that DG schemes are significantly less sensitive than finite‐volume methods to mesh‐shock misalignment in the given setting and (b) evaluate the sensitivity of surface heating predictions to various types of no‐slip isothermal wall boundary conditions. To handle strong shocks prevalent in this flow regime, intra‐element solution variations are used to detect discontinuities and smooth artificial viscosity is employed for stabilization. We discuss the algorithmic developments for extending a standard DG discretization of the compressible Navier–Stokes equations with the calorically perfect gas model to the thermally perfect gas model. We apply the resulting DG formulation to several hypersonic test cases, including inviscid flow over a cylinder and viscous flow over a double cone. Where appropriate, our results are benchmarked against those obtained with state‐of‐the‐art finite‐volume hypersonic flow solvers. The final case entails high‐speed particle‐laden flow over a blunt body. The results of this test illuminate how the consideration of variable thermodynamic properties influences the prediction of the particle phase.

21. A TensorFlow simulation framework for scientific computing of fluid flows on tensor processing units

    M. Wang, M. Ihme, Y. Chen, and J. R. Anderson

    Computer Physics Communications, 2022

    Abs DOI Open Access

    A computational fluid dynamics (CFD) simulation framework for fluid-flow prediction is developed on the Tensor Processing Unit (TPU) platform. The TPU architecture is featured with accelerated dense matrix multiplication, large high bandwidth memory, and a fast inter-chip interconnect, making it attractive for high-performance scientific computing. The CFD framework solves the variable-density Navier-Stokes equation using a low-Mach approximation, and the governing equations are discretized by a finite-difference method on a collocated structured mesh. It uses the graph-based TensorFlow as the programming paradigm. The accuracy and performance of this framework is studied both numerically and analytically, specifically focusing on effects of TPU-native single precision floating point arithmetic. The algorithm and implementation are validated with canonical 2D and 3D Taylor-Green vortex simulations. To demonstrate the capability for simulating turbulent flows, simulations are conducted for two configurations, namely decaying homogeneous isotropic turbulence and a turbulent planar jet. Both simulations show good statistical agreement with reference solutions. The performance analysis shows a linear weak scaling and a superlinear strong scaling up to a full TPU v3 pod with 2048 cores.

22. LES of HCCI combustion of iso-octane/air in a flat-piston rapid compression machine

    W. T. Chung, N. Ly, and M. Ihme

    Proceedings of the Combustion Institute, 2022

    DOI Open Access
23. An Enrichment Wall Model for the Spectral Element Method

    S. Brill, P. Pal, M. Ameen, C. Xu, and M. Ihme

    AIAA SCITECH 2022 Forum, 2022

    Abs DOI

    View Video Presentation: https://doi.org/10.2514/6.2022-1206.vid An enrichment wall-model for spectral element method (SEM) is developed and implemented in the high-order Nek5000 CFD solver. The enrichment method augments the polynomial solution with a non-polynomial enrichment function that represents a feature in the flow that cannot be accurately resolved with the polynomial basis. The method is applied to wall-modeling by enriching the polynomial solution in the wall-adjacent elements with an analytical law-of-the-wall function representing the mean velocity near the wall. This enrichment function is able to represent the high-gradients near the wall which allows the polynomial modes to represent the turbulent fluctuations. It is shown that the proposed method greatly improves solution accuracy on under resolved grids as compared to the traditional spectral element method by better capturing the shear stress at the wall and reducing unphysical oscillations. Additionally, the enrichment wall-model shows good agreement for coarser grids than traditional shear stress wall-models. The method is applied to turbulent channel flow simulations for a range of Reynolds numbers in Reynolds-averaged Navier-Stokes and large-eddy simulations. The results show great improvements in the prediction of turbulent shear stress and the log-law profile.

24. Wall heat transfer and flame structure transitions in stagnating spray flames

    D. Mohaddes, and M. Ihme

    Proceedings of the Combustion Institute, 2022

    DOI
25. Towards real-time predictions of large-scale wildfire scenarios using a fully coupled atmosphere-fire physical modelling framework

    Q. Wang, M. Ihme, Y. Chen, V. Yang, F. Sha, and J. Anderson

    In Imprensa da Universidade de Coimbra eBooks, 2022

    Abs DOI Open Access

    With the changing climate, fire-exclusion, and expansion of wildland-urban interfaces, the frequency and severity of wildfires are expected to increase, putting substantial stress on fire management and authorities to mitigate the risk of wildfires. Improved physical models in conjunction with advanced high-performance computing resources offer new opportunities for operational use in examining potential fire-spread scenarios and planning. This work presents an open-source, high-fidelity modelling framework for simulating large-scale wildfire scenarios, taking into consideration atmospheric/fire coupling, complex terrain, and heterogeneous fuel loading. The framework is implemented using the TensorFlow programming environment on tensor processing units (TPUs). TPUs are a dedicated high-performance computing architecture to accelerate machine-learning applications and high-performance scientific computing. This framework solves the Favre-filtered reacting Navier-Stokes equations and the unclosed terms describing turbulence/chemistry interaction and turbulence transport are modelled using large-eddy simulation (LES) closures. Wildfire dynamics is described by a one-step solid-fuel pyrolysis/combustion model that is coupled to atmospheric flow dynamics using a Boussinesq-type approximation. A second-order finite-difference discretization is employed in a variable-density, low-Mach number formulation to discretize the governing equations, and an immersed-boundary method is adapted to represent complex terrain. In conjunction with the coupled atmosphere/fire model and physical models for turbulence/atmosphere/fire interaction, the resulting simulation framework enables high-resolution simulations (with spatial resolution below 2m) of large-scale fires that cover up to  100,000 acres. Following the summary of validation results against a prescribed fire experiment to assess the overall accuracy at well-controlled conditions, we employ this coupled atmosphere/fire modelling framework to simulate a large-scale wildfire scenario that is representative of the 2017 California Tubb’s fire. To this end, we extract the terrain of the North Bay region of Calistoga and Santa Rosa, spanning an area of 20×20 km2, and consider a North-Eastern wind. The simulation results illustrate the rapid fire-spread dynamics and the coupling of the fire with the terrain and atmosphere. With relevance to operational and research applications that include parametric studies to examine effects of wind, fuel-density, other environmental factors, and fire-management strategies, we discuss the scalability and further extensions of the physical fidelity towards enabling real-time applications on TPU-compute architectures.

26. The Bearable Lightness of Big Data: Towards Massive Public Datasets in Scientific Machine Learning

    W. T. Chung, K. S. Jung, J. H. Chen, and M. Ihme

    arXiv (Cornell University), 2022

    Abs DOI Open Access

    In general, large datasets enable deep learning models to perform with good accuracy and generalizability. However, massive high-fidelity simulation datasets (from molecular chemistry, astrophysics, computational fluid dynamics (CFD), etc. can be challenging to curate due to dimensionality and storage constraints. Lossy compression algorithms can help mitigate limitations from storage, as long as the overall data fidelity is preserved. To illustrate this point, we demonstrate that deep learning models, trained and tested on data from a petascale CFD simulation, are robust to errors introduced during lossy compression in a semantic segmentation problem. Our results demonstrate that lossy compression algorithms offer a realistic pathway for exposing high-fidelity scientific data to open-source data repositories for building community datasets. In this paper, we outline, construct, and evaluate the requirements for establishing a big data framework, demonstrated at https://blastnet.github.io/, for scientific machine learning.

27. Development of a Fidelity-Adaptive Combustion Modeling Framework in a Rule-Based All-Speed Implicit Flow Solver

    J. Wright, S. Thakur, C. Neal, and M. Ihme

    AIAA SCITECH 2022 Forum, 2022

    Abs DOI

    View Video Presentation: https://doi.org/10.2514/6.2022-2249.vid The work presented here involves the implementation of a fidelity-adaptive combustion modeling framework based on the Pareto-Efficient Combustion (PEC) model developed by Ihme and co-workers [Combustion and Flame, Vol 162, Issue 11, 2015, pages 4208–4230.) into an all-speed implicit flow solver called Loci-STREAM for fidelity-adaptive combustion modeling of rocket injectors. The objective is to develop a high-fidelity, high-performance multiphysics simulation capability to enhance current simulation capability of unsteady turbulent reacting flows involving cryogenic propellants in space propulsion applications. The PEC model utilizes a combustion submodel assignment, combining flamelet-based combustion models (such as inert-mixing models, equilibrium chemistry, diffusion-flame Flamelet/Progress Variable (FPV) or premixed-flame models) for computationally efficient characterization of quasi one-dimensional, steady, and equilibrated combustion regimes, with combustion models of higher physical fidelity (such as detailed or reduced/lumped chemistry models) for accurate representation of topologically complex combustion regions (associated with flame-anchoring, autoignition, flame-liftoff, thermoacoustic coupling, and non-equilibrium combustion processes) that are not adequately represented by flamelet models. Algorithmic and implementation details of the PEC model in Loci-STREAM are presented, followed by the results of its application to a tribrachial flame and a shear coaxial rocket injector configuration.

2021

1. Imaging the short-lived hydroxyl-hydronium pair in ionized liquid water

   M. Lin, N. Singh, S. Liang, M. Mo, J. P. F. Nunes, K. Ledbetter, J. Yang, M. Kozina, S. Weathersby, X. Shen, A. A. Cordones, T. Wolf, C. D. Pemmaraju, M. Ihme, and X. Wang

   Science, 2021

   Abs DOI Open Access

   Capturing OH(H 3 O + ) in ionized water Recent advances in liquid-phase ultrafast electron diffraction techniques make it possible to observe what has only been theoretically presumed to occur at short times upon interaction of ionizing radiation with liquid water. Lin et al . provide direct evidence for capturing the short-lived radical-cation pair OH(H 3 O + ), which has been hypothesized for years to form after ionization of liquid water but was not structurally resolved previously (see the Perspective by Cao et al .). The authors trace dissociation and subsequent nonradiative structural relaxation around the ionization center. —YS

2. General Drag Coefficient for Flow over Spherical Particles

   N. Singh, M. D. Kroells, C. Li, E. J. Ching, M. Ihme, C. J. Hogan, and T. E. Schwartzentruber

   AIAA Journal, 2021

   Abs DOI

   A generalized physics-based expression for the drag coefficient of spherical particles moving in a fluid is developed. The proposed correlation incorporates essential rarefied effects, low-speed hydrodynamics, and shock-wave physics to accurately model the particle-drag force for a wide range of Mach and Knudsen numbers (and therefore Reynolds number). Owing to the basis of the derivation in physics-based scaling laws, the proposed correlation embeds gas-specific properties and has explicit dependence on the ratio of specific heat capacities. The correlation is applicable for arbitrary particle relative velocity, particle diameter, gas pressure, gas temperature, and surface temperature. Compared with existing drag models, the correlation is shown to more accurately reproduce a wide range of experimental data. Finally, the new correlation is applied to simulate particle trajectories in high-speed dusty flows, relevant to a spacecraft entering the Martian atmosphere. The enhanced surface heat flux due to particle impact is found to be sensitive to the particle drag model.

3. Analysis of droplet evaporation in isotropic turbulence through droplet-resolved DNS

   M. Dodd, D. Mohaddes, A. Ferrante, and M. Ihme

   International Journal of Heat and Mass Transfer, 2021

   DOI Open Access
4. Data-assisted combustion simulations with dynamic submodel assignment using random forests

   W. T. Chung, A. Mishra, N. Perakis, and M. Ihme

   Combustion and Flame, 2021

   DOI Open Access
5. Structural analysis of biomass pyrolysis and oxidation using in-situ X-ray computed tomography

   É. Boigné, N. R. Bennett, A. Wang, and M. Ihme

   Combustion and Flame, 2021

   DOI
6. Hot surface ignition of a wall-impinging fuel spray: Modeling and analysis using large-eddy simulation

   D. Mohaddes, P. Boettcher, and M. Ihme

   Combustion and Flame, 2021

   DOI Open Access
7. Sensitivity of Hypersonic Dusty Flows to Physical Modeling of the Particle Phase

   E. J. Ching, M. Barnhardt, and M. Ihme

   Journal of Spacecraft and Rockets, 2021

   Abs DOI

   The presence of dust in the Mars atmosphere can adversely affect heat shields of entry vehicles by enhancing erosion and increasing surface heat fluxes. However, the accuracy of numerical simulations investigating these effects is limited due to a lack of well-validated physical models of the particle phase. To address this issue, the sensitivities of computational predictions of dust-induced heating augmentation to different parameters and components of the disperse-phase model are evaluated. In particular, the drag correlation, the Nusselt-number correlation, various contributions to momentum and energy transfer, and particle diameter are examined. Numerical simulations are performed using an Euler–Lagrange methodology, in which the carrier gas is solved with a discontinuous Galerkin method. Very strong sensitivity to the drag correlation, moderate sensitivity to the Nusselt-number correlation, and strong sensitivity to the particle diameter are found. In addition, the quasi-steady drag force and heating rate are the most significant contributors to interphase momentum and energy transfer, respectively. These results can contribute to improving models and identifying knowledge gaps and uncertainties in the appropriate dust conditions.

8. Effects of evaporation on chemical reactions in counterflow spray flames

   W. Xie, W. Wu, Z. Ren, H. Liu, and M. Ihme

   Physics of Fluids, 2021

   Abs DOI

   The role of evaporation on chemical reactions in counterflow spray flames remains a key issue due to the non-linear inter-phase interactions through mass, momentum, and energy conservation. An extended chemical explosive mode analysis to illustrate the role of evaporation (ECEMA) was previously developed by separating the evaporation source term from non-chemical terms and projecting them onto the chemical explosive mode (CEM). Evaporation was found to promote chemical reactions when the fuel supply effect dominated the evaporative cooling and to inhibit reactions otherwise. In this work, ECEMA is applied to one-dimensional laminar and three-dimensional turbulent counterflow spray flames. For laminar cases, ECEMA is applied to multi-modal spray flame solutions including the distributed, collocated, and cool flame. The analysis for the collocated and cool flame shows similar behavior, namely, that evaporation inhibits chemical reactions due to heat absorption near the fuel injection region and enhances chemistry around the zero-crossing CEM. For the distributed flame, the promotion effect is observed for most of the domain. For the turbulent counterflow flame, three CEM regions are identified, namely, a hybrid region, an inhibition region, and a promotion region. In particular, for the hybrid region near the spray injection side, the contribution of evaporation to chemical reactions changes from inhibition to promotion. For the inhibition region in the middle, significant suppressing of chemical reactions by evaporation is observed. In the promotion region near the oxidizer side, evaporation remains promoting chemical reactions. For the case investigated, the dominant combustion modes are the assisted ignition and local extinction.

9. Development of a particle collision algorithm for discontinuous Galerkin simulations of compressible multiphase flows

   E. J. Ching, and M. Ihme

   Journal of Computational Physics, 2021

   DOI
10. Infrasound Radiation From Impulsive Volcanic Eruptions: Nonlinear Aeroacoustic 2D Simulations

    L. M. Watson, E. M. Dunham, D. Mohaddes, J. Labahn, T. Jaravel, and M. Ihme

    Journal of Geophysical Research Solid Earth, 2021

    Abs DOI

    Abstract Infrasound observations are increasingly used to constrain properties of volcanic eruptions. In order to better interpret infrasound observations, however, there is a need to better understand the relationship between eruption properties and sound generation. Here we perform two‐dimensional computational aeroacoustic simulations where we solve the compressible Navier‐Stokes equations for pure‐air with a large‐eddy simulation approximation. We simulate idealized impulsive volcanic eruptions where the exit velocity is specified and the eruption is pressure‐balanced with the atmosphere. Our nonlinear simulation results are compared with the commonly used analytical linear acoustics model of a compact monopole source radiating acoustic waves isotropically in a half space. The monopole source model matches the simulations for low exit velocities ( m/s or where is the Mach number); however, the two solutions diverge as the exit velocity increases with the simulations developing lower peak amplitude, more rapid onset, and anisotropic radiation with stronger infrasound signals recorded above the vent than on Earth’s surface. Our simulations show that interpreting ground‐based infrasound observations with the monopole source model can result in an underestimation of the erupted volume for eruptions with sonic or supersonic exit velocities. We examine nonlinear effects and show that nonlinear effects during propagation are relatively minor for the parameters considered. Instead, the dominant nonlinear effect is advection by the complex flow structure that develops above the vent. This work demonstrates the need to consider anisotropic radiation patterns and jet dynamics when interpreting infrasound observations, particularly for eruptions with sonic or supersonic exit velocities.

11. Efficient projection kernels for discontinuous Galerkin simulations of disperse multiphase flows on arbitrary curved elements

    E. J. Ching, and M. Ihme

    Journal of Computational Physics, 2021

    DOI
12. Towards the Suitability of Information Entropy as an LES Quality Indicator

    L. Engelmann, M. Ihme, I. Wlokas, and A. Kempf

    Flow Turbulence and Combustion, 2021

    Abs DOI Open Access

    Abstract The Shannon entropy is a rigorous measure to evaluate the complexity in dynamical systems. Shannon entropy can be directly calculated from any set of experimental or numerical data and yields the uncertainty of a given dataset. Originating from information theory, the concept can be generalized from assessing the uncertainty in a message to any dynamical system. Following the concept of ergodicity, turbulence forms another class of dynamical systems, which is generally assessed using statistical measures. The quantification of resolution quality is a crucial aspect in assessing turbulent-flow simulations. While a vast variety of statistical measures for the evaluation of resolution is available, measures closer representing the dynamics of a turbulent systems, such as the Wasserstein metric or the Ljapunov exponent become popular. This study investigates how the Shannon entropy can lead to useful insights in the quality of turbulent-flow simulations. The Shannon entropy is calculated based on distributions, which enables the direct evaluation from unsteady flow simulations or by post-processing. A turbulent channel flow and a planar turbulent jet are used as validation tests. The Shannon entropy is calculated for turbulent velocity- and scalar-fields and correlations with physical quantities, such as turbulent kinetic energy and passive scalars, are investigated. It is shown that the spatial structure of the Shannon entropy can be related to flow phenomena. This is illustrated by the investigation of the entropy of the velocity fluctuations, passive scalars and turbulent kinetic energy. Grid studies reveal the Shannon entropy as a converging measure. It is demonstrated, that classical turbulent-kinetic-energy-based quality measures struggle with the identification of insufficient resolution, while the Shannon entropy has demonstrated potential to form a solid basis for LES quality assessment.

13. Limitations of flamelet formulation for modeling turbulent pool fires

    B. Wu, M. Ihme, and X. Zhao

    Combustion and Flame, 2021

    DOI
14. A discontinuous Galerkin method for wall-modeled large-eddy simulations

    Y. Lv, X. I. A. Yang, G. I. Park, and M. Ihme

    Computers & Fluids, 2021

    DOI
15. Parametric analysis of core-noise from a realistic gas-turbine combustor for cruise and take-off conditions

    C. Shao, D. Brouzet, N. Rock, and M. Ihme

    Applications in Energy and Combustion Science, 2021

    Abs DOI Open Access

    Combustion noise from a realistic gas-turbine combustor geometry is investigated parametrically using a hybrid simulation framework that combines large-eddy simulation and a linearized Euler formulation. The effect of operating conditions on the relative noise-source contributions arising from direct and indirect noise is examined by considering take-off and cruise conditions. To quantify the predictions of the combustion flow field, comparisons with available experimental data for velocity and spray droplet diameter are performed. Analysis of combustion dynamics shows that tonal combustion noise from a thermoacoustic instability is present at both operating conditions, which is consistent with experimental data. The unsteady combustion and dilution are the main sources for the fluctuations of pressure, temperature and mixture composition at the combustor exit. While the thermo-chemical properties do not significantly change between the two operating conditions, the simulation results show that the level of unsteadiness in the flow field is significantly higher for the take-off condition. This leads to an overall increase in the noise emissions by up to 20 dB at take-off compared to the cruise condition. Indirect noise arising from compositional inhomogeneities is found to exceed the entropy noise for both operating conditions. Phase cancellation between composition noise and temperature-induced noise is shown to result in an overall reduction of the indirect combustion-noise emissions.

16. Quantitative X-ray computed tomography: Prospects for detailed in-situ imaging in bench-scale fire measurements

    É. Boigné, N. R. Bennett, A. Wang, and M. Ihme

    Fire Safety Journal, 2021

    DOI
17. CFD Modeling of Lean Blowout and Ignition Fuel Sensitivity

    M. S. Anand, J. A. Lovett, J. A. Moder, T. Wey, M. Ihme, L. Esclapez, P. Ma, S. Menon, A. Panchal, V. R. Hasti, J. P. Gore, P. Kundu, S. Som, V. Raman, Y. Tang, F. Xu, and V. Sankaran

    In American Institute of Aeronautics and Astronautics, Inc. eBooks, 2021

    DOI
18. Using adjoint-based optimization to enhance ignition in non-premixed jets

    U. A. Qadri, L. Magri, M. Ihme, and P. J. Schmid

    Proceedings of the Royal Society A Mathematical Physical and Engineering Sciences, 2021

    Abs DOI Open Access

    Gradient-based optimization is used to reliably and optimally induce ignition in three examples of laminar non-premixed mixture configurations. Using time-integrated heat release as a cost functional, the non-convex optimization problem identified optimal energy source locations that coincide with the stoichiometric local mixture fraction surface for short optimization horizons, while for longer horizons, the hydrodynamics plays an increasingly important role and a balance between flow and chemistry features determines non-trivial optimal ignition locations. Rather than identifying a single optimal ignition location, the results of this study show that there may be several equally good ignition locations in a given flow configuration.

19. Development of a discontinuous Galerkin solver using Legion for heterogeneous high-performance computing architectures

    K. Bando, S. Brill, E. Slaughter, M. Sekachev, A. Aiken, and M. Ihme

    AIAA Scitech 2021 Forum, 2021

    Abs DOI

    View Video Presentation: https://doi.org/10.2514/6.2021-0140.vid This work discusses the development, verification and performance assessment of a discontinuous Galerkin solver for the compressible Navier-Stokes equations using the Legion programming system. This is motivated by (i) the potential of this family of high-order numer- ical methods to accurately and efficiently realize scale-resolving simulations on unstructured grids and (ii) the desire to accommodate the utilization of emerging compute platforms that exhibit increased parallelism and heterogeneity. As a task-based programming model specifically designed for performance portability across distributed heterogeneous architectures, Legion represents an interesting lternative to the traditional approach of using Message Passing Interface for massively parallel computational physics solvers. Following detailed discussion of the implementation, the high-order convergence of the solver is demonstrated by a suite of canonical test cases and good strong scaling behavior is obtained. This work constitutes a first step towards a research platform that is able to be deployed and efficiently run on modern supercomputers.

20. Toward Numerical Investigation of Ignition and Combustion Transition in a Subscale LOX/Methane Rocket Combustor

    M. Bonanni, D. Mohaddes, N. Ly, N. Perakis, J. Hardi, M. Börner, and M. Ihme

    AIAA Scitech 2021 Forum, 2021

    Abs DOI

    View Video Presentation: https://doi.org/10.2514/6.2021-1141.vid LOX/methane is a promising alternative to hypergolic fuels in spacecraft thrusters, featuring advantages such as nontoxicity, high specific impulse, and low cost. Its reliable ignition, however, is a challenging problem due to its dependence on the local mixture composition, its requirement of overcoming ionization energies, and its process of transition to a sustained combustion mode. Experimental research has focused on laser pulses as an ignition mechanism for these thrusters, and has demonstrated significant viability. This work presents a numerical study of a reacting LOX/methane thruster using large eddy simulation combined with a Lagrangian description of the LOX spray. This model is successfully employed to simulate reacting flow in the Reaction Control System (RCS) breadboard combustor at DLR, achieving spray behavior that is qualitatively similar to the pre-ignition experimental data. This work represents a forward step towards simulating the startup process of this thruster via laser ignition.

21. Discontinuous Galerkin simulations of dusty flows over a full-scale capsule during Mars atmospheric entry

    E. J. Ching, and M. Ihme

    AIAA Scitech 2021 Forum, 2021

    Abs DOI

    View Video Presentation: https://doi.org/10.2514/6.2021-1518.vid Suspended dust particles can have adverse effects on heat shields during Mars atmospheric entry. For instance, they can increase surface heat fluxes and surface recession. Current studies on this subject often make many simplifications, focus on one or two specific physical processes, or include flow conditions different from those on Mars. To address this, we perform simulations of hypersonic dusty flows over a full-scale entry capsule based on a realistic trajectory. The simulations are performed with an Euler-Lagrange methodology in which the carrier gas solution is obtained with a discontinuous Galerkin method. The effects of various factors, such as particle size distribution, drag correlation, and two-way coupling, on dust-induced heating augmentation and erosion are assessed.

22. Interpretable data-driven methods for subgrid-scale closure in LES for transcritical LOX/GCH4 combustion

    W. T. Chung, A. Mishra, and M. Ihme

    Combustion and Flame, 2021

    DOI Open Access
23. Time integration considerations for the solution of reacting flows using discontinuous Galerkin methods

    B. Bornhoft, E. J. Ching, and M. Ihme

    AIAA Scitech 2021 Forum, 2021

    Abs DOI

    View Video Presentation: https://doi.org/10.2514/6.2021-0745.vid In this work, we present intermediate steps to develop a discontinuous Galerkin (DG) discretization in both space and time applicable to multi-component reacting flows. Here, we choose the ADER time integration method for its compact stencil, arbitrary order of accuracy in space and time, and its ability to handle stiff source terms such as those present in reacting flows. In this paper, we verify the scheme for an isentropic vortex, assess its behavior for discontinuous properties, and compare the scheme to explicit methods under a simplified stiff source term. We conclude this paper by conducting a demonstration of an overdriven detonation with a single-step irreversible reaction mechanism.

24. Heat transfer augmentation by recombination reactions in turbulent reacting boundary layers at elevated pressures

    N. Perakis, O. Haidn, and M. Ihme

    International Journal of Heat and Mass Transfer, 2021

    DOI Open Access
25. Evaluation of Electron Tomography Reconstruction Methods for a Barnett Shale

    L. Frouté, É. Boigné, M. Ihme, and A. R. Kovscek

    Proceedings of the 9th Unconventional Resources Technology Conference, 2021

    DOI
26. Numerical Analyses of Transcritical Evaporation using Diffuse-Interface Method

    N. Ly, and M. Ihme

    International Conference on Liquid Atomization and Spray Systems (ICLASS), 2021

    Abs DOI Open Access

    Liquid fuel placed in a supercritical environment (with respect to the fuel’s critical properties) can undergo different evaporation processes that is determined by the droplet’s surface temperature T s in comparison with the critical mixing temperature T c . In the transcritical regime, the ambient condition is such that at initial time, T s < T c , a sharp liquid-vapor interface exists, and evaporation occurs at subcritical condition. Eventually, the surface temperature approaches the critical mixing temperature, and the evaporation process becomes supercritical, with no distinct liquid-vapor interface. Traditional methods for simulating this complex evaporation behavior employ separate formulations for the liquid and vapor phases during subcritical evaporation and a continuous formulation during supercritical evaporation. Here, we developed a holistic formulation for the simulation of transcritical evaporation that is valid for both sub-and super-critical evaporation regimes using a diffuse-interface method derived from Linear Gradient Theory. The formulation allows for direct modeling of surface tension and interface thickness during the process of interface thickening as T s T c . The diffuse-interface model was employed in simulations of a nanoscale transcritical planar evaporation configuration. Furthermore, the ability of the diffuse-interface model to resolve both macroscale fluid behavior and nanoscale liquid-vapor interface structure was demonstrated.

27. Accelerating High-fidelity Combustion Simulations with Classification Algorithms.

    W. T. Chung, A. Mishra, N. Perakis, and M. Ihme

    National Conference on Artificial Intelligence, 2021
28. An Enriched Spectral Element Wall-Model for Wall-Modeled Large-Eddy Simulations

    S. Brill, P. Pal, M. Ameen, C. Xu, and M. Ihme

    Bulletin of the American Physical Society, 2021
29. Heat transfer augmentation by recombination reactions in turbulent\nreacting boundary layers at elevated pressures

    N. Perakis, O. Haidn, and M. Ihme

    arXiv (Cornell University), 2021

    Abs DOI Open Access

    A study of a reacting boundary layer flow with heat transfer at conditions\ntypical for configurations at elevated pressures has been performed using a set\nof direct numerical simulations. Effects of wall temperatures are investigated,\nrepresentative for cooled walls of gas turbines and sub-scale rocket engines\noperating with hydrocarbon as fuels. The results show that exothermic chemical\nreactions induced by the low-enthalpy in the boundary layer take place\npredominantly in the logarthimic sub-layer. The majority of the heat release is\nattributed to the exothermic recombination of OH and CO to produce CO2 and H2O.\nThe recombination reactions result in an increase of the wall heat loads by up\nto 20% compared to the inert flow. The gas composition experiences strong\ndeviations from the chemical equilibrium conditions. In fact, a quenching of\nthe major species is observed within the viscous sub-layer and the transition\nregion. Analysis of chemical time-scales shows that the location of quenched\ncomposition coincides with the region where the Damkoehler number decreases\nbelow unity. Within the viscous sub-layer, a secondary reaction zone is\ndetected, involving the production of formyl and formaldehyde radicals that\nprovide an additional source of energy release. The analysis of the reaction\npaths showed that reactions with zero activation energy are responsible for\nthis change in gas composition, which also account for the initial branching of\nhydrocarbon fuels decomposition according to previous auto-ignition studies.\nThe effect of the secondary recombination reactions is more prominent for the\nlower wall temperature case. Finally, the role of turbulent fluctuations on the\nspecies net chemical production rates is evaluated, showing a strong\ncorrelation between species and temperature fluctuations.\n

2020

1. Between supercritical liquids and gases – Reconciling dynamic and thermodynamic state transitions

   D. T. Banuti, M. Raju, and M. Ihme

   The Journal of Supercritical Fluids, 2020

   DOI
2. Modeling Heat-Shield Erosion due to Dust Particle Impacts for Martian Entries

   G. Palmer, E. J. Ching, M. Ihme, D. Allofs, and A. Gülhan

   Journal of Spacecraft and Rockets, 2020

   Abs DOI

   Heat-shield design for spacecraft entering the atmosphere of Mars may be affected by the presence of atmospheric dust. Particle impacts with sufficient kinetic energy can cause spallation damage to the heat shield that must be estimated. The dust environment in terms of particle size distribution and number density can be inferred from ground-based or atmospheric observations at Mars. Using a Lagrangian approach, the particle trajectories through the shock layer can be computed using a set of coupled ordinary differential equations. The dust particles are small enough that noncontinuum effects must be accounted for when computing the drag coefficient and heat transfer to the particle surface. Surface damage correlations for impact crater diameter and penetration depth are presented for fused silica, AVCOAT, shuttle tiles, cork, and Norcoat® Liège. The cork and Norcoat Liège correlations are new and were developed in this study. The modeling equations presented in this paper are applied to compute the heat-shield erosion due to dust particle impacts on the ExoMars Schiaparelli entry capsule during dust storm conditions.

3. Modeling Heatshield Erosion due to Dust Particle Impacts for Martian Entries

   G. Palmer, E. J. Ching, M. Ihme, D. Kerkhoff, and A. Guelhan

   AIAA Scitech 2020 Forum, 2020

   Abs DOI

   Heatshield design for spacecraft entering the atmosphere of Mars may be affected by the presence of atmospheric dust. Particle impacts with sufficient kinetic energy can cause spallation damage to the heatshield that must be estimated. The dust environment in terms of particle size distribution and number density can be inferred from ground-based or atmospheric observations at Mars. Using a Lagrangian approach, the particle trajectories through the shock layer can be computed using a set of coupled ordinary differential equations. The dust particles are small enough that non-continuum effects must be accounted for when computing the drag coefficient and heat transfer to the particle surface. Surface damage correlations for impact crater diameter and penetration depth are presented for fused-silica, AVCOAT, Shuttle tiles, cork, and Norcoat Liège. The cork and Norcoat Liège correlations are new and were developed in this study. The modeling equations presented in this paper are applied to compute the heatshield erosion due to dust particle impacts on the ExoMars Schiaparelli entry capsule during dust storm conditions.

4. Thermodynamic cycle analysis of superadiabatic matrix-stabilized combustion for gas turbine engines

   D. Mohaddes, C. T. Chang, and M. Ihme

   Energy, 2020

   DOI
5. Pore-resolved simulations of porous media combustion with conjugate heat transfer

   J. C. Ferguson, S. Sobhani, and M. Ihme

   Proceedings of the Combustion Institute, 2020

   DOI
6. Experimental feasibility of tailored porous media burners enabled via additive manufacturing

   S. Sobhani, P. Muhunthan, É. Boigné, D. Mohaddes, and M. Ihme

   Proceedings of the Combustion Institute, 2020

   DOI
7. StanShock: a gas-dynamic model for shock tube simulations with non-ideal effects and chemical kinetics

   K. Grogan, and M. Ihme

   Shock Waves, 2020

   DOI
8. Simultaneous in-situ measurements of gas temperature and pyrolysis of biomass smoldering via X-ray computed tomography

   É. Boigné, N. R. Bennett, A. Wang, K. Mohri, and M. Ihme

   Proceedings of the Combustion Institute, 2020

   DOI
9. Additive Manufacturing of Tailored Macroporous Ceramic Structures for High‐Temperature Applications

   S. Sobhani, S. M. Allan, P. Muhunthan, É. Boigné, and M. Ihme

   Advanced Engineering Materials, 2020

   Abs DOI Open Access

   Pathlines of fluid flow are strategically guided through additively manufactured ceramic structures with specified porosity and pore diameter variations. With additional tunable parameters such as printing layer thickness and ceramic particle size, tailored porous structures hold opportunities for application-specific designs in thermofluidic systems. Further details can be found in the article number 2000158 by Sadaf Sobhani and co-workers.

10. Additive Manufacturing of Tailored Macroporous Ceramic Structures for High‐Temperature Applications

    S. Sobhani, S. M. Allan, P. Muhunthan, É. Boigné, and M. Ihme

    Advanced Engineering Materials, 2020

    Abs DOI

    Macroporous ceramic materials are ubiquitous in numerous energy‐conversion and thermal‐management systems. The morphology and material composition influence the effective thermophysical properties of macroporous ceramic structures and interphase transport in interactions with the working fluid. Therefore, tailoring these properties can enable significant performance enhancements by modulating thermal transport, reactivity, and stability. However, conventional ceramic‐matrix fabrication techniques limit the ability for tailoring the porous structure and optimizing the performance of these systems, such as by introducing anisotropic morphologies, pore‐size gradations, and variations in pore connectivity and material properties. Herein, an integrated framework is proposed for enabling the design, optimization, and fabrication of tailored ceramic porous structures by combining computational modeling, mathematically defined surfaces, and lithography‐based additive manufacturing. The benefits of pore‐structure tailoring are illustrated experimentally for interstitial combustion in a porous‐media burner operating with a smoothly graded matrix structure. In addition, a remarkable range of achievable thermal conductivities for a single material is demonstrated with tuning of the fabrication process, thus providing unique opportunities for modulating thermal transport properties of porous‐ceramic structures.

11. Analysis of low-temperature chemistry in a turbulent swirling spray flame near lean blow-out

    D. Mohaddes, W. Xie, and M. Ihme

    Proceedings of the Combustion Institute, 2020

    DOI Open Access
12. Kinetics for the hydrolysis of Ti(OC3H7)4: A molecular dynamics simulation study

    J. Wei, A. Ostadhossein, S. Li, and M. Ihme

    Proceedings of the Combustion Institute, 2020

    DOI
13. Stability diagram and blow-out mechanisms of turbulent non-premixed combustion

    D. Li, and M. Ihme

    Proceedings of the Combustion Institute, 2020

    DOI
14. Modeling Adsorption in Silica Pores via Minkowski Functionals and Molecular Electrostatic Moments

    F. Simeski, A. Boelens, and M. Ihme

    Energies, 2020

    Abs DOI Open Access

    Capillary condensation phenomena are important in various technological and environmental processes. Using molecular simulations, we study the confined phase behavior of fluids relevant to carbon sequestration and shale gas production. As a first step toward translating information from the molecular to the pore scale, we express the thermodynamic potential and excess adsorption of methane, nitrogen, carbon dioxide, and water in terms of the pore’s geometric properties via Minkowski functionals. This mathematical reconstruction agrees very well with molecular simulations data. Our results show that the fluid molecular electrostatic moments are positively correlated with the number of adsorption layers in the pore. Moreover, stronger electrostatic moments lead to adsorption at lower pressures. These findings can be applied to improve pore-scale thermodynamic and transport models.

15. Carbon oxidation in turbulent premixed jet flames: A comparative experimental and numerical study of ethylene, n-heptane, and toluene

    D. I. Pineda, L. Paxton, N. Perakis, C. Wei, S. Luna, H. Kahouli, M. Ihme, F. N. Egolfopoulos, and R. M. Spearrin

    Combustion and Flame, 2020

    DOI
16. Structural analysis and regime diagrams of laminar counterflow spray flames with low-temperature chemistry

    W. Xie, P. Govindaraju, Z. Ren, and M. Ihme

    Proceedings of the Combustion Institute, 2020

    DOI
17. Uncertainty quantification of combustion noise by generalized polynomial chaos and state-space models

    C. Silva, P. Pettersson, G. Iaccarino, and M. Ihme

    Combustion and Flame, 2020

    DOI Open Access
18. Analysis of core-noise contributions in a realistic gas-turbine combustor operated near lean blow-out

    C. Shao, K. Maeda, and M. Ihme

    Proceedings of the Combustion Institute, 2020

    DOI
19. Investigation of CO recombination in the boundary layer of CH4/O2 rocket engines

    N. Perakis, O. Haidn, and M. Ihme

    Proceedings of the Combustion Institute, 2020

    DOI Open Access
20. Pareto-efficient combustion framework for predicting transient ignition dynamics in turbulent flames: Application to a pulsed jet-in-hot-coflow flame

    Q. Douasbin, M. Ihme, and C. M. Arndt

    Combustion and Flame, 2020

    DOI Open Access
21. Comparison of algorithms for simulating multi-component reacting flows using high-order discontinuous Galerkin methods

    K. Bando, M. Sekachev, and M. Ihme

    AIAA Scitech 2020 Forum, 2020

    Abs DOI

    This work compares two approaches for simulating reacting flows using high-order discontinuous Galerkin methods. On the one hand, the double flux method was developed in order to overcome the well-known issue of pressure oscillations at contact interfaces when variable thermodynamic properties are used. This quasi-conservative scheme was previously applied to high-speed inviscid reacting flows but potential issues with lower-Mach flows and viscous effects were not discussed. On the other hand, the fully conservative formulation, although inconsistent with pressure equilibrium at contact interfaces, allows a conservative scheme consistent with the sub-cell resolution of high-order schemes. These two approaches are compared on several test cases and their strength and weaknesses are analyzed.

22. An Enriched Basis Discontinuous Galerkin Method for Shocks and High-Gradient Features in Fluid Mechanics

    S. Brill, and M. Ihme

    AIAA Scitech 2020 Forum, 2020

    Abs DOI

    Discontinuous Galerkin methods struggle to efficiently represent high-gradient features without spurious oscillations due to their polynomial basis. This paper discusses the development of an enriched basis discontinuous Galerkin method to better resolve high-gradient features in compressible flows. The standard polynomial basis is augmented with problem-specific non-polynomial enrichment functions to represent underresolved features. The enrichment functions are incorporated into the basis using a partition of unity to keep the methodology consistent. The method was tailored for shock capturing and wall-modeling of turbulent flows. The performance is evaluated with shock tube problems and Reynolds-averaged Navier-Stokes turbulent channel simulations. The results show this methodology allows for sharp representation of shocks within elements and for larger elements and fewer degrees of freedom to be used in the near wall region of turbulent channel flows.

23. Smooth projection kernels for Euler-Lagrange simulations on arbitrary elements computed with discontinuous Galerkin schemes

    E. J. Ching, and M. Ihme

    AIAA Scitech 2020 Forum, 2020

    Abs DOI

    In this work, we develop smooth kernels for Euler-Lagrange simulations on arbitrary elements under a high-order discontinuous Galerkin framework. These kernels are employed for projecting the action of the particles to the Eulerian mesh. Instead of commonly used isotropic kernels (such as a Gaussian kernel), we construct an anisotropic polynomial-based smoothing function that preserves the compactness of the discontinuous Galerkin method on high-aspect-ratio elements and reduces computational cost. At the same time, it circumvents Gibbs-type noise/instabilities associated with the Dirac delta function and low-order kernels. Specifically, the shape of the kernel is elliptical, which can be constructed from a grid-implied metric that encodes information about mesh size and orientation. We also discuss a strategy for dealing with walls compatible with arbitrary curved elements. We apply the proposed methodology to different types of meshes and supersonic dusty flow over a flat plate.

24. Deep Learning Models for Predicting Wildfires from Historical Remote-Sensing Data

    F. Huot, R. Hu, M. Ihme, Q. Wang, J. Burge, T. Lu, J. Hickey, Y. Chen, and J. R. Anderson

    arXiv (Cornell University), 2020

    Abs DOI Open Access

    Identifying regions that have high likelihood for wildfires is a key component of land and forestry management and disaster preparedness. We create a data set by aggregating nearly a decade of remote-sensing data and historical fire records to predict wildfires. This prediction problem is framed as three machine learning tasks. Results are compared and analyzed for four different deep learning models to estimate wildfire likelihood. The results demonstrate that deep learning models can successfully identify areas of high fire likelihood using aggregated data about vegetation, weather, and topography with an AUC of 83%.

25. Convolutional LSTM Neural Networks for Modeling Wildland Fire Dynamics

    J. Burge, M. Bonanni, M. Ihme, and R. Hu

    arXiv (Cornell University), 2020

    Abs DOI Open Access

    As the climate changes, the severity of wildland fires is expected to worsen. Models that accurately capture fire propagation dynamics greatly help efforts for understanding, responding to and mitigating the damages caused by these fires. Machine learning techniques provide a potential approach for developing such models. The objective of this study is to evaluate the feasibility of using a Convolutional Long Short-Term Memory (ConvLSTM) recurrent neural network to model the dynamics of wildland fire propagation. The machine learning model is trained on simulated wildfire data generated by a mathematical analogue model. Three simulated datasets are analyzed, each with increasing degrees of complexity. The simplest dataset includes a constant wind direction as a single confounding factor, whereas the most complex dataset includes dynamic wind, complex terrain, spatially varying moisture content and heterogenous vegetation density distributions. We examine how effective the ConvLSTM can learn the fire-spread dynamics over consecutive time steps. It is shown that ConvLSTMs can capture local fire transmission events, as well as the overall fire dynamics, such as the rate at which the fire spreads. Finally, we demonstrate that ConvLSTMs outperform other network architectures that have previously been used to model similar wildland fire dynamics.

26. A General Drag Coefficient for Flow over a Sphere

    N. Singh, M. D. Kroells, C. Li, E. J. Ching, M. Ihme, C. J. Hogan, and T. E. Schwartzentruber

    arXiv (Cornell University), 2020

    Abs DOI Open Access

    A generalized physics-based expression for the drag coefficient of spherical particles moving in a fluid is derived. The proposed correlation incorporates essential rarefied physics, low-speed hydrodynamics, and shock-wave physics to accurately model the particle-drag force for a wide range of Mach and Knudsen numbers (and therefore Reynolds number) a particle may experience. Owing to the basis of the derivation in physics-based scaling laws, the proposed correlation embeds gas-specific properties and has explicit dependence on the ratio of specific heat capacities at constant pressure and constant volume. The correlation is applicable for arbitrary particle relative velocity, particle diameter, gas pressure, gas temperature, and surface temperature. Compared to existing drag models, the correlation is shown to more accurately reproduce a wide range of experimental data. Finally, the new correlation is applied to simulate dust particles’ trajectories in high-speed flow, relevant to a spacecraft entering the Martian atmosphere. The enhanced surface heat flux due to particle impact is found to be sensitive to the particle drag model.

27. Parametric Study of Hot Surface Ignition of an Impinging Fuel Spray Using Large-Eddy Simulation

    D. Mohaddes, and M. Ihme

    Bulletin of the American Physical Society, 2020
28. Requirements Towards Predictive Simulations of Turbulent Reacting Flows

    M. Ihme

    arXiv (Cornell University), 2020

    Abs DOI Open Access

    Significant progress has been made on the model development for simulating turbulent reacting flows. As a consequence, we are currently in a position where key-physical aspects of fairly complex combustion processes are well understood at a qualitative and – in many cases – also at a quantitative level. Examples are the prediction of temperature and major species, statistically stationary flames, gas-phase combustion, turbulent transport, and turbulence/flame coupling. However, current challenges lie in capturing transient processes and stability boundaries, minor species and emissions, multiphase flows and phase-transition, as well as multidimensional flame/flow interactions that may involve flame curvature effects, stratification, partially premixing, or flame-wall interaction. With this, the question arises what steps need to be taken to elevate the current state of modeling capabilities in order to address these deficiencies? This paper seeks to address this question. We begin by reviewing the current state of combustion model approaches, our quest for improving existing models, the separation of errors arising from numerical discretization and physical models, and ideas on model evaluations. We then proceed by examining concepts on quantitative model evaluations, requirements on predictability, quantities of interest, and cost/accuracy trade-offs. We close by introducing recent concepts that assimilate time-resolved measurements into numerical simulations for state estimation, model evaluation, and parameter determination.

29. Pareto-optimal modeling of autoignition in a turbulent methane jet-in-hot-coflow configuration

    Q. Douasbin, and M. Ihme

    Bulletin of the American Physical Society, 2020
30. Flame-front kinematics in porous media analyzed via 2D simulations

    É. Boigné, S. Sobhani, J. C. Ferguson, and M. Ihme

    Bulletin of the American Physical Society, 2020
31. On core noise for the typical operating conditions in a realistic gas turbine combustor

    C. Shao, K. Maeda, and M. Ihme

    Bulletin of the American Physical Society, 2020
32. Ensemble Predictions of Wildfire Spread Through TPU-Compatible TensorFlow Acceleration

    M. Bonanni, and M. Ihme

    Bulletin of the American Physical Society, 2020
33. Storage Capacity, Sorption and Mass Transfer in Shale

    F. Simeski, N. Singh, J. Wu, T. T. Tsotsis, K. Jessen, and M. Ihme

    AGU Fall Meeting 2020, 2020
34. Reacting Simulations of Multi-mode Combustion in a Rapid Compression Machine under HCCI and SICI Modes

    N. Ly, and M. Ihme

    APS Division of Fluid Dynamics Meeting Abstracts, 2020
35. Improvements upon Characterizing the Thermal Boundary Layer in Turbulent, Transcritical Channel Flows

    J. Guo, Y. Xiang, and M. Ihme

    APS Division of Fluid Dynamics Meeting Abstracts, 2020
36. Deep learning-based assignment of combustion submodels for large-eddy simulation

    W. T. Chung, A. Mishra, N. Perakis, and M. Ihme

    mediaTUM (Technical University of Munich), 2020
37. Examining the Structure of Supercritical CO 2 using X-ray Raman Spectroscopy and Atomistic Scale Modeling

    P. Muhunthan, S. Sainio, Δ. Σωκαράς, and M. Ihme

    AGU Fall Meeting Abstracts, 2020
38. Phase Transitions and Condensation in Tight Porous Materials

    T. Lehmann, K. Covington, E. Smith, A. K. Goroncy, V. Alvarado, F. Simeski, A. Boelens, M. Ihme, S. Hu, K. Jessen, and T. T. Tsotsis

    AGU Fall Meeting Abstracts, 2020

2019

1. Functionalization of 2D materials for enhancing OER/ORR catalytic activity in Li–oxygen batteries

   A. Ostadhossein, J. Guo, F. Simeski, and M. Ihme

   Communications Chemistry, 2019

   Abs DOI Open Access

   Abstract A major barrier toward the practical application of lithium-oxygen batteries is the high overpotential caused by the precipitation of oxygen-reduction products at the cathode, resulting in poor cyclability. By combining first-principle calculations and reactive molecular dynamics simulations, we show that surface functionalization of 2D MXene nanosheets offers a high degree of tunability of the catalytic activity for oxygen-reduction and oxygen-evolution reactions (ORR/OER). We show that the controlled creation of active vacancy sites on the MXene surface enhances ORR in excess of a factor of 60 compared to graphene-based cathode materials. Furthermore, we find that increasing the ratio of fluorine vs. oxygen termination of the functionalized Ti 4 N 3 -MXene catalyst reduces the charge overpotential by up to 70% and 80% compared with commercial platinum-on-carbon and graphene catalysts, respectively. These results provide direct guidance toward the rational design of functionalized 2D materials for modulating the catalytic activity for a wide range of electrocatalytic applications.

2. Efficient time-stepping techniques for simulating turbulent reactive flows with stiff chemistry

   H. Wu, P. Ma, and M. Ihme

   Computer Physics Communications, 2019

   DOI
3. Ensemble Kalman Filter for Assimilating Experimental Data into Large-Eddy Simulations of Turbulent Flows

   J. W. Labahn, H. Wu, S. Harris, B. Coriton, J. H. Frank, and M. Ihme

   Flow Turbulence and Combustion, 2019

   DOI Open Access
4. On the numerical behavior of diffuse-interface methods for transcritical real-fluids simulations

   P. Ma, H. Wu, D. T. Banuti, and M. Ihme

   International Journal of Multiphase Flow, 2019

   DOI
5. A two-way coupled Euler-Lagrange method for simulating multiphase flows with discontinuous Galerkin schemes on arbitrary curved elements

   E. J. Ching, S. Brill, M. Barnhardt, and M. Ihme

   Journal of Computational Physics, 2019

   DOI
6. Experimental investigation of lean premixed pre-vaporized liquid-fuel combustion in porous media burners at elevated pressures up to 20 bar

   S. Sobhani, J. Legg, D. Bartz, J. Kojima, C. T. Chang, J. D. Sullivan, J. P. Moder, and M. Ihme

   Combustion and Flame, 2019

   DOI
7. Molecular diffusion and phase stability in high-pressure combustion

   M. Yao, J.-P. Hickey, P. Ma, and M. Ihme

   Combustion and Flame, 2019

   DOI
8. A regularized deconvolution model for sub-grid dispersion in large eddy simulation of turbulent spray flames

   M. Wang, X. Zhao, and M. Ihme

   Combustion and Flame, 2019

   DOI
9. Examination of diesel spray combustion in supercritical ambient fluid using large-eddy simulations

   W. T. Chung, P. Ma, and M. Ihme

   International Journal of Engine Research, 2019

   Abs DOI Open Access

   High-pressure conditions in diesel engines can often surpass the thermodynamic critical limit of the working fluid. Consequently, the injection of fuel at these conditions can lead to complex behaviors that remain only incompletely understood. This study is concerned with investigating the application of a diffuse-interface method in conjunction with a finite-rate chemistry model in large-eddy simulations of diesel spray injection and ignition in a supercritical ambient environment. The presented numerical approach offers the capability of simulating these complex conditions without the need for parameter tuning that is commonly employed in spray-breakup models. Numerical simulations of inert and reacting n-dodecane sprays — under the Engine Combustion Network Spray A and Spray D configurations — are studied, and results are compared with experimental data for liquid/vapor penetration lengths and ignition timing. In addition, parametric studies are performed to identify flow sensitivities arising from the variation in nozzle diameters between both injectors, along with the impact of low-temperature oxidation on ignition in Spray D simulations. Spray A simulations are found to be insensitive to turbulence, and predictions for penetration length and ignition behavior are in good agreement with experiments. In contrast, Spray D predictions for penetration length and ignition delay demonstrated significant sensitivities to in-nozzle turbulence, introducing uncertainty to the predicted results and stipulating the need for quantitative measurements for model evaluation.

10. A regularized deconvolution method for turbulent closure modeling in implicitly filtered large-eddy simulation

    M. Wang, and M. Ihme

    Combustion and Flame, 2019

    DOI Open Access
11. Closure of the scalar dissipation rate in the spray flamelet equations through a transport equation for the gradient of the mixture fraction

    H. Olguin, A. Scholtissek, S. González, F. Gonzalez, M. Ihme, C. Hasse, and E. Gutheil

    Combustion and Flame, 2019

    DOI
12. Data Assimilation and Optimal Calibration in Nonlinear Models of Flame Dynamics

    H. Yu, T. Jaravel, M. Ihme, M. P. Juniper, and L. Magri

    Volume 4B: Combustion, Fuels, and Emissions, 2019

    Abs DOI Open Access

    Abstract We propose an on-the-fly statistical learning method to take a qualitative reduced-order model of the dynamics of a premixed flame and make it quantitatively accurate. This physics-informed data-driven method is based on the statistically optimal combination of (i) a reduced-order model of the dynamics of a pre-mixed flame with a level-set method, (ii) high-quality data, which can be provided by experiments and/or high-fidelity simulations, and (iii) assimilation of the data into the reduced-order model to improve the prediction of the dynamics of the premixed flame. The reduced-order model learns the state and the parameters of the premixed flame on the fly with the ensemble Kalman filter, which is a Bayesian filter used, for example, in weather forecasting. The proposed method and algorithm are applied to two test cases with relevance to reacting flows and instabilities. First, the capabilities of the framework are demonstrated in a twin experiment, where the assimilated data is produced from the same model as that used in prediction. Second, the assimilated data is extracted from a high-fidelity reacting-flow direct numerical simulation (DNS), which provides the reference solution. The results are analyzed by using Bayesian statistics, which robustly provide the level of confidence in the calculations from the reduced-order model. The versatile method we propose enables the optimal calibration of computationally inexpensive reduced-order models in real time when experimental data becomes available, for example, from gas-turbine sensors.

13. Data Assimilation and Optimal Calibration in Nonlinear Models of Flame Dynamics

    H. Yu, T. Jaravel, M. Ihme, M. P. Juniper, and L. Magri

    Journal of Engineering for Gas Turbines and Power, 2019

    Abs DOI

    Abstract We propose an on-the-fly statistical learning method to take a qualitative reduced-order model of the dynamics of a premixed flame and make it quantitatively accurate. This physics-informed data-driven method is based on the statistically optimal combination of (i) a reduced-order model of the dynamics of a premixed flame with a level-set method, (ii) high-quality data, which can be provided by experiments and/or high-fidelity simulations, and (iii) assimilation of the data into the reduced-order model to improve the prediction of the dynamics of the premixed flame. The reduced-order model learns the state and the parameters of the premixed flame on the fly with the ensemble Kalman filter, which is a Bayesian filter used, for example, in weather forecasting. The proposed method and algorithm are applied to two test cases with relevance to reacting flows and instabilities. First, the capabilities of the framework are demonstrated in a twin experiment, where the assimilated data are produced from the same model as that used in prediction. Second, the assimilated data are extracted from a high-fidelity reacting-flow direct numerical simulation (DNS), which provides the reference solution. The results are analyzed by using Bayesian statistics, which robustly provide the level of confidence in the calculations from the reduced-order model. The versatile method we propose enables the optimal calibration of computationally inexpensive reduced-order models in real-time when experimental data become available, for example, from gas-turbine sensors.

14. Numerical Analysis of Heat and Mass Transfer Coupled With Gaseous Fuel Injection in Reactive Porous Media

    Z. Cheng, J. Yang, Z. Guo, P. Fu, M. Ihme, and Q. Wang

    Journal of Heat Transfer, 2019

    Abs DOI

    Abstract Iron ore sintering is a typical application of reactive porous media combustion (RPMC) which has been widely reported. In this work, a computational model of RPMC is developed, in which heat and mass transfer, as well as main chemical subprocesses in reactive porous media are incorporated. A gaseous-fuel injection method is examined to enable better heat pattern. However, the imbalance of heat distribution in the flow direction caused by internally recirculating heat released via a solid matrix is still problematic on energy efficiency. Through computations, it is observed that the heat pattern and melting quantity index (MQI) are sensitive on gaseous-fuel concentration, providing a possibility of improving the heat distribution imbalance. Numerical results are presented to demonstrate the benefits of fuel segregation. Finally, more reasonable heat pattern is observed by combinations of gaseous-fuel segregation with hot air and oxygen enrichment. The dynamics of gaseous/solid-fuel combustion zones caused by the hot air and oxygen enrichment would contribute to an expansion of melting zone near the inlet, producing a more reasonable and uniform heat distribution in a sintering bed.

15. Sensitivity study of high-speed dusty flows over blunt bodies simulated using a discontinuous Galerkin method

    E. J. Ching, and M. Ihme

    AIAA Scitech 2019 Forum, 2019

    DOI
16. Requirements Towards Predictive Simulations of Turbulent Combustion

    M. Ihme

    AIAA Scitech 2019 Forum, 2019

    DOI
17. Compressible Flamelet Model with Thickened Flame Closure in an All-Speed Combustion Solver

    S. Thakur, J. Wright, and M. Ihme

    AIAA Scitech 2019 Forum, 2019

    DOI
18. Experimental Investigation of Combustion in Porous Media Burners with Tailored Matrix-Structure Using Additive Manufacturing

    P. Muhunthan, S. Sobhani, É. Boigné, D. Mohaddes, and M. Ihme

    Bulletin of the American Physical Society, 2019
19. Determining dynamic content of turbulent flow LES using the Lyapunov exponent

    J. W. Labahn, G. Nastac, L. Magri, and M. Ihme

    Cambridge University Engineering Department Publications Database, 2019

    Abs DOI Open Access

    Direct Numerical Simulation (DNS) and Large-Eddy Simulation (LES) have been em- ployed for computing turbulent flows . While DNS resolves all turbulent scales involved in the dynamics with no physical modeling, LES represents the energy contained in the large scales, and effects of the smaller scales are taken into account either explicitly through a subgrid scale model or implicitly through the numerical dissipation of the nu- merical method. Besides numerical algorithms, two factors determine the quality of LES: the physical model or dissipation of the subgrid scales (SGS), which are filtered out in the governing equations, and the filter width, which describes the numerical resolution of the resolved scales.

20. Experimental investigation of solid fuel smoldering using 3D X-ray computed tomography with gas-phase temperature measurements.

    É. Boigné, and M. Ihme

    Bulletin of the American Physical Society, 2019
21. Flame structure analysis and flame stabilization in a turbulent swirling spray flame

    D. Mohaddes, W. Xie, and M. Ihme

    Bulletin of the American Physical Society, 2019
22. Analysis of Thermal Boundary-Layer Structure and Scale-Dependence in Transcritical Flows at Turbulent Conditions

    J. Guo, Y. Xiang, and M. Ihme

    Bulletin of the American Physical Society, 2019
23. Modeling Heatshield Erosion Due to Dust Particle Impacts for a Martian Entry Vehicle

    G. Palmer, E. J. Ching, M. Ihme, D. Kerkhoff, and A. Gülhan

    NASA STI Repository (National Aeronautics and Space Administration), 2019

    Abs Open Access

    Because planetary missions to Mars take years from initial design to arrival at Mars, and because of the unpredictability of major global dust storms, the de-sign of the thermal protection system (TPS) of a Mars entry vehicle requires an estimation for the potential damage caused by dust particle impacts on the heat-shield. This paper will review previous analytical and experimental approaches to modeling dust particle ero-sion and will compare the legacy models against more modern computational techniques and new dust ero-sion models that will be based on upcoming experi-ments in the German Aerospace Center (DLR) GBK facility. The various models will be compared by incorporating them into the Icarus material response code applied to a representative vehicle entering the Martian atmosphere.

24. High-order discontinuous Galerkin methods for multi-physics flow simulations on exascale computing systems

    M. Ihme, K. Bando, E. J. Ching, and A. Aiken

    APS Division of Fluid Dynamics Meeting Abstracts, 2019
25. Development of a Four-Way-Coupled Lagrangian Particle Method for Discontinuous Galerkin Schemes

    E. J. Ching, and M. Ihme

    APS Division of Fluid Dynamics Meeting Abstracts, 2019
26. An Enriched-Basis Discontinuous Galerkin Method for Wall-Modeled Large-Eddy Simulations

    S. Brill, and M. Ihme

    APS Division of Fluid Dynamics Meeting Abstracts, 2019
27. Reacting flow simulations using high-order discontinuous Galerkin methods

    K. Bando, M. Ihme, and M. Sekachev

    APS Division of Fluid Dynamics Meeting Abstracts, 2019
28. Numerical Predictions of Dust-Induced Heat Flux Augmentation in Hypersonic Blunt-Body Flows Using a Discontinuous Galerkin Multiphase Flow Solver

    E. J. Ching, M. Barnhardt, and M. Ihme

    NASA Technical Reports Server (NASA), 2019

    Abs

    Recent interest in human-scale missions to Mars has motivated the need for high-fidelity simulations of reentry flows. During a dust storm, there can be high levels of suspended dust in the Martian atmosphere, which cannot only enhance erosion of thermal protection systems but also transfer energy and momentum to the shock layer, thereby significantly augmenting the surface heat flux. Second-order finite-volume schemes are typically employed for hypersonic flow simulations, but such schemes suffer from a number of disadvantages. An attractive alternative is discontinuous Galerkin methods, which benefit from arbitrarily high spatial order of accuracy, geometric flexibility, and other properties. To enable accurate computations of high-speed particle-laden flows, an Euler-Lagrange methodology was developed in which the Eulerian field of the carrier gas is calculated using a discontinuous Galerkin scheme while the disperse phase is treated with Lagrangian particle tracking. We discuss challenges associated with coupling these two formulations and how to handle them. Momentum and energy transfer between the carrier gas and the particle phase is considered, and the importance of accounting for interparticle collisions is assessed. In addition, we describe the physical model of the particle phase and examine effects of its uncertainties on the numerical solution. We demonstrate the performance of the Euler-Lagrange method in representative testcases, with focus on the accurate prediction of particle trajectories and heating augmentation. Quantitative comparisons with experiments are provided.

2018

1. Shock capturing for discontinuous Galerkin methods with application to predicting heat transfer in hypersonic flows

   E. J. Ching, Y. Lv, P. A. Gnoffo, M. Barnhardt, and M. Ihme

   Journal of Computational Physics, 2018

   DOI
2. Phase transitions of ordered ice in graphene nanocapillaries and carbon nanotubes

   M. Raju, A. C. T. Duin, and M. Ihme

   Scientific Reports, 2018

   Abs DOI Open Access

   New phase diagrams for water confined in graphene nanocapillaries and single-walled carbon nanotubes (CNTs) are proposed, identifying ice structures, their melting points and revealing the presence of a solid-liquid critical point. For quasi-2D water in nanocapillaries, we show through molecular-dynamics simulations that AA stacking in multilayer quasi-2D ice arises from interlayer hydrogen-bonding and is stable up to three layers, thereby explaining recent experimental observations. Detailed structural and energetic analyses show that quasi-2D water can freeze discontinuously through a first-order phase transition or continuously with a critical point. The first-order transition line extends to a continuous transition line, defined by a sharp transition in diffusivity between solid-like and liquid-like regimes. For quasi-1D water, confined in CNTs, we observe the existence of a similar critical point at intermediate densities. In addition, an end point is identified on the continuous-transition line, above which the solid and liquid phases deform continuously. The solid-liquid phase transition temperatures in CNTs are shown to be substantially higher than 273 K, confirming recent Raman spectroscopy measurements. We observe ultrafast proton and hydroxyl transport in quasi-1D and -2D ice at 300 K, exceeding those of bulk water up to a factor of five, thereby providing possible applications to fuel-cells and electrolyzers.

3. Structure of wall-bounded flows at transcritical conditions

   P. Ma, X. I. A. Yang, and M. Ihme

   Physical Review Fluids, 2018

   Abs DOI

   Direct numerical simulations of channel flow at transcritical conditions with density change in the flow field are conducted. We find a regime with useful semi-local scaling as well as streamwise velocity and energy spectrum scaling which support the attached eddy model at transcritical conditions.

4. Large-eddy simulations of transcritical injection and auto-ignition using diffuse-interface method and finite-rate chemistry

   P. Ma, H. Wu, T. Jaravel, L. Bravo, and M. Ihme

   Proceedings of the Combustion Institute, 2018

   DOI
5. Modulation of heat transfer for extended flame stabilization in porous media burners via topology gradation

   S. Sobhani, D. Mohaddes, É. Boigné, P. Muhunthan, and M. Ihme

   Proceedings of the Combustion Institute, 2018

   DOI
6. Data assimilation using high-speed measurements and LES to examine local extinction events in turbulent flames

   J. W. Labahn, H. Wu, B. Coriton, J. H. Frank, and M. Ihme

   Proceedings of the Combustion Institute, 2018

   DOI
7. Atomistic and continuum scale modeling of functionalized graphyne membranes for water desalination

   M. Raju, P. Govindaraju, A. C. T. Duin, and M. Ihme

   Nanoscale, 2018

   Abs DOI

   , which is up to three orders of magnitude larger than commercial seawater reverse osmosis (RO) membranes and up to ten times that of nanoporous graphene. Pore chemistry, functionalization and geometry are shown to play a critical role in modulating the water flux, and these observations are explained by water velocity, density, and energy barriers in the pores. The atomistic scale investigations are complemented by upscale continuum analysis to examine the performance of these membranes in application to cross-flow RO systems. This upscale analysis, however, shows that the significant increase in permeability, observed from MD simulations, does not fully translate to current RO systems due to transport limitations. Nevertheless, upscale calculations predict that the higher permeability of graphyne membranes would allow up to six times higher permeate recovery or up to 6% less energy consumption as compared to thin-film composite membranes at currently accessible operating conditions. Significantly higher energy savings and permeate recovery can be achieved if higher feed-flow rates can be realized.

8. Nonadiabatic Flamelet Formulation for Predicting Wall Heat Transfer in Rocket Engines

   P. Ma, H. Wu, M. Ihme, and J.-P. Hickey

   AIAA Journal, 2018

   Abs DOI Open Access

   A flamelet-based combustion model is proposed for the prediction of wall heat transfer in rocket engines and confined combustion systems. To account for convective heat loss due to the interaction of the flame with the wall, a permeable thermal boundary condition is introduced in the counterflow diffusion flame configuration. The solution of the resulting nonadiabatic flame structure forms a three-dimensional manifold, which is parameterized in terms of mixture fraction, progress variable, and temperature. The performance of the model is first evaluated through a direct numerical simulation analysis of an diffusion flame that is stabilized at an inert isothermal wall. The developed nonadiabatic flamelet model is shown to accurately predict the temperature, chemical composition, and wall heat transfer. Combined with a presumed probability density function closure, the model is then applied to large-eddy simulation of a single-injector rocket combustor to examine effects of heat transfer on the turbulent flame structure in rocket engines.

9. Assessment of differential diffusion effects in flamelet modeling of oxy-fuel flames

   S. Gierth, F. Hunger, S. Popp, H. Wu, M. Ihme, and C. Hasse

   Combustion and Flame, 2018

   DOI
10. Thermodynamic structure of supercritical LOX–GH2 diffusion flames

    D. T. Banuti, P. Ma, J.-P. Hickey, and M. Ihme

    Combustion and Flame, 2018

    DOI
11. Analysis of transient blow-out dynamics in a swirl-stabilized combustor using large-eddy simulations

    P. Ma, H. Wu, J. W. Labahn, T. Jaravel, and M. Ihme

    Proceedings of the Combustion Institute, 2018

    DOI
12. Numerical study of the ignition behavior of a post-discharge kernel in a turbulent stratified crossflow

    T. Jaravel, J. W. Labahn, B. Sforzo, J. Seitzman, and M. Ihme

    Proceedings of the Combustion Institute, 2018

    DOI Open Access
13. Large eddy simulations of diesel-fuel injection and auto-ignition at transcritical conditions

    M. Ihme, P. Ma, and L. Bravo

    International Journal of Engine Research, 2018

    Abs DOI Open Access

    Large eddy simulations of transcritical injection and auto-ignition of n-dodecane in a combustion chamber are performed. To this end, a diffuse-interface method is employed that solves the compressible multi-species conservation equations, and a cubic state equation together with real-fluid transport properties is employed to describe the transcritical fluid state. The reaction chemistry is represented by a finite-rate chemistry model involving a 33-species reduced mechanism for n-dodecane. Compared to commonly employed two-phase approaches, the method presented in this work does not introduce tunable parameters for spray-breakup. Large eddy simulation calculations are performed by considering the Spray A single-hole injector at non-reacting and reacting conditions at a pressure of 60 bar and temperatures between 800 and 1200 K. Quantitative comparisons with measurements for liquid and vapor penetration lengths are performed for non-reacting conditions, and sensitivity to threshold values on mixture fraction are examined. The analysis of reacting flow simulations focuses on comparisons of the instantaneous temperature and species fields for OH and CH 2 O at 800 and 900 K, respectively. Quantitative comparisons with measurements for ignition delay and lift-off heights as a function of ambient temperature are performed. To examine the transient ignition phase, comparisons of radially integrated OH profiles obtained from the simulations with reported measurements for OH* are performed, showing good agreement. These results show that the large eddy simulation modeling framework adequately reproduces the corresponding ignition processes, which are relevant to realistic diesel-fuel injection systems.

14. Error-controlled kinetics reduction based on non-linear optimization and sensitivity analysis

    T. Jaravel, H. Wu, and M. Ihme

    Combustion and Flame, 2018

    DOI
15. Coupling of turbulence on the ignition of multicomponent sprays

    P. Govindaraju, T. Jaravel, and M. Ihme

    Proceedings of the Combustion Institute, 2018

    DOI
16. Identification of governing physical processes of irregular combustion through machine learning

    K. Grogan, and M. Ihme

    Shock Waves, 2018

    DOI
17. A new ignition time model applied to super knock

    K. Grogan, and M. Ihme

    Proceedings of the Combustion Institute, 2018

    DOI
18. Efficient time stepping for reactive turbulent simulations with stiff chemistry

    H. Wu, P. Ma, and M. Ihme

    2018 AIAA Aerospace Sciences Meeting, 2018

    Abs DOI

    A combination of a steady-state preserving operator splitting method and a semi-implicit integration scheme is proposed for efficient time stepping in simulations of unsteady reacting flows, such as turbulent flames, using detailed chemical kinetic mechanisms. The operator splitting is based on the Simpler balanced splitting method, which is constructed with improved stability properties and reduced computational cost. The method is shown to be capable of stable and accurate prediction of ignition and extinction for reaction-diffusion systems near critical conditions. The ROK4E scheme is designed for semi-implicit integration of spatially independent chemically reacting systems. Being a Rosenbrock-Krylov method, ROK4E utilizes the low-rank approximation of the Jacobian to reduce the cost for integrating the system of ODEs that have relative few stiff components. The efficiency of the scheme is further improved via the careful choice of coefficients to require three right-hand-side evaluations over four stages. Combing these two methods, efficient calculation is achieved for large-scale parallel simulations of turbulent flames.

19. Transcritical mixing and auto-ignition of n-dodecane fuel using a diffuse interface method

    L. Bravo, P. Ma, M. Ihme, and K. A. Kerner

    2018 Joint Propulsion Conference, 2018

    DOI
20. Pareto-efficient combustion modeling for improved CO-emission prediction in LES of a piloted turbulent dimethyl ether jet flame

    H. Wu, P. Ma, T. Jaravel, and M. Ihme

    Proceedings of the Combustion Institute, 2018

    DOI
21. Assessment of spray combustion models in large-eddy simulations of a polydispersed acetone spray flame

    Q. Wang, T. Jaravel, and M. Ihme

    Proceedings of the Combustion Institute, 2018

    DOI
22. X-ray computed tomography for flame-structure analysis of laminar premixed flames

    É. Boigné, P. Muhunthan, D. Mohaddes, Q. Wang, S. Sobhani, W. S. Hinshaw, and M. Ihme

    Combustion and Flame, 2018

    DOI Open Access
23. Application of Pareto-efficient combustion modeling framework to large eddy simulations of turbulent reacting flows

    H. Wu, P. Ma, T. Jaravel, and M. Ihme

    2018 AIAA Aerospace Sciences Meeting, 2018

    Abs DOI

    In the application of the combustion models based on low-dimensional manifolds (for instance flamelet models) to large-eddy simulation (LES) of reactive turbulent flows, the modeling simplifications of the combustion process is a critical source of uncertainty in addition to those due to the turbulent closure model and numerical discretization. The ability to quantitatively assess this uncertainty in absence of the reference result is vital to the reliable and predictive simulations of practical combustion this http URL the present study, the Pareto-efficient combustion (PEC) framework is extended to adaptive LES combustion simulations of turbulent flames. The key component of the PEC framework is the so-called manifold drift term. Its extension LES is proposed to make such assessment by examining the compliance of a particular combustion model in describing a quantity of interest with respect to the underlying flow field representation. With the focus on improving predictions of CO emissions of flamelet-based combustion models, this work employs PEC to augment the flamelet/progress variable (FPV) model through local sub-model assignment of the finite-rate chemistry (FRC) model. To this end, a series of LES-PEC calculations are performed on a piloted partially-premixed dimethyl ether flame (DME-D), using a combination of FPV and FRC models. The drift term is utilized in the PEC framework to the estimate the model related error for quantities of interest. The PEC approach is demonstrated to be capable of significantly improving the prediction of CO emissions compared with the monolithic FPV simulation. The improved accuracy is achieved by enriching the FPV model with FRC in regions where the lower-order model is determined insufficient through the evaluation of drift terms.

24. Lyapunov exponent and Wasserstein metric as validation tools for assessing short-time dynamics and quantitative model evaluation of large-eddy simulation

    H. Wu, P. Ma, Y. Lv, and M. Ihme

    2018 AIAA Aerospace Sciences Meeting, 2018

    Abs DOI

    In this work, methods for the evaluation of LES-quality and LES-accuracy are presented, which include the Lyapunov exponent for the analysis of short-time predictability of LES-calculation and the Wasserstein metric for the quantitative assessment of simulation results. Both methods are derived and evaluated in application to the Volvo test case. Both the non-reacting and reacting cases are calculated. For the non- reacting cases, good agreement with the experimental data is achieved by solvers at high numerical resolution. The reacting cases are more challenging due to the small length scale of the flame and the suppression of sinuous mode of absolute instability by the density ratio. The analysis of the turbulent simulation data using the concept of the Lyapunov exponent and the Wasserstein metric provides a more quantitative approach to assess the mesh dependency of the simulation results. The convergence of the Lyapunov exponent is shown to be a more sensitive and stronger indication of mesh-independence. Though grid convergence for the reacting cases cannot be reached with the chosen resolutions, the Lyapunov exponents and the Wasserstein metric are shown to be capable of identifying quantity-specific sensitivities with respect to the numerical resolution, while requiring significantly less computational resources than acquiring profiles of conventional turbulent statistics.

25. Direct numerical simulations of turbulent channel flow under transcritical conditions

    P. Ma, X. I. A. Yang, and M. Ihme

    2018 AIAA Aerospace Sciences Meeting, 2018

    Abs DOI Open Access

    Turbulent flows under transcritical conditions are present in regenerative cooling systems of rocker engines and extraction processes in chemical engineering. The turbulent flows and the corresponding heat transfer phenomena in these complex processes are still not well understood experimentally and numerically. The objective of this work is to investigate the turbulent flows under transcritical conditions using DNS of turbulent channel flows. A fully compressible solver is used in conjunction with a Peng-Robinson real-fluid equation of state to describe the transcritical flows. A channel flow with two isothermal walls is simulated with one heated and one cooled boundary layers. The grid resolution adopted in this study is slightly finer than that required for DNS of incompressible channel flows. The simulations are conducted using both fully (FC) and quasi-conservative (QC) schemes to assess their performance for transcritical wall-bounded flows. The instantaneous flows and the statistics are analyzed and compared with the canonical theories. It is found that results from both FC and QC schemes qualitatively agree well with noticeable difference near the top heated wall, where spurious oscillations in velocity can be observed. Using the DNS data, we then examine the usefulness of Townsend attached eddy hypothesis in the context of flows at transcritical conditions. It is shown that the streamwise energy spectrum exhibits the inverse wavenumber scaling and that the streamwise velocity structure function follows a logarithmic scaling, thus providing support to the attached eddy model at transcritical conditions.

26. Numerical analysis on mixing processes for transcritical real-fluid simulations

    P. Ma, H. Wu, D. T. Banuti, and M. Ihme

    2018 AIAA Aerospace Sciences Meeting, 2018

    Abs DOI Open Access

    The accurate and robust simulation of transcritical real-fluid flows is crucial for many engineering applications. Diffused interface methods are frequently employed and several numerical schemes have been developed for simulating transcritical flows. These schemes can be categorized into two types, namely fully conservative and quasi-conservative schemes. An adaptive scheme which is a hybrid of the two is developed in this study. By considering several numerical test cases, it is shown that different schemes predict distinctly different mixing behaviors. It is shown that the mixing processes follow the isobaric-adiabatic and isobaric-isochoric mixing models for fully and quasi-conservative schemes, respectively, and the adaptive scheme yields a mixing behavior that spans both models. The distinct mixing behaviors are a consequence of numerical diffusion instead of physical diffusion and can be attributed to insufficient numerical spatial resolution. This work provides a better understanding on the interpretation of numerical simulation results and the mixing models that are commonly used to study transcritical flows.

27. Development of a Lagrangian Particle Tracking Method for High-Order Discontinuous Galerkin Schemes

    E. J. Ching, S. Brill, Y. Lv, and M. Ihme

    2018 AIAA Aerospace Sciences Meeting, 2018

    DOI
28. Variational multiscale SGS modeling for LES using a high-order discontinuous Galerkin method

    K. Bando, F. Naddei, M. D. L. L. Plata, and M. Ihme

    HAL (Le Centre pour la Communication Scientifique Directe), 2018

    Abs Open Access

    International audience

29. Generalized chaos expansion of state space models for uncertainty quanti cation in thermoacoustics

    C. Silva, P. Pettersson, I. Gianluca, and M. Ihme

    2018
30. Computational Modeling and Analysis of Diesel-fuel Injection and Autoignition at Transcritical Conditions

    M. Ihme, P. Ma, and L. Bravo

    arXiv (Cornell University), 2018

    Abs DOI Open Access

    The need for improved engine efficiencies has motivated the development of high-pressure combustion systems, in which operating conditions achieve and exceed critical conditions. Associated with these conditions are strong variations in thermo-transport properties as the fluid undergoes phase transition, and two-stage ignition with low-temperature combustion. Accurately simulating these physical phenomena at real-fluid environments remains a challenge. By addressing this issue, a high-fidelity LES-modeling framework is developed to conduct simulations of transcritical fuel spray mixing and auto-ignition at high-pressure conditions. The simulation is based on a recently developed diffused interface method that solves the compressible multi-species conservation equations along with a Peng-Robinson state equation and real-fluid transport properties. LES analysis is performed for non-reacting and reacting spray conditions targeting the ECN Spray A configuration at chamber conditions with a pressure of 60 bar and temperatures between 900 K and 1200 K to investigate effects of the real-fluid environment and low-temperature chemistry. Comparisons with measurements in terms of global spray parameters (i.e., liquid and vapor penetration lengths) are shown to be in good agreement. Analysis of the mixture fraction distributions in the dispersed spray region demonstrates the accuracy in modelling the turbulent mixing behavior. Good agreement of the ignition delay time and the lift-off length is obtained from simulation results at different ambient temperature conditions and the formation of intermediate species is captured by the simulations, indicating that the presented numerical framework adequately reproduces the corresponding low- and high-temperature ignition processes under high-pressure conditions, which are relevant to realistic diesel-fuel injection systems.

31. Computational Modeling and Analysis of Diesel-fuel Injection and\nAutoignition at Transcritical Conditions

    M. Ihme, P. C. Ma, and L. Bravo

    arXiv (Cornell University), 2018

    Abs DOI Open Access

    The need for improved engine efficiencies has motivated the development of\nhigh-pressure combustion systems, in which operating conditions achieve and\nexceed critical conditions. Associated with these conditions are strong\nvariations in thermo-transport properties as the fluid undergoes phase\ntransition, and two-stage ignition with low-temperature combustion. Accurately\nsimulating these physical phenomena at real-fluid environments remains a\nchallenge. By addressing this issue, a high-fidelity LES-modeling framework is\ndeveloped to conduct simulations of transcritical fuel spray mixing and\nauto-ignition at high-pressure conditions. The simulation is based on a\nrecently developed diffused interface method that solves the compressible\nmulti-species conservation equations along with a Peng-Robinson state equation\nand real-fluid transport properties. LES analysis is performed for non-reacting\nand reacting spray conditions targeting the ECN Spray A configuration at\nchamber conditions with a pressure of 60 bar and temperatures between 900 K and\n1200 K to investigate effects of the real-fluid environment and low-temperature\nchemistry. Comparisons with measurements in terms of global spray parameters\n(i.e., liquid and vapor penetration lengths) are shown to be in good agreement.\nAnalysis of the mixture fraction distributions in the dispersed spray region\ndemonstrates the accuracy in modelling the turbulent mixing behavior. Good\nagreement of the ignition delay time and the lift-off length is obtained from\nsimulation results at different ambient temperature conditions and the\nformation of intermediate species is captured by the simulations, indicating\nthat the presented numerical framework adequately reproduces the corresponding\nlow- and high-temperature ignition processes under high-pressure conditions,\nwhich are relevant to realistic diesel-fuel injection systems.\n

32. Modeling of sub-grid dispersion in large-eddy simulation using regularized deconvolution method

    Q. Wang, X. Zhao, and M. Ihme

    2018 AIAA Aerospace Sciences Meeting, 2018

    DOI
33. Correction: Modeling of sub-grid dispersion in large-eddy simulation using regularized deconvolution method

    Q. Wang, X. Zhao, and M. Ihme

    2018 AIAA Aerospace Sciences Meeting, 2018

    DOI
34. Transcritical Mixing and Auto-Ignition of n-dodecane Liquid Fuel using a Diffused Interface Method

    L. Bravo, P. Ma, and M. Ihme

    arXiv (Cornell University), 2018

    Abs DOI Open Access

    High-fidelity simulations of transcritical spray mixing and auto ignition in a combustion chamber are performed at high pressure and temperature conditions using a recently developed finite rate LES solver. The simulation framework is based on a diffused-interface (DI) method that solves the compressible multi-species conservation equations along with the Peng Robinson state equation and real-fluid transport properties. A finite volume approach with entropy stable scheme is employed to accurate simulate the non-linear real fluid flow. LES analysis is performed for non-reacting and reacting spray conditions targeting the ECN Spray A configuration at chamber conditions with a pressure of 60 bar and temperatures between 800 K and 1200 K to investigate effects of the real-fluid environment and low-temperature chemistry. Comparisons with measurements in terms of global spray parameters and mixture fraction distributions demonstrates the accuracy in modeling the turbulent mixing behavior. Good overall agreement of the auto-ignition process is obtained from simulation results at different ambient temperature conditions and the formation of intermediate species is captured by the simulations, indicating that the presented numerical framework adequately reproduces the corresponding low-and-high-temperature ignition processes under high-pressure conditions that are relevant to realistic diesel fuel injection systems.

35. Correction: Transcritical mixing and auto-ignition of n-dodecane fuel using a diffuse interface method

    L. Bravo, P. Ma, M. Ihme, and K. A. Kerner

    2018 Joint Propulsion Conference, 2018

    DOI
36. What is the potential of high-order methods for predicting turbulent combustion at highly turbulentconditions?

    M. Ihme

    Bulletin of the American Physical Society, 2018
37. Flow field statistics and scaling in random 2D porous media

    S. Sobhani, S. V. Apte, and M. Ihme

    Bulletin of the American Physical Society, 2018
38. Characterizing polydispersity in counterflow spray flames

    P. Govindaraju, and M. Ihme

    Bulletin of the American Physical Society, 2018
39. An Euler-Lagrange formulation for simulating high-speed dusty flows using discontinuous Galerkin schemes

    E. J. Ching, S. Brill, Y. Lv, and M. Ihme

    Bulletin of the American Physical Society, 2018
40. Data assimilation and parameter estimation of thermoacoustic instabilities in a ducted premixed flame

    H. Yu, T. Jaravel, M. Ihme, F. Garita, M. P. Juniper, and L. Magri

    Bulletin of the American Physical Society, 2018
41. An observable regularization of reactive compressible flows

    B. Aboulhasanzadeh, K. Mohseni, and M. Ihme

    Bulletin of the American Physical Society, 2018
42. Computational Modeling of Accidental Fire Spread in Under-Ventilated Compartments

    D. Mohaddes, and M. Ihme

    Bulletin of the American Physical Society, 2018
43. Microexplosions in Multicomponent Fuel Droplets

    T. Yau, P. Govindaraju, and M. Ihme

    Bulletin of the American Physical Society, 2018
44. An Enriched Discontinuous Galerkin Method for Representing High-Gradient Features in Fluid Dynamics

    S. Brill, and M. Ihme

    Bulletin of the American Physical Society, 2018
45. Development of synchrotron X-ray computed tomography for quantitative measurements of gas-phase temperature in reacting flows

    É. Boigné, D. M. Khorassani, P. Muhunthan, S. Sobhani, D. Parkinson, H. Barnard, and M. Ihme

    Bulletin of the American Physical Society, 2018
46. Large-scale space definition for the DG-VMS method based on energy transfer analyses

    F. Naddei, M. D. L. L. Plata, É. Lamballais, V. Couaillier, M. Massot, and M. Ihme

    HAL (Le Centre pour la Communication Scientifique Directe), 2018

    Abs Open Access

    International audience

2017

1. Fuel effects on lean blow-out in a realistic gas turbine combustor

   L. Esclapez, P. Ma, E. Mayhew, R. Xu, S. Stouffer, T. Lee, H. Wang, and M. Ihme

   Combustion and Flame, 2017

   DOI
2. An entropy-stable hybrid scheme for simulations of transcritical real-fluid flows

   P. Ma, Y. Lv, and M. Ihme

   Journal of Computational Physics, 2017

   DOI
3. Widom Lines in Binary Mixtures of Supercritical Fluids

   M. Raju, D. T. Banuti, P. Ma, and M. Ihme

   Scientific Reports, 2017

   Abs DOI Open Access

   Recent experiments on pure fluids have identified distinct liquid-like and gas-like regimes even under supercritical conditions. The supercritical liquid-gas transition is marked by maxima in response functions that define a line emanating from the critical point, referred to as Widom line. However, the structure of analogous state transitions in mixtures of supercritical fluids has not been determined, and it is not clear whether a Widom line can be identified for binary mixtures. Here, we present first evidence for the existence of multiple Widom lines in binary mixtures from molecular dynamics simulations. By considering mixtures of noble gases, we show that, depending on the phase behavior, mixtures transition from a liquid-like to a gas-like regime via distinctly different pathways, leading to phase relationships of surprising complexity and variety. Specifically, we show that miscible binary mixtures have behavior analogous to a pure fluid and the supercritical state space is characterized by a single liquid-gas transition. In contrast, immiscible binary mixture undergo a phase separation in which the clusters transition separately at different temperatures, resulting in multiple distinct Widom lines. The presence of this unique transition behavior emphasizes the complexity of the supercritical state to be expected in high-order mixtures of practical relevance.

4. Similarity law for Widom lines and coexistence lines

   D. T. Banuti, M. Raju, and M. Ihme

   Physical review. E, 2017

   Abs DOI

   The coexistence line of a fluid separates liquid and gaseous states at subcritical pressures, ending at the critical point. Only recently, it became clear that the supercritical state space can likewise be divided into regions with liquidlike and gaslike properties, separated by an extension to the coexistence line. This crossover line is commonly referred to as the Widom line, and is characterized by large changes in density or enthalpy, manifesting as maxima in the thermodynamic response functions. Thus, a reliable representation of the coexistence line and the Widom line is important for sub- and supercritical applications that depend on an accurate prediction of fluid properties. While it is known for subcritical pressures that nondimensionalization with the respective species critical pressures p_cr and temperatures T_cr only collapses coexistence line data for simple fluids, this approach is used for Widom lines of all fluids. However, we show here that the Widom line does not adhere to the corresponding states principle, but instead to the extended corresponding states principle. We resolve this problem in two steps. First, we propose a Widom line functional based on the Clapeyron equation and derive an analytical, species specific expression for the only parameter from the Soave-Redlich-Kwong equation of state. This parameter is a function of the acentric factor ω and compares well with experimental data. Second, we introduce the scaled reduced pressure p_r^* to replace the previously used reduced pressure p_r=p/p_cr. We show that p_r^* is a function of the acentric factor only and can thus be readily determined from fluid property tables. It collapses both subcritical coexistence line and supercritical Widom line data over a wide range of species with acentric factors ranging from -0.38 (helium) to 0.34 (water), including alkanes up to n-hexane. By using p_r^*, the extended corresponding states principle can be applied within corresponding states principle formalism. Furthermore, p_r^* provides a theoretical foundation to compare Widom lines of different fluids.

5. Seven questions about supercritical fluids - towards a new fluid state diagram

   D. T. Banuti, M. Raju, P. Ma, M. Ihme, and J.-P. Hickey

   55th AIAA Aerospace Sciences Meeting, 2017

   DOI
6. Lyapunov exponent as a metric for assessing the dynamic content and predictability of large-eddy simulations

   G. Nastac, J. W. Labahn, L. Magri, and M. Ihme

   Physical Review Fluids, 2017

   Abs DOI Open Access

   Temporal evolution of the solution separation of two initially infinitesimally perturbed simulations for a turbulent flow. This metric provides assessment of the Lyapunov exponent as a measure for the predictability time and dynamic content of turbulent flow simulations.

7. Non-equilibrium wall-modeling for internal combustion engine simulations with wall heat transfer

   P. Ma, M. Greene, V. Sick, and M. Ihme

   International Journal of Engine Research, 2017

   Abs DOI

   Heat transfer affects the performance and phasing of internal combustion engines. Correlations and equilibrium wall-function models are typically employed in engine simulations to predict heat transfer. However, many studies have shown that significant errors are expected, owing to the failure of fundamental assumptions in deriving equilibrium wall-function models. Non-equilibrium wall models provide a more accurate way of describing the near-wall region of in-cylinder flows. In this study, simultaneous high-speed high-resolution particle image velocimetry and heat-flux measurements are conducted in an optically accessible engine. The experiments are performed under both motored and fired conditions at two different engine speeds. The experimental data are utilized to assess the performance of different models in predicting the thermoviscous boundary layer. These models include commonly used heat transfer correlations, equilibrium and modified wall-function models, and a recently developed non-equilibrium wall model. It is found that the equilibrium wall-function model significantly underpredicts the heat flux under both motored and fired conditions. By considering heat release effects in the boundary layer, the non-equilibrium wall model is shown to be able to adequately capture the structure and dynamics of both momentum and thermal boundary layers in comparison with experimental measurements, demonstrating its improved performance over previously employed correlation functions and the equilibrium model.

8. A general probabilistic approach for the quantitative assessment of LES combustion models

   R. C. Johnson, H. Wu, and M. Ihme

   Combustion and Flame, 2017

   DOI Open Access
9. High-order discontinuous Galerkin method for applications to multicomponent and chemically reacting flows

   Y. Lv, and M. Ihme

   Acta Mechanica Sinica, 2017

   DOI
10. Numerical framework for transcritical real-fluid reacting flow simulations using the flamelet progress variable approach

    P. Ma, D. T. Banuti, J.-P. Hickey, and M. Ihme

    55th AIAA Aerospace Sciences Meeting, 2017

    Abs DOI Open Access

    An extension to the classical FPV model is developed for transcritical real-fluid combustion simulations in the context of finite volume, fully compressible, explicit solvers. A double-flux model is developed for transcritical flows to eliminate the spurious pressure oscillations. A hybrid scheme with entropy-stable flux correction is formulated to robustly represent large density ratios. The thermodynamics for ideal-gas values is modeled by a linearized specific heat ratio model. Parameters needed for the cubic EoS are pre-tabulated for the evaluation of departure functions and a quadratic expression is used to recover the attraction parameter. The novelty of the proposed approach lies in the ability to account for pressure and temperature variations from the baseline table. Cryogenic LOX/GH2 mixing and reacting cases are performed to demonstrate the capability of the proposed approach in multidimensional simulations. The proposed combustion model and numerical schemes are directly applicable for LES simulations of real applications under transcritical conditions.

11. MVP-Workshop Contribution: Modeling of Volvo bluff body flame experiment

    H. Wu, P. Ma, Y. Lv, and M. Ihme

    55th AIAA Aerospace Sciences Meeting, 2017

    DOI
12. A Flamelet Model with Heat-Loss Effects for Predicting Wall-Heat Transfer in Rocket Engines

    P. Ma, H. Wu, M. Ihme, and J.-P. Hickey

    53rd AIAA/SAE/ASEE Joint Propulsion Conference, 2017

    DOI
13. Investigation of Lean Combustion Stability, Pressure Drop, and Material Durability in Porous Media Burners

    S. Sobhani, B. Haley, D. Bartz, J. Dunnmon, J. D. Sullivan, and M. Ihme

    Volume 5C: Heat Transfer, 2017

    Abs DOI

    The operational stability and thermal durability of combustion in two-zone porous media burners (PMBs) is examined experimentally and computationally. Long-term material durability tests at constant and cycled on-off conditions are performed, along with a characterization of combustion stability, pressure drop and pollutant emissions for a range of equivalence ratios, mass flow rates, and burner setups. Experimental thermocouple temperature measurements and pressure drop data are presented and compared to results obtained from one-dimensional volume-averaged simulations. Experimental and model results show good agreement for temperature profiles and pressure drop evaluated using the Darcy-Forchheimer equation with Ergun’s relations. Enhanced flame stability is observed for burners with Yttria-stabilized Zirconia Alumina (YZA) upstream and Silicon Carbide (SiC) in the downstream combustion zone. Measurements of product gas concentrations illustrate highest emissions of CO at conditions close to flash-back and, as expected, higher NOx emissions with increasing equivalence ratios.

14. Phase separation analysis in supercritical injection using large-eddy-simulation and vapor-liquid-equilibrium

    D. T. Banuti, P. Ma, and M. Ihme

    53rd AIAA/SAE/ASEE Joint Propulsion Conference, 2017

    DOI
15. Formulation of optimal surrogate descriptions of fuels considering sensitivities to experimental uncertainties

    P. Govindaraju, and M. Ihme

    Combustion and Flame, 2017

    DOI
16. Large-Eddy Simulations of Fuel Effect on Gas Turbine Lean Blow-out

    L. Esclapez, P. Ma, E. Mayhew, R. Xu, S. Stouffer, T. Lee, H. Wang, and M. Ihme

    55th AIAA Aerospace Sciences Meeting, 2017

    DOI
17. Flamelet regime characterization for non-premixed turbulent combustion simulations

    W. L. Chan, and M. Ihme

    Combustion and Flame, 2017

    DOI Open Access
18. On underresolved simulations of compressible turbulence using an entropy-bounded DG method: Solution stabilization, scheme optimization, and benchmark against a finite-volume solver

    Y. Lv, P. Ma, and M. Ihme

    Computers & Fluids, 2017

    DOI
19. Effects of Nozzle Helmholtz Number on Indirect Combustion Noise by Compositional Perturbations

    L. Magri, J. O’Brien, and M. Ihme

    Journal of Engineering for Gas Turbines and Power, 2017

    Abs DOI Open Access

    By modeling a multicomponent gas, a new source of indirect combustion noise is identified, which is named compositional indirect noise. The advection of mixture inhomogeneities exiting the gas-turbine combustion chamber through subsonic and supersonic nozzles is shown to be an acoustic dipole source of sound. The level of mixture inhomogeneity is described by a difference in composition with the mixture fraction. An n-dodecane mixture, which is a kerosene fuel relevant to aeronautics, is used to evaluate the level of compositional noise. By relaxing the compact-nozzle assumption, the indirect noise is numerically calculated for Helmholtz numbers up to 2 in nozzles with linear velocity profile. The compact-nozzle limit is discussed. Only in this limit, it is possible to derive analytical transfer functions for (i) the noise emitted by the nozzle and (ii) the acoustics traveling back to the combustion chamber generated by accelerated compositional inhomogeneities. The former contributes to noise pollution, whereas the latter has the potential to induce thermoacoustic oscillations. It is shown that the compositional indirect noise can be at least as large as the direct noise and entropy noise in choked nozzles and lean mixtures. As the frequency with which the compositional inhomogeneities enter the nozzle increases, or as the nozzle spatial length increases, the level of compositional noise decreases, with a similar, but not equal, trend to the entropy noise. The noisiest configuration is found to be a compact supersonic nozzle.

20. Effects of Nozzle Helmholtz Number on Indirect Combustion Noise by Compositional Perturbations

    L. Magri, J. O’Brien, and M. Ihme

    Volume 2C: Turbomachinery, 2017

    Abs DOI

    By modeling a multi-component gas, a new source of indirect combustion noise is identified, which is named compositional indirect noise. The advection of mixture inhomogeneities exiting the gas-turbine combustion chamber through subsonic and supersonic nozzles is shown to be an acoustic dipole source of sound. The level of mixture inhomogeneity is described by a difference in composition with the mixture fraction. An n-dodecane mixture, which is a kerosene fuel relevant to aeronautics, is used to evaluate the level of compositional noise. By relaxing the compact-nozzle assumption, the indirect noise is numerically calculated for Helmholtz numbers up to 2 in nozzles with linear velocity profile. The compact-nozzle limit is discussed. Only in this limit, it is possible to derive analytical transfer functions for (i) the noise emitted by the nozzle and (ii) the acoustics travelling back to the combustion chamber generated by accelerated compositional inhomogeneities. The former contributes to noise pollution, whereas the latter has the potential to induce thermoacoustic oscillations. It is shown that the compositional indirect noise can be at least as large as the direct noise and entropy noise in chocked nozzles and lean mixtures. As the frequency with which the compositional inhomogeneities enter the nozzle increases, or as the nozzle spatial length increases, the level of compositional noise decreases, with a similar, but not equal, trend to the entropy noise. The noisiest configuration is found to be a compact supersonic nozzle.

21. Application of Pareto-efficient combustion modeling framework to large eddy simulations of turbulent reacting flows

    H. Wu, and M. Ihme

    Bulletin of the American Physical Society, 2017
22. Development of Discontinuous Galerkin method for Hypersonic Heating Prediction

    E. J. Ching, Y. Lv, and M. Ihme

    55th AIAA Aerospace Sciences Meeting, 2017

    DOI
23. Species Dependency of the Compositional Indirect Noise Mechanism

    J. O’Brien, and M. Ihme

    Volume 2C: Turbomachinery, 2017

    Abs DOI

    Indirect combustion noise is a growing concern for aviation engine designers. It is typically associated with the distortion of “hot spots” (entropy structures) and vortical structures that generate excess noise as they are convected by an accelerating mean flow. Recently, it has been shown theoretically that fluctuations in a gas’s chemical composition can act as an additional source of indirect noise in aviation engines. This work examines this composition noise mechanism, focusing on the underlying chemical effects that drive this source of indirect noise. Since the mechanism has yet to be confirmed experimentally, this paper begins by applying the theory to inert mixtures of noble gases and air, in an attempt to guide experimentalists by identifying the set of operating conditions that will produce the strongest acoustic response. Turning from non-reacting to reacting flows, the paper next examines the sensitivity of the compositional noise mechanism to fuel type, testing several common fuels. It is found that, while there is a substantial difference between hydrogen and hydrocarbon fuels, overall noise levels vary only slightly between different hydrocarbon fuels. Additionally, there appears to be a common underlying structure to the response of a product-gas mixture generated by burning a fuel, which is explained through linearized theory and confirmed with numerical results. Lastly, the physics of composition noise is examined at the species-specific level, attempting to provide a link between individual combustion products and changes in a mixture’s propensity to generate indirect noise. The sensitivity of individual species can be explained by a combination of differences between the species and mixture’s Gibbs free energy and strong gradients in product gas concentration with mixture fraction. However, by analyzing the species dependency of combustion products at several different mean mixture fractions, it is found that no single species dominates the noise generation over the combustor’s entire range, but rather the most acoustically active species varies strongly with local stoichiometry.

24. Experimental Investigation of Flame Stability in Porous Media Burners

    D. Mohaddes, S. Sobhani, É. Boigné, P. Muhunthan, and M. Ihme

    Bulletin of the American Physical Society, 2017
25. Determining flow and simulation dependent properties of turbulent flows based on an analysis of the Lyapunov exponent

    J. W. Labahn, G. Nastac, L. Magri, and M. Ihme

    Bulletin of the American Physical Society, 2017
26. Compositional inhomogeneities as a source of indirect combustion noise: Physical mechanisms and fuel effects

    M. Ihme, J. O’Brien, and J. Kim

    2017
27. MVP-Workshop Contribution: Modeling of Volvo Bluff Flame Experiment and Comparison of Finite-Volume and Discontinuous-Galerkin Schemes

    H. Wu, P. Ma, Y. Lv, and M. Ihme

    arXiv (Cornell University), 2017

    Abs DOI Open Access

    The Volvo burner features the canonical configuration of a bluff-body stabilized premixed flame. This configuration was studied experimentally under the Volvo Flygmotor AB program. Two cases are considered in this study: a non-reacting case with an inlet flow speed of 16.6 m/s and a reacting case with equilibrium ratio of 0.65 and inflow speed of 17.3 m/s. The characteristic vortex shedding in the wake behind the bluff body is present in the non-reacting case, while two oscillation modes are intermittently present in the reacting case. A series of large-eddy simulations are performed on this configuration using two solvers, one using a high-resolution finite-volume (FV) scheme and the other featuring a high-order discontinuous-Galerkin (DG) discretization. The FV calculations are conducted on hexahedral meshes with three different resolution (4mm, 2mm, and 1mm). The DG calculations are performed using two different polynomial orders on the same tetrahedral mesh. For the non-reacting cases, good agreement with respect to the experimental data is achieved by both solvers at high numerical resolution. The reacting cases are calculated using a two-step global mechanism in combination with the thickened-flame model. Reasonable agreement with experiments is obtained by both solvers at higher resolution. Models for combustion-turbulence interaction are necessary for the reacting case as it contains the length scale of the flame, which is smaller than the grid resolution in all calculations. The impact of such models on the flame stability and flow/flame dynamics is the subject of future research.

28. Modeling and Simulation of Diesel Injection at Transcritical Conditions

    P. Ma, M. Ihme, and L. Bravo

    arXiv (Cornell University), 2017

    Abs DOI Open Access

    The need for improved engine efficiencies has motivated the development of high-pressure combustion systems, in which operating conditions achieve and exceed critical conditions. Associated with these conditions are large thermodynamic gradients and strong variations in transport properties as the fluid undergoes mixing and phase transition. Accurately simulating these real-fluid environments remains a main challenge. Different modeling approaches have been employed, which can be categorized as diffused and sharp interface methods. The objective of this study is to examine the diffused interface method for simulating diesel-fuel injection at conditions related to the supercritical regime. To this end, a recently developed compressible real-fluid solver for transcritical conditions is employed. Simulations of an ECN-relevant diesel-fuel injector are performed and predictions for instantaneous and statistical flow-field results are compared against available measurements. It is expected that results from this analysis will be useful in identifying limitations of current modeling techniques and in improving physical and numerical models for high-pressure injection systems.

29. Numerical methods to prevent pressure oscillations in transcritical flows

    P. C. Ma, Y. Lv, and M. Ihme

    arXiv (Cornell University), 2017

    Abs DOI Open Access

    The accurate and robust simulation of transcritical real-fluid effects is crucial for many engineering applications, such as fuel injection in internal combustion engines, rocket engines and gas turbines. For example, in diesel engines, the liquid fuel is injected into the ambient gas at a pressure that exceeds its critical value, and the fuel jet will be heated to a supercritical temperature before combustion takes place. This process is often referred to as transcritical injection. The largest thermodynamic gradient in the transcritical regime occurs as the fluid undergoes a liquid-like to a gas-like transition when crossing the pseudo-boiling line (Yang 2000, Oschwald et al. 2006, Banuti 2015). The complex processes during transcritical injection are still not well understood. Therefore, to provide insights into high-pressure combustion systems, accurate and robust numerical simulation tools are required for the characterization of supercritical and transcritical flows.

30. Instability Analysis of the Separated Boundary Layer in Shock Tubes

    K. Grogan, and M. Ihme

    55th AIAA Aerospace Sciences Meeting, 2017

    DOI
31. Calibration of X-ray computed tomography (XCT) using a flat flame burner

    P. Muhunthan, S. Sobhani, É. Boigné, D. Mohaddes, W. S. Hinshaw, and M. Ihme

    Bulletin of the American Physical Society, 2017
32. Lagrangian Particle Tracking in a Discontinuous Galerkin Method for Hypersonic Reentry Flows in Dusty Environments

    E. J. Ching, Y. Lv, and M. Ihme

    Bulletin of the American Physical Society, 2017
33. Towards Wall-Modeled LES using High-Order Discontinuous Galerkin Methods

    Y. Lv, X. Yang, G. I. Park, and M. Ihme

    Bulletin of the American Physical Society, 2017
34. DNS of Multicomponent Spray Combustion

    P. Govindaraju, T. Jaravel, and M. Ihme

    Bulletin of the American Physical Society, 2017
35. Investigation of pore-scale flow physics in porous media burners

    S. Sobhani, P. Muhunthan, É. Boigné, D. Mohaddes, and M. Ihme

    Bulletin of the American Physical Society, 2017
36. Comparison of Differential Diffusion Flamelet Modeling Approaches in Turbulent Oxy-Fuel Flames

    S. Gierth, F. Hunger, S. Popp, M. Ihme, and C. Hasse

    2017
37. Pore-scale and topology analysis of flame stabilization inside inert porous media using X-ray microtomography

    É. Boigné, P. Muhunthan, D. M. Khorassani, S. Sobhani, D. Parkinson, H. Barnard, and M. Ihme

    Bulletin of the American Physical Society, 2017
38. Assessment of sub-grid scale dispersion closure with regularized deconvolution method in a particle-laden turbulent jet

    M. Wang, X. Zhao, and M. Ihme

    Bulletin of the American Physical Society, 2017
39. Modeling wall-bounded flows at transcritical conditions

    P. Ma, X. Yang, and M. Ihme

    Bulletin of the American Physical Society, 2017
40. Modal Analysis of Direct Core Noise in a Model Combustor

    J. O’Brian, F. Bake, and M. Ihme

    elib (German Aerospace Center), 2017

    Abs

    Combustion noise is a growing concern for gas turbine designers, especially the presence \nof prominent tonal structures or thermoacoustic instabilities. High fidelity simulations such as compressible \nlarge eddy simulation (LES) have demonstrated great potential in predicting the behavior \nof these reacting flows, but these methods generate massive amounts of data and efficient techniques \nare required to isolate the behavior and underlying physics of acoustic structures. Multiple modaldecomposition \ntechniques have gained prominence across a variety of fields for identifying the underlying \ndynamics of complex systems, including the fast Fourier Transform (FFT), proper orthogonal \ndecomposition (POD), and dynamic mode decomposition (DMD). Here, these three techniques \nare applied to the pressure field taken from a compressible LES of a gas turbine model combustor \n(GTMC). Comparisons are made between the structures identified by each algorithm as well as the \ncapacity of each technique to identify mode amplitude and assign cardinality to the modes. Lastly, \ncomments are offered about the advantages and disadvantages, including robustness, of the methods \nstudied.

41. The coupling of high-speed high resolution experimental data and LES through data assimilation techniques

    S. Harris, J. W. Labahn, J. H. Frank, and M. Ihme

    Bulletin of the American Physical Society, 2017
42. Numerical study of the ignition behavior of a post-discharge kernel injected into a turbulent stratified cross-flow

    T. Jaravel, J. W. Labahn, and M. Ihme

    Bulletin of the American Physical Society, 2017
43. Investigation of Initial Droplet Distribution and Importance of Secondary Breakup Model on Lean Blowout Predictions of a Model Gas Turbine Combustor

    J. W. Labahn, P. Ma, L. Esclapez, and M. Ihme

    2017

2016

1. Combustion and Engine-Core Noise

   M. Ihme

   Annual Review of Fluid Mechanics, 2016

   Abs DOI

   The implementation of advanced low-emission aircraft engine technologies and the reduction of noise from airframe, fan, and jet exhaust have made noise contributions from an engine core increasingly important. Therefore, meeting future ambitious noise-reduction goals requires the consideration of engine-core noise. This article reviews progress on the fundamental understanding, experimental analysis, and modeling of engine-core noise; addresses limitations of current techniques; and identifies opportunities for future research. After identifying core-noise contributions from the combustor, turbomachinery, nozzles, and jet exhaust, they are examined in detail. Contributions from direct combustion noise, originating from unsteady combustion, and indirect combustion noise, resulting from the interaction of flow-field perturbations with mean-flow variations in turbine stages and nozzles, are analyzed. A new indirect noise-source contribution arising from mixture inhomogeneities is identified by extending the theory. Although typically omitted in core-noise analysis, the impact of mean-flow variations and nozzle-upstream perturbations on the jet-noise modulation is examined, providing potential avenues for future core-noise mitigation.

2. Spectral kinetic energy transfer in turbulent premixed reacting flows

   C. Towery, A. Poludnenko, J. Urzay, J. O’Brien, M. Ihme, and P. E. Hamlington

   Physical review. E, 2016

   Abs DOI

   Spectral kinetic energy transfer by advective processes in turbulent premixed reacting flows is examined using data from a direct numerical simulation of a statistically planar turbulent premixed flame. Two-dimensional turbulence kinetic-energy spectra conditioned on the planar-averaged reactant mass fraction are computed through the flame brush and variations in the spectra are connected to terms in the spectral kinetic energy transport equation. Conditional kinetic energy spectra show that turbulent small-scale motions are suppressed in the burnt combustion products, while the energy content of the mean flow increases. An analysis of spectral kinetic energy transfer further indicates that, contrary to the net down-scale transfer of energy found in the unburnt reactants, advective processes transfer energy from small to large scales in the flame brush close to the products. Triadic interactions calculated through the flame brush show that this net up-scale transfer of energy occurs primarily at spatial scales near the laminar flame thermal width. The present results thus indicate that advective processes in premixed reacting flows contribute to energy backscatter near the scale of the flame.

3. The cross-scale physical-space transfer of kinetic energy in turbulent premixed flames

   J. O’Brien, C. Towery, P. E. Hamlington, M. Ihme, A. Poludnenko, and J. Urzay

   Proceedings of the Combustion Institute, 2016

   DOI
4. Compositional inhomogeneities as a source of indirect combustion noise

   L. Magri, J. O’Brien, and M. Ihme

   Journal of Fluid Mechanics, 2016

   Abs DOI Open Access

   The generation of indirect combustion noise by compositional inhomogeneities is examined theoretically. For this, the compact-nozzle theory of Marble & Candel ( J. Sound Vib. , vol. 55 (2), 1977, pp. 225–243) is extended to a multi-component gas mixture, and the chemical potential function is introduced as an additional acoustic source mechanism. Transfer functions for subcritical and supercritical nozzle flows are derived, and the contribution of compositional noise is compared to entropy noise and direct noise by considering an idealized nozzle downstream of the combustor exit. It is shown that compositional noise is dependent on the local mixture composition and can exceed entropy noise for fuel-lean conditions and supercritical nozzle flows. This suggests that the compositional indirect noise requires potential consideration with the implementation of low-emission combustors.

5. The role of preferential evaporation on the ignition of multicomponent fuels in a homogeneous spray/air mixture

   A. Stagni, L. Esclapez, P. Govindaraju, A. Cuoci, T. Faravelli, and M. Ihme

   Proceedings of the Combustion Institute, 2016

   DOI Open Access
6. An investigation of internal flame structure in porous media combustion via X-ray Computed Tomography

   J. Dunnmon, S. Sobhani, M. Wu, R. Fahrig, and M. Ihme

   Proceedings of the Combustion Institute, 2016

   DOI
7. Development and Analysis of Wall Models for Internal Combustion Engine Simulations Using High-speed Micro-PIV Measurements

   P. Ma, T. Ewan, C. Jainski, L. Lu, A. Dreizler, V. Sick, and M. Ihme

   Flow Turbulence and Combustion, 2016

   DOI
8. Classification and lift-off height prediction of non-premixed MILD and autoignitive flames

   M. J. Evans, P. R. Medwell, H. Wu, A. Stagni, and M. Ihme

   Proceedings of the Combustion Institute, 2016

   DOI Open Access
9. An entropy-residual shock detector for solving conservation laws using high-order discontinuous Galerkin methods

   Y. Lv, Y. C. See, and M. Ihme

   Journal of Computational Physics, 2016

   DOI
10. Regimes describing shock boundary layer interaction and ignition in shock tubes

    K. Grogan, and M. Ihme

    Proceedings of the Combustion Institute, 2016

    DOI
11. Compliance of combustion models for turbulent reacting flow simulations

    H. Wu, and M. Ihme

    Fuel, 2016

    DOI
12. Regularized deconvolution method for turbulent combustion modeling

    Q. Wang, and M. Ihme

    Combustion and Flame, 2016

    DOI Open Access
13. Group contribution method for multicomponent evaporation with application to transportation fuels

    P. Govindaraju, and M. Ihme

    International Journal of Heat and Mass Transfer, 2016

    DOI
14. Sub- or Supercritical? A flamelet analysis of high pressure rocket propellant injection

    D. T. Banuti, P. Ma, J.-P. Hickey, and M. Ihme

    52nd AIAA/SAE/ASEE Joint Propulsion Conference, 2016

    DOI
15. High-Fidelity Simulations of Fuel Injection and Atomization of a Hybrid Air-Blast Atomizer

    P. Ma, L. Esclape, S. Carbajal, M. Ihme, T. Buschhagen, S. Naik, J. P. Gore, and R. P. Lucht

    54th AIAA Aerospace Sciences Meeting, 2016

    DOI
16. Numerical investigation of soot-flame-vortex interaction

    B. Franzelli, A. Cuoci, A. Stagni, M. Ihme, T. Faravelli, and S. Candel

    Proceedings of the Combustion Institute, 2016

    DOI
17. Multiple-scale thermo-acoustic stability analysis of a coaxial jet combustor

    L. Magri, Y. C. See, O. Tammisola, M. Ihme, and M. P. Juniper

    Proceedings of the Combustion Institute, 2016

    DOI Open Access
18. Investigation of the mechanisms of jet-engine core noise using large-eddy simulation

    J. O’Brien, J. Kim, and M. Ihme

    54th AIAA Aerospace Sciences Meeting, 2016

    DOI
19. Large-Eddy Simulation of a Turbulent Lifted Flame in a Vitiated Co-Flow

    M. Ihme, and Y. C. See

    Deep Blue (University of Michigan), 2016

    Abs Open Access

    Large-eddy simulation (LES) of a lifted flame in a vitiated co-flow has been performed using an unsteady flamelet/progress variable (UFPV) model. This model is an extension to the steady flamelet/progress variable approach, and describes the transient autoignition process of the lifted flame through the unsteady flamelet model. The particular advan-tage of this model is that it eliminates the flamelet time scale, and all thermochemical quantities are parameterized by mixture fraction, reaction progress parameter, and stoi-chiometric scalar dissipation rate. For application to LES, a presumed probability density function closure is employed, in which a beta-distribution is used for the mixture fraction, a statistically most-likely distribution is employed for the reaction progress parameter, and the distribution of the stoichiometric scalar dissipation rate is modeled by a Dirac delta function. Compared to the steady flamelet/progress variable model, predictions from the UFPV model show significant improvements, and the spatial evolution of the flame igni-tion process and lift-off height is in good agreement with experimental data. Flow field structure, statistical results, and scatter data are compared with experimental data, and potential improvements of the model are discussed. I.

20. Entropy Residual as a Feature-Based Adaptation Indicator for Simulations of Unsteady Flow<br />

    E. J. Ching, Y. Lv, and M. Ihme

    54th AIAA Aerospace Sciences Meeting, 2016

    DOI
21. Analysis of Combustion Closure Assumptions in a Dual-Mode Scramjet Combustor

    W. L. Chan, and M. Ihme

    54th AIAA Aerospace Sciences Meeting, 2016

    Abs DOI Open Access

    Peer Reviewed

22. Assessement of high order DG methods for LES of free stream transonic turbulence

    K. Hillewaert, J. Cagnone, S. Murman, A. Garai, L. Yu, and M. Ihme

    ORBi (University of Liège), 2016

    Abs Open Access

    This paper describes validation work for high-order discontinuous Galerkin Methods on Large Eddy Simulation (LES) of canonical test cases including shocks, namely compressible homogeneous isotropic turbulence and homogeneous turbulence passing a shock.Three different DG methods are considered, and the impact of their shock-stabilization mechanisms is evaluated, as a preliminary step towards defining an optimized strategy.

23. Efficient partially implicit integration method for stiff chemistry in high-fidelity simulations of turbulent reacting flows

    H. Wu, and M. Ihme

    Bulletin of the American Physical Society, 2016
24. Compressible Flamelet Model in a Rule-Based Turbulent Combustion Solver

    S. Thakur, J. Wright, and M. Ihme

    54th AIAA Aerospace Sciences Meeting, 2016

    Abs DOI

    A robust unsteady turbulent compressible combustion modeling capability developed in a CFD solver called Loci-STREAM is presented in this paper. It utilizes the flamelet methodology extended to account for compressibility. This solver integrates proven numerical methods for generalized grids and state-of-the-art physical models in a rule-based programming framework called Loci which allows: (a) seamless integration of multidisciplinary physics in a unified manner, and (b) automatic handling of parallel computing. The objective is to be able to routinely simulate unsteady combustion in complex geometries such as liquid rocket engines requiring large unstructured grids and complex multidisciplinary physics.

25. Optimal ignition placement using nonlinear adjoint looping

    U. A. Qadri, P. J. Schmid, L. Magri, and M. Ihme

    Bulletin of the American Physical Society, 2016
26. LES modeling of scalar transport based on high-order discontinuous finite-element method: Assessment of implicit LES and scalar variance modeling

    Y. Lv, and M. Ihme

    46th AIAA Fluid Dynamics Conference, 2016

    DOI
27. Reduction of Shock Capturing Error in Discontinuous Galerkin Schemes for Hypersonic Flow Simulations

    E. J. Ching, Y. Lv, and M. Ihme

    The 7th International Conference on Computational Methods (ICCM2016), 2016

    Abs

    Motivated by recent interest in simulations of hypersonic flows under re-entry conditions, a robust and accurate shock capturing approach for high-order discontinuous Galerkin methods is developed. Artificial viscosity is localized via a shock detector based on the entropy residual, which converges to zero in smooth regions and assumes nonzero values in non-smooth regions. The piecewise-constant artificial viscosity field is then diffused in order to generate a smooth profile to avoid spurious oscillations of the solution near the shock (Barter and Darmofal, 2010). To improve accuracy, this method is combined with mesh adaptation and a procedure for optimizing the artificial viscosity field. The performance of this approach is investigated in numerical tests of hypersonic flow over 2D and 3D cylinders. Results concerning heat transfer predictions demonstrate that this shock capturing scheme is relatively insensitive to errors caused by misalignment between unstructured meshes and the shock.

28. Generation of indirect combustion noise by compositional inhomogeneities

    L. Magri, J. O’Brien, and M. Ihme

    Bulletin of the American Physical Society, 2016
29. Characterization of transcritical and supercritical droplet vaporization regimes using computations

    P. Govindaraju, D. T. Banuti, P. Ma, M. Raju, and M. Ihme

    Bulletin of the American Physical Society, 2016
30. LES of a bluff-body stabilized premixed flame using discontinuous Galerkin scheme

    Y. Lv, and M. Ihme

    Bulletin of the American Physical Society, 2016
31. LES Investigation of Core Noise Mechanisms inside a Combustor-Nozzle System

    J. O’Brien, F. Bake, J. Kim, and M. Ihme

    elib (German Aerospace Center), 2016

    Abs Open Access

    The aim of this work is to expand knowledge of core noise physics through the study of a representative aviation-type combustor with a converging-diverging nozzle attached to the exhaust. First, a fully compressible LES of the entire flowpath is performed and validated against experimental measurements. From this calculation, the time history of the flow is sampled in a plane near the nozzle entrance in order to construct a library of representative fluctuations that are potential precursors to the direct and indirect noise observed at the nozzle outlet. This data is then used as an inflow for a series of separate nozzle simulations in which fluctuations in pressure, temperature ("hot spots"), and mixture composition are imposed separately to isolate their effect on the emitted noise. This methodology allows quantitative investigation of core-noise physics that lower-order models do not, including: the effect of non-linearity of high-amplitude perturbations, superposition of forcing types, the impact of the spatial structure of the perturbations, and the restriction to low-frequency perturbations and calorically perfect gas assumption. The calculations also represent the first time that variations in mixture composition have been shown to induce downstream noise in a high-fidelity, 3D simulation.

32. Investigation of lean combustion stability and pressure drop in porous media burners

    S. Sobhani, B. Haley, D. Bartz, J. Dunnmon, J. D. Sullivan, and M. Ihme

    Bulletin of the American Physical Society, 2016
33. Evaluation of subgrid dispersion models for LES of spray flames

    M. Wang, X. Zhao, L. Esclapez, P. Govindaraju, and M. Ihme

    Bulletin of the American Physical Society, 2016
34. Wavelet multi-resolution analysis of energy transfer in turbulent premixed flames

    J. Kim, M. Bassenne, C. Towery, A. Poludnenko, P. E. Hamlington, M. Ihme, and J. Urzay

    Bulletin of the American Physical Society, 2016
35. Discontinuous Galerkin method for predicting heat transfer in hypersonic environments

    E. J. Ching, Y. Lv, and M. Ihme

    Bulletin of the American Physical Society, 2016
36. The latent heat of vaporization of supercritical fluids

    D. T. Banuti, M. Raju, J.-P. Hickey, and M. Ihme

    Bulletin of the American Physical Society, 2016
37. Analysis of Porous Media as Inlet Concept for Rotating Detonation Engines

    K. Grogan, and M. Ihme

    Bulletin of the American Physical Society, 2016
38. Flamelet Regime Diagram for Turbulent Combustion Simulations

    W. L. Chan, M. Ihme, H. Kolla, and J. Chen

    Bulletin of the American Physical Society, 2016
39. Compositional inhomogeneities as a source of indirect noise in subsonic and supersonic nozzles

    L. Magri, J. O’Brien, and M. Ihme

    Cambridge University Engineering Department Publications Database, 2016

    Abs DOI Open Access

    Engine-core noise in aeronautical gas-turbines is commonly divided into direct and indirect noise (Strahle 1978; Dowling & Mahmoudi 2015; Ihme 2017). Direct combustion noise is a source of self-noise, and describes the generation of acoustic pressure uctuations by unsteady heat release in the combustion chamber (Figure 1). In contrast, indirect combustion noise represents an induced noise-source mechanism that arises from the interaction between non-acoustic perturbations exiting the combustion chamber and downstream engine components. The indirect noise generation by temperature inhomogeneities arising from hot and cold spots is referred to as entropy noise (Marble & Candel 1977a), and indirect noise from vorticity uctuations is referred to as vorticity noise (Cumpsty 1979). Once sound has been generated, its propagation through the engine core depends on mean ow gradients and the geometry, which distort, di ract and re ect the acoustic propagation. Contributions of indirect noise to the overall core-noise emission have been examined theoretically and experimentally. These studies focused on separating out the contributions to noise from the (i) direct transmission and (ii) entropy noise. Di erent techniques have been employed to determine the transfer functions, including compact nozzle theories (Marble & Candel 1977a) and expansion methods (Stow et al. 2002; Goh & Morgans 2011; Moase et al. 2007; Giauque et al. 2012; Dur an & Moreau 2013). These theoretical investigations were supported by experimental studies (Bake et al. 2009; Kings & Bake 2010). These studies showed that indirect combustion noise requires consideration in the analysis of engine-core noise and can exceed the contribution from direct noise under some circumstances (see, e.g., Dowling & Mahmoudi 2015).

2015

1. Ignition regimes in rapid compression machines

   K. Grogan, S. S. Goldsborough, and M. Ihme

   Combustion and Flame, 2015

   DOI Open Access
2. A Pareto-efficient combustion framework with submodel assignment for predicting complex flame configurations

   H. Wu, Y. C. See, M. Wang, and M. Ihme

   Combustion and Flame, 2015

   DOI Open Access
3. Entropy-bounded discontinuous Galerkin scheme for Euler equations

   Y. Lv, and M. Ihme

   Journal of Computational Physics, 2015

   DOI Open Access
4. Numerical Modeling of Soot Production in Aero-Engine Combustors Using Large Eddy Simulations

   B. Franzelli, É. Riber, B. Cuenot, and M. Ihme

   Volume 4B: Combustion, Fuels and Emissions, 2015

   Abs DOI

   Numerical simulations are regarded as an essential tool for improving the design of combustion systems since they can provide information that is complementary to experiments. However, although numerical simulations have already been successfully applied to the prediction of temperature and species concentration in turbulent flames, the production of soot is far from being conclusive due to the complexity of the processes involved in soot production. In this context, first Large Eddy Simulations (LES) of soot production in turbulent flames are reported in the literature in laboratory-scale configurations, thereby confirming the feasibility of the approach. However numerous modeling and numerical issues have not been completely solved. Moreover, validation of the models through comparisons with measurements in realistic complex flows typical of aero-engines is still rare. This work therefore proposes to evaluate the LES approach for the prediction of soot production in an experimental swirl-stabilized non-premixed ethylene/air aero-engine combustor, for which soot and flame data are available. Two simulations are carried out using a two-equation soot model to compare the performance of a hybrid chemical description (reduced chemistry for the flame structure/tabulated chemistry for soot precursor chemistry) to a classical full tabulation method. Discrepancies of soot concentration between the two LES calculations will be analyzed and the sensitivity to the chemical models will be investigated.

5. On the generalisation of the mixture fraction to a monotonic mixing-describing variable for the flamelet formulation of spray flames

   B. Franzelli, A. Vié, and M. Ihme

   Combustion Theory and Modelling, 2015

   Abs DOI Open Access

   Spray flames are complex combustion configurations that require the consideration of competing processes between evaporation, mixing and chemical reactions. The classical mixture-fraction formulation, commonly employed for the representation of gaseous diffusion flames, cannot be used for spray flames owing to its non-monotonicity. This is a consequence of the presence of an evaporation source term in the corresponding conservation equation. By addressing this issue, a new mixing-describing variable, called the effective composition variable η, is introduced to enable the general analysis of spray-flame structures in composition space. This quantity combines the gaseous mixture fraction Zg and the liquid-to-gas mass ratio Zl, and is defined as . This new expression reduces to the classical mixture-fraction definition for gaseous systems, thereby ensuring consistency. The versatility of this new expression is demonstrated in application to the analysis of counterflow spray flames. Following this analysis, this effective composition variable is employed for the derivation of a spray-flamelet formulation. The consistent representation in both effective composition space and physical space is guaranteed by construction and the feasibility of solving the resulting spray-flamelet equations in this newly defined composition space is demonstrated numerically. A model for the scalar dissipation rate is proposed to close the derived spray-flamelet equations. The laminar one-dimensional counterflow spray-flamelet equations are numerically solved in η-space and compared to the physical-space solutions. It is shown that the hysteresis and bifurcation characterising the flame structure response to variations of droplet diameter and strain rate are correctly reproduced by the proposed composition-space formulation.

6. An SMLD Joint PDF Model for Turbulent Non-Premixed Combustion Using the Flamelet Progress-Variable Approach

   A. Coclite, G. Pascazio, P. De Palma, L. Cutrone, and M. Ihme

   Flow Turbulence and Combustion, 2015

   DOI Open Access
7. Supercritical and Transcritical Real-Fluid Mixing in Diesel Engine Applications

   P. S. C, L. Bravo, and M. Ihme

   2015

   Abs

   Abstract : A numerical framework for simulating the mixing of supercritical and transcritical fluids with large density ratios is presented. The accurate and robust modeling of real-fluid effects on the fuel/air mixing process is critical in characterizing engine combustion. To describe this mixing process, the double-flux model is extended to systems with real-fluid state equations. For this, an effective specific heat ratio is introduced to eliminate the spurious pressure oscillations that are caused by the nonlinearity of the real-fluid equation of state. High-order WENO spatial discretization scheme is used for the real-fluid flow calculations to account for the large density gradients associated with these flows. One-dimensional single and multicomponent advection test cases and two-dimensional supercritical nitrogen jet mixing and transcritical n-dodecane injection test cases are conducted to show the capability of the present study in application to real-fluid supercritical and transcritical flows. Energy conservation errors, generated due to the double-flux formulation, are acceptable.

8. Characterizing spray flame–vortex interaction: A spray spectral diagram for extinction

   B. Franzelli, A. Vié, and M. Ihme

   Combustion and Flame, 2015

   DOI Open Access
9. Detailed Simulations of Weak-to-Strong Ignition of a H2/O2/Ar Mixture in Shock-Tubes

   M. Ihme, Y. Sun, and R. Deiterding

   In 29th International Symposium on Shock Waves 1, 2015

   DOI
10. Characterization of scalar mixing in dense gaseous jets using X-ray computed tomography

    J. Dunnmon, S. Sobhani, T. W. Kim, A. R. Kovscek, and M. Ihme

    Experiments in Fluids, 2015

    DOI
11. A general and robust high-order numerical framework for shock-capturing: entropy-bounding, shock detection and artificial viscosity

    Y. Lv, Y. C. See, and M. Ihme

    53rd AIAA Aerospace Sciences Meeting, 2015

    Abs DOI

    A high-order shock-capturing framework is presented, that includes three key components, namely entropybounding, shock detection and artificial viscosity. The discontinuous Galerkin scheme is used to provide the discretization foundation, although the entire methodology is applicable to any discontinuous scheme. In this framework, entropy-bounding guarantees the numerical robustness by completely avoiding the appearance of non-physical quantities. An entropy-residual based shock detector is developed, which is able to accurately locate discontinuities. Building on the first two components, artificial viscosity is introduced locally to reduce spurious oscillations. By conducting Fourier analysis, a algebraic relation for the artificial viscosity is derived, which is linked to the eigen-spetrum of the numerical discretization. The resulting numerical framework offers a good balance between shock-capturing and computational efficiency for explicit time integration. Several test cases are conducted to confirm the benefits of using this method.

12. Linear Analysis of Jet-Engine Core Noise Based upon High-Fidelity Combustor and Turbine Simulations

    J. O’Brien, J. Kim, and M. Ihme

    53rd AIAA Aerospace Sciences Meeting, 2015

    DOI
13. A Fidelity Adaptive Modeling Framework for Combustion Systems Based on Model Trust-Region

    H. Wu, Y. C. See, Q. Wang, and M. Ihme

    53rd AIAA Aerospace Sciences Meeting, 2015

    Abs DOI

    In numerical predictions of turbulent reacting flows, the selection of a particular combustion model remains an area where there is no well-established systematic procedures. Often, expert knowledge and even experimental measurements are required to make an informed decision on the particular choice of a combustion model. Furthermore, the computational cost that is associated with a combustion model is another constraint in the selection process. To date, there has yet to be a general high fidelity combustion model that is computationally tractable for a large class of realistic combustion devices. In this work, a fidelity adaptive modeling (FAM) framework for combustion problems is developed to alleviate some of these issues. With minimal user input, this modeling framework is designed to automatically utilize the most appropriate combustion model locally in reacting flow simulations. This framework is demonstrated in an application to steady tribrachial flame, representing a challenging test case to single-regime reaction-diffusion manifold models. By utilizing reaction-diffusion manifold models in regions where they are accurate for a user-defined threshold, simulations with the FAM approach are able to reproduce results comparable to the detailed chemistry simulation. ∗Research Assistant, Center for Turbulence Research, Stanford University, AIAA Member. †Research Assistant, Flow Physics and Computational Engineering, Stanford University, AIAA Member. ‡Professor, Center for Turbulence Research, Stanford University, AIAA Member.

14. Linear Analysis of Engine Core Noise Using a Hybrid Modeling Approach

    J. O’Brien, J. Kim, and M. Ihme

    21st AIAA/CEAS Aeroacoustics Conference, 2015

    DOI
15. Hybrid Analysis of Engine Core Noise

    J. O’Brien, J. Kim, and M. Ihme

    Bulletin of the American Physical Society, 2015
16. Multiscale Interactions and Backscatter in Premixed Combustion

    P. E. Hamlington, C. Towery, J. O’Brien, A. Poludnenko, J. Urzay, and M. Ihme

    Bulletin of the American Physical Society, 2015
17. LES of combustion dynamics near blowout in a realistic gas-turbine combustor

    L. Esclapez, M. B. Nik, P. Ma, J. O’Brien, S. Carbajal, and M. Ihme

    Bulletin of the American Physical Society, 2015
18. High-Energy X-ray Absorption Diagnostics as an Experimental Combustion Technique

    J. Dunnmon, S. Sobhani, W. S. Hinshaw, R. Fahrig, and M. Ihme

    Bulletin of the American Physical Society, 2015
19. Analysis of Fuel Injection and Atomization of a Hybrid Air-Blast Atomizer.

    P. Ma, L. Esclape, T. Buschhagen, S. Naik, J. P. Gore, R. P. Lucht, and M. Ihme

    Bulletin of the American Physical Society, 2015
20. LES-Modeling of a Partially Premixed Flame using a Deconvolution Turbulence Closure

    M. Wang, H. Wu, and M. Ihme

    Bulletin of the American Physical Society, 2015
21. Evaporation and Combustion Characteristics of Multicomponent Fuels

    P. Govindaraju, A. Stagni, and M. Ihme

    Bulletin of the American Physical Society, 2015
22. LES of Mild Combustion using Pareto-efficient Combustion Adaptation

    H. Wu, M. P. Evans, and M. Ihme

    Bulletin of the American Physical Society, 2015
23. Adaptive entropy-constrained discontinuous Galerkin method for simulation of turbulent flows

    Y. Lv, and M. Ihme

    Bulletin of the American Physical Society, 2015
24. Viscous Shear Layers Formed by Non-Bifurcating Shock Waves in Shock-Tubes

    K. Grogan, and M. Ihme

    Bulletin of the American Physical Society, 2015
25. A metric for assessing the dynamic content of large-eddy simulations

    G. Nastac, and M. Ihme

    Bulletin of the American Physical Society, 2015
26. Modeling Gas Dynamic Effects in Shock-Tubes for Reaction Kinetics Measurements

    K. Grogan, M. Wang, and M. Ihme

    53rd AIAA Aerospace Sciences Meeting, 2015

    DOI
27. Multicomponent Droplet Evaporation using Group Contribution Methods with Application to Fuel Blends

    P. Govindaraju, Q. Wang, and M. Ihme

    2015

2014

1. Discontinuous Galerkin method for multicomponent chemically reacting flows and combustion

   Y. Lv, and M. Ihme

   Journal of Computational Physics, 2014

   DOI
2. Subgrid-scale backscatter in reacting and inert supersonic hydrogen–air turbulent mixing layers

   J. O’Brien, J. Urzay, M. Ihme, P. Moin, and A. Saghafian

   Journal of Fluid Mechanics, 2014

   Abs DOI

   Abstract This study addresses the dynamics of backscatter of kinetic energy in the context of large-eddy simulations (LES) of high-speed turbulent reacting flows. A priori analyses of direct numerical simulations (DNS) of reacting and inert supersonic, time-developing, hydrogen–air turbulent mixing layers with complex chemistry and multicomponent diffusion are conducted here in order to examine the effects of compressibility and combustion on subgrid-scale (SGS) backscatter of kinetic energy. The main characteristics of the aerothermochemical field in the mixing layer are outlined. A selfsimilar period is identified in which some of the turbulent quantities grow in a quasi-linear manner. A differential filter is applied to the DNS flow field to extract filtered quantities of relevance for the large-scale kinetic-energy budget. Spatiotemporal analyses of the flow-field statistics in the selfsimilar regime are performed, which reveal the presence of considerable amounts of SGS backscatter. The dilatation field becomes spatially intermittent as a result of the high-speed compressibility effect. In addition, the large-scale pressure-dilatation work is observed to be an essential mechanism for the local conversion of thermal and kinetic energies. A joint probability density function (PDF) of SGS dissipation and large-scale pressure-dilatation work is provided, which shows that backscatter occurs primarily in regions undergoing volumetric expansion; this implies the existence of an underlying physical mechanism that enhances the reverse energy cascade. Furthermore, effects of SGS backscatter on the Boussinesq eddy viscosity are studied, and a regime diagram demonstrating the relationship between the different energy-conversion modes and the sign of the eddy viscosity is provided along with a detailed budget of the volume fraction in each mode. A joint PDF of SGS dissipation and SGS dynamic-pressure dilatation work is calculated, which shows that high-speed compressibility effects lead to a decorrelation between SGS backscatter and negative eddy viscosities, which increases for increasingly large values of the SGS Mach number and filter width. Finally, it is found that the combustion dynamics have a marginal impact on the backscatter and flow-dilatation distributions, which are mainly dominated by the high-Mach-number effects.

3. Weak and strong ignition of hydrogen/oxygen mixtures in shock-tube systems

   K. Grogan, and M. Ihme

   Proceedings of the Combustion Institute, 2014

   DOI
4. Large eddy simulation of a partially-premixed gas turbine model combustor

   Y. C. See, and M. Ihme

   Proceedings of the Combustion Institute, 2014

   DOI
5. Analysis of segregation and bifurcation in turbulent spray flames: A 3D counterflow configuration

   A. Vié, B. Franzelli, Y. Gao, T. Lu, H. Wang, and M. Ihme

   Proceedings of the Combustion Institute, 2014

   DOI Open Access
6. Tabulated chemistry approach for diluted combustion regimes with internal recirculation and heat losses

   J. Lamouroux, M. Ihme, B. Fiorina, and O. Gicquel

   Combustion and Flame, 2014

   DOI
7. A multi-scale asymptotic scaling and regime analysis of flamelet equations including tangential diffusion effects for laminar and turbulent flames

   A. Scholtissek, W. L. Chan, H. Xu, F. Hunger, H. Kolla, J. H. Chen, M. Ihme, and C. Hasse

   Combustion and Flame, 2014

   DOI
8. Coupling of flame geometry and combustion instabilities based on kilohertz formaldehyde PLIF measurements

   P. M. Allison, Y. Chen, M. Ihme, and J. F. Driscoll

   Proceedings of the Combustion Institute, 2014

   DOI Open Access
9. Computational analysis of re-ignition and re-initiation mechanisms of quenched detonation waves behind a backward facing step

   Y. Lv, and M. Ihme

   Proceedings of the Combustion Institute, 2014

   DOI
10. Large-Eddy Simulations of a Dual-Mode Scramjet Combustor: Operating Point "A" of University of Virginia’s Scramjet Experiments

    W. L. Chan, and M. Ihme

    52nd Aerospace Sciences Meeting, 2014

    Abs DOI Open Access

    Peer Reviewed

11. Assessment of model assumptions and budget terms of the unsteady flamelet equations for a turbulent reacting jet-in-cross-flow

    W. L. Chan, H. Kolla, J. H. Chen, and M. Ihme

    Combustion and Flame, 2014

    DOI Open Access
12. Instability of elliptic liquid jets: Temporal linear stability theory and experimental analysis

    G. Amini, Y. Lv, A. Dolatabadi, and M. Ihme

    Physics of Fluids, 2014

    Abs DOI

    The instability dynamics of inviscid liquid jets issuing from elliptical orifices is studied, and effects of the surrounding gas and the liquid surface tension on the stability behavior are investigated. A dispersion relation for the zeroth azimuthal (axisymmetric) instability mode is derived. Consistency of the analysis is confirmed by demonstrating that these equations reduce to the well-known dispersion equations for the limiting cases of round and planar jets. It is shown that the effect of the ellipticity is to increase the growth rate over a large range of wavenumbers in comparison to those of a circular jet. For higher Weber numbers, at which capillary forces have a stabilizing effect, the growth rate decreases with increasing ellipticity. Similar to circular and planar jets, increasing the density ratio between gas and liquid increases the growth of disturbances significantly. These theoretical investigations are complemented by experiments to validate the local linear stability results. Comparisons of predicted growth rates with measurements over a range of jet ellipticities confirm that the theoretical model provides a quantitatively accurate description of the instability dynamics in the Rayleigh and first wind-induced regimes.

13. Modeling of Non-Equilibrium Homogeneous Turbulence in Rapidly Compressed Flows

    P. E. Hamlington, and M. Ihme

    Flow Turbulence and Combustion, 2014

    DOI
14. Quantitative model-based imaging of mid-infrared radiation from a turbulent nonpremixed jet flame and plume

    B. A. Rankin, M. Ihme, and J. P. Gore

    Combustion and Flame, 2014

    DOI Open Access
15. Effects of flow-field and mixture inhomogeneities on the ignition dynamics in continuous flow reactors

    H. Wu, and M. Ihme

    Combustion and Flame, 2014

    DOI
16. Effects of finite-rate chemistry and detailed transport on the instability of jet diffusion flames

    Y. C. See, and M. Ihme

    Journal of Fluid Mechanics, 2014

    Abs DOI

    Abstract Local linear stability analysis has been shown to provide valuable information about the response of jet diffusion flames to flow-field perturbations. However, this analysis commonly relies on several modelling assumptions about the mean flow prescription, the thermo-viscous-diffusive transport properties, and the complexity and representation of the chemical reaction mechanisms. In this work, the effects of these modelling assumptions on the stability behaviour of a jet diffusion flame are systematically investigated. A flamelet formulation is combined with linear stability theory to fully account for the effects of complex transport properties and the detailed reaction chemistry on the perturbation dynamics. The model is applied to a methane–air jet diffusion flame that was experimentally investigated by Füri et al. ( Proc. Combust. Inst. , vol. 29, 2002, pp. 1653–1661). Detailed simulations are performed to obtain mean flow quantities, about which the stability analysis is performed. Simulation results show that the growth rate of the inviscid instability mode is insensitive to the representation of the transport properties at low frequencies, and exhibits a stronger dependence on the mean flow representation. The effects of the complexity of the reaction chemistry on the stability behaviour are investigated in the context of an adiabatic jet flame configuration. Comparisons with a detailed chemical-kinetics model show that the use of a one-step chemistry representation in combination with a simplified viscous-diffusive transport model can affect the mean flow representation and heat release location, thereby modifying the instability behaviour. This is attributed to the shift in the flame structure predicted by the one-step chemistry model, and is further exacerbated by the representation of the transport properties. A pinch-point analysis is performed to investigate the stability behaviour; it is shown that the shear-layer instability is convectively unstable, while the outer buoyancy-driven instability mode transitions from absolutely to convectively unstable in the nozzle near field, and this transition point is dependent on the Froude number.

17. Large Eddy Simulation of Supercritical Mixing and Combustion for Rocket Applications

    J.-P. Hickey, and M. Ihme

    52nd Aerospace Sciences Meeting, 2014

    Abs DOI

    We report on the implementation of the real fluid capabilities to CharlesX, the in-house, unstructured, large eddy simulation code used at the Center for Turbulence Research at Stanford University. A conceptually distinct implementation was needed for the puremixing and the flamelet/progress-variable (FPV) model combustion case. For the nonreacting simulations, a Newton-Raphson based iterative algorithm is used to determine the temperature from the transported density and energy. For the reacting simulations, an extended flamelet table is used that tabulates the departure functions as well as the compressibility factor. These tabulated parameters are used to correct the transported thermodynamic properties. The real fluid extension to CharlesX was used to investigate a non-reacting and a reacting case. In both of these cases, a second-order essentially nonoscillatory (ENO) schemes is locally applied to the flux computation on the faces identified with a dual-threshold relative density sensor. This avoids spurious oscillations of the numerical solution with limited numerical dissipation. This preliminary work illustrates the capability CharlesX to capture the important physics in a typical rocket engine configuration.

18. LES Investigation of Flow Field Sensitivity in a Gas Turbine Model Combustor

    Y. C. See, and M. Ihme

    52nd Aerospace Sciences Meeting, 2014

    Abs DOI Open Access

    Peer Reviewed

19. Characterization of Scalar Mixing in Dense Gaseous Jets Using X-Ray Computed Tomography

    J. Dunnmon, T. W. Kim, A. R. Kovscek, R. Fahrig, and M. Ihme

    Bulletin of the American Physical Society, 2014
20. A spray-flamelet formulation using an effective composition-space variable

    B. Franzelli, A. V. ’e, and M. Ihme

    Bulletin of the American Physical Society, 2014
21. Consideration of Turbulence Effects in One-Dimensional Laminar Flamelet Equations

    W. L. Chan, and M. Ihme

    Bulletin of the American Physical Society, 2014
22. Supercritical fluid mixing in Diesel Engine Applications

    L. Bravo, P. Ma, M. S. Kurman, M. Tess, M. Ihme, and C.-B. Kweon

    Bulletin of the American Physical Society, 2014
23. Analysis of Wall Models for Internal Combustion Engine Simulations Using High-speed Micro-PIV Measurements

    P. Ma, T. Ewan, C. Jainski, A. Dreizler, L. Lu, V. Sick, and M. Ihme

    Bulletin of the American Physical Society, 2014
24. A New Approach for Imposing Artificial Viscosity for Explicit Discontinuous Galerkin Scheme

    Y. C. See, Y. Lv, and M. Ihme

    Bulletin of the American Physical Society, 2014
25. Positivity-preserving and entropy-bounded Discontinuous Galkerin method for conservation laws

    Y. Lv, and M. Ihme

    Bulletin of the American Physical Society, 2014
26. A Trust-Region Constrained Fidelity Adaptive Combustion Model

    Y. C. See, H. Wu, Q. Wang, and M. Ihme

    Bulletin of the American Physical Society, 2014
27. Numerical Investigation of Detonation Transmission Mechanism based on High-order Discontinuous Galerkin Scheme

    Y. Lv, and M. Ihme

    52nd Aerospace Sciences Meeting, 2014

    DOI
28. Modeling Gas-Dynamic Effects in Shock-Tubes for Reaction-Kinetic Measurements

    K. Grogan, Q. Wang, and M. Ihme

    Bulletin of the American Physical Society, 2014

    Abs

    The model is derived from the reactive NavierStokes equations where losses are accounted for by additional sink terms in the momentum and energy equations. The model was demonstrated to replicate experiments and was shown to perform well against substantially more computationally expensive unsteady Reynolds-averaged Navier-Stokes models. Additionally, a kinetic rate measurement experiment was simulated using the model. From this, non-idealities due to the boundary layer were quantied to yield a 23% discrepancy in activation energy between the actual kinetic rate and that obtained from the simulation.

29. Multiple space-scale global analysis for hydrodynamic/thermoacoustic instability in low Mach number combustion chambers

    L. Magri, O. Tammisola, Y. C. See, M. Ihme, and M. P. Juniper

    Bulletin of the American Physical Society, 2014
30. Engine core noise analysis using an hybrid modeling approach

    J. O’Brien, J. Kim, and M. Ihme

    Bulletin of the American Physical Society, 2014

2013

1. Large Eddy Simulation of Shear Coaxial Rocket Injector: Real Fluid Effects

   J.-P. Hickey, P. Ma, M. Ihme, and S. Thakur

   49th AIAA/ASME/SAE/ASEE Joint Propulsion Conference, 2013

   Abs DOI

   The implementation and verification of real-fluid effects towards the high-fidelity large eddy simulation of rocket combustors is reported. The non-ideal fluid behavior is modeled using a cubic Peng-Robinson equation of state; a thermodynamically consistent approach is used to convert conserved into primitive variables. The viscosity is estimated by Chung et al.’s method in the supercritical gas phase. In the transcritical liquid phase, a simple, accurate and efficient method to estimate the viscosity as a function of temperature and pressure is proposed. The highly non-linear coupling of the primitive thermodynamic variables requires special consideration in regions of high-density gradients to avoid spurious numerical oscillations. The characterization of the non-linearity of the equation of state identifies the regions of high sensitivity. In these regions, small relative changes in the pressure lead to significant changes in density and/or temperature, therefore, numerical instabilities tend to be amplified in these regions. To avoid non-physical oscillations, a first-order and second-order essentially non-oscillatory (ENO) schemes are locally applied to the flux computation on the faces identified with a dual-threshold relative density sensor. The evaluation of the sensor and capabilities of the non-oscillatory schemes on canonical test cases are presented. Finally, these schemes are used to model two canonical cases.

2. Large-eddy simulation of a piloted premixed jet burner

   Y. Chen, and M. Ihme

   Combustion and Flame, 2013

   DOI
3. Effect of gravity on capillary instability of liquid jets

   G. Amini, M. Ihme, and A. Dolatabadi

   Physical Review E, 2013

   Abs DOI

   The effect of gravity on the onset and growth rate of capillary instabilities in viscous liquid jets is studied. To this end, a spatial linear stability analysis of Cosserat’s equations is performed using a multiscale expansion technique. A dispersion relation and expressions for the perturbation amplitude are derived to evaluate the growth rate of the most unstable axisymmetric disturbance mode. Modeling results are compared with classical results in the limit of zero Bond number, confirming the validity of this approach. Expressions for the critical Weber number, demarcating the transition between convective and absolute instability are derived as functions of capillary and Bond numbers. Parametric investigations for a range of relevant operating conditions (characterized by capillary, Weber, and Bond numbers) are performed to examine the jet breakup and the perturbation growth rate. In addition to the physical insight that is obtained from this investigation, the results that are presented in this work could also be of relevance as test cases for the algorithmic development and the verification of high-fidelity multiphase simulation codes.

4. Detailed Simulations of Shock-Bifurcation and Ignition of an Argon-diluted Hydrogen/Oxygen Mixture in a Shock Tube

   M. Ihme, Y. Sun, and R. Deiterding

   51st AIAA Aerospace Sciences Meeting including the New Horizons Forum and Aerospace Exposition, 2013

   Abs DOI Open Access

   Detailed simulations of the bifurcation and ignition of an Argon-diluted Hydrogen/Oxy-gen mixture in the two-stage weak ignition regime are performed. An adaptive mesh-refinement (AMR) technique is employed to resolve all relevant physical scales that are associated with the viscous boundary-layer, the reaction front, and the shock-wave. A high-order hybrid WENO/central-differencing method is used as spatial discretization scheme, and a detailed chemical mechanism is employed to describe the combustion of the H2/O2 mixture. The operating conditions considered in this study are p5 = 5 bar and T5 = 1100 K, and fall in the third explosion limit. The computations show that the mixing of the thermally stratified fluid, carrying different momentum and enthalpy, introduces inhomogeneities in the core-region behind the reflected shock. These inhomogeneities act as localized ignition kernels. During the induction period, these kernels slowly expand and eventually transition to a detonation wave that rapidly consumes the unburned mixture. In competition with this detonation wave are the presence of secondary ignition kernels that appear in the unreacted core-region between reflected shock and detonation wave. I.

5. Backscatter of turbulent kinetic energy in chemically-reacting compressible flows

   J. Urzay, M. Ihme, P. Moin, and A. Saghafian

   2013

   Abs

   This study addresses the dynamics of kinetic-energy backscatter in the context of large-eddy simulations (LES) of turbulent chemically-reacting compressible flows. As a case study, a-priori analyses of direct numerical simulations (DNS) of reacting and inert supersonic, time-developing, hydrogen-air turbulent mixing layers with complex chemistry and multi-component diffusion are conducted herein to examine the effects of compressibility and combustion on subgrid-scale (SGS) backscatter of kinetic energy. General formulations of SGS backscatter with dilatation are provided, including an LESbased energy-transfer diagram that illustrates the conversion dynamics. Lastly, influences of SGS backscatter on the Boussinesq eddy viscosity are analyzed. 2. Background The energy-cascade hypothesis predicts that turbulent kinetic energy is generated at the largest scales in a flow and then transferred to progressively smaller and smaller scales until it is dissipated by molecular viscosity. This energy cascade typically holds in a statistically-averaged sense, but it does not always describe the local behavior of a turbulent flow. The turbulent dissipation associated with the smallest, viscous scales, is actually the difference between two energy fluxes, namely, the forwardscatter, corresponding to the classical energy cascade, and the backscatter, a reversal of this process in which energy is transferred from the small scales back to the large scales (Lesieur & �

6. Liquid Jet Instability Under Gravity Effects

   G. Amini, and M. Ihme

   51st AIAA Aerospace Sciences Meeting including the New Horizons Forum and Aerospace Exposition, 2013

   Abs DOI Open Access

   Peer Reviewed

7. Discontinuous Galerkin Method for Compressible Viscous Reacting Flow

   Y. Lv, and M. Ihme

   21st AIAA Computational Fluid Dynamics Conference, 2013

   Abs DOI

   In the present study, a discontinuous Galerkin (DG) framework is developed to simulate chemically reacting ows. The algorithm combines a double-ux method to account for variable thermodynamic properties, a Strang-splitting scheme for the sti reaction chemistry, a robust WENO-based shock limiter, and the non-linear viscous-diusive transport is discretized using the BR2 method. The algorithm is veried and validated by considering a series of one- and two-dimensional test cases, and results are compared with self-similarity solutions and experiments to examine critical algorithmic components. These cases include low-Mach deagration systems and supersonic inviscid and viscous problems. Multi-dimensional congurations consider the shock-ame interaction and detonation initiation process. It is shown that the reactive DG-method provides an accurate description of key-physical mechanisms that control the ignition onset in conned detonation systems.

8. Development of Discontinuous Galerkin Method for Detonation and Supersonic Combustion

   Y. Lv, and M. Ihme

   51st AIAA Aerospace Sciences Meeting including the New Horizons Forum and Aerospace Exposition, 2013

   Abs DOI Open Access

   Peer Reviewed

9. Parallel Compressible Solver for Unsteady Turbulent Combustion in Rocket Injectors Using Flamelet Models

   S. Thakur, J. Wright, and M. Ihme

   21st AIAA Computational Fluid Dynamics Conference, 2013

   DOI
10. Large-Eddy Simulation of a Gas Turbine Model Combustor

    Y. C. See, and M. Ihme

    51st AIAA Aerospace Sciences Meeting including the New Horizons Forum and Aerospace Exposition, 2013

    Abs DOI Open Access

    Peer Reviewed

11. Large-eddy simulations of real-fluid effects in rocket engine combustors

    P. Ma, J.-P. Hickey, and M. Ihme

    Bulletin of the American Physical Society, 2013
12. Modeling of Unsteady Heat Transfer in Flame-Wall Interaction

    H. Wu, and M. Ihme

    Bulletin of the American Physical Society, 2013
13. The know unknowns: Detailed simulations and low-order modeling to characterize facility-induced non-idealities in chemical-kinetics experiments

    M. Ihme

    Bulletin of the American Physical Society, 2013
14. Analysis of Subgrid-Scale Backscatter in Turbulent Reacting Flows

    J. O’Brien, J. Urzay, M. Ihme, P. Moin, and A. Saghafian

    Bulletin of the American Physical Society, 2013
15. Boundary Layer Effects on Ignition in a Shock-Tube System

    K. Grogan, and M. Ihme

    Bulletin of the American Physical Society, 2013

2012

1. Regularization of reaction progress variable for application to flamelet-based combustion models

   M. Ihme, L. Shunn, and J. Zhang

   Journal of Computational Physics, 2012

   DOI Open Access
2. Large-Eddy Simulation of a Jet-in-Hot-Coflow Burner Operating in the Oxygen-Diluted Combustion Regime

   M. Ihme, J. Zhang, G. He, and B. B. Dally

   Flow Turbulence and Combustion, 2012

   DOI
3. Acoustic characterization of a partially-premixed gas turbine model combustor: Syngas and hydrocarbon fuel comparisons

   P. M. Allison, J. F. Driscoll, and M. Ihme

   Proceedings of the Combustion Institute, 2012

   DOI
4. On the role of turbulence and compositional fluctuations in rapid compression machines: Autoignition of syngas mixtures

   M. Ihme

   Combustion and Flame, 2012

   DOI
5. On the Generation of Direct Combustion Noise in Turbulent Non-Premixed Flames

   M. Ihme, and H. Pitsch

   International Journal of Aeroacoustics, 2012

   Abs DOI

   Generation of combustion noise in an unconfined turbulent non-premixed flame is investigated. For this, a model is developed, combining Lighthill’s acoustic analogy with a flamelet-based combustion model to consistently express all thermochemical quantities by a set of reduced scalars. The model is applied in a large-eddy simulation, and the acoustic pressure in the far field is obtained from an integral solution. Three relevant acoustic source terms with different source characteristics and Mach number scaling are identified. The spatial distribution and spectral characteristics of the acoustic sources are analyzed, and it is shown that the acoustic source due to chemical reaction is the main noise contributor, and is located in the upper part of the flame. Contributions from the acoustic sources due to Reynolds stresses and fluctuating mass flux are found to be virtually insignificant at low frequencies. Discrepancies in the prediction of high-frequency sound pressure level in the jet forward direction were analyzed and are attributed to high-frequency acoustic refraction effects due to variations in sound speed. The directivity exhibits a weak directionality in the 30° forward direction, and some phase cancellation between individual acoustic sources is evident.

6. Acoustic Behavior of a Partially-Premixed Gas Turbine Model Combustor

   P. M. Allison, J. F. Driscoll, and M. Ihme

   50th AIAA Aerospace Sciences Meeting including the New Horizons Forum and Aerospace Exposition, 2012

   Abs DOI Open Access

   Combustion instabilities in gas turbine engines often give rise to acoustic resonances. These resonances occur as manifestations of different acoustic modes, of which a single or multiple modes may be present. In this work, the acoustic behavior of a model gas turbine combustor, developed at DLR Stuttgart by Meier, was investigated using both syn(thetic) gas and standard hydrocarbon fuels. Syngas displayed significantly different behavior than hydrocarbon fuels, even when the laminar flame speeds of the fuels were matched. The following operating parameters were systematically varied: equivalence ratio, air mass flow rate, burner temperature, flame speed, and fuel type. The results did not correspond to any one known instability mechanism. It is concluded that, in the current burner configuration, integrated-acoustics occur that involve a combination of mechanisms. These include organ tone resonances, Helmholtz-type instability, and convective-acoustic effects. Another possible factor is the fluid mechanical switching observed between the two swirler air passages, due to blockage caused by flame position and shape. Flame characteristics such as anchoring and liftoff height appear to play a major role in the determination of instability strength. As hydrogen content in syngas is increased, a lifted flame eventually anchors, resulting in a drastic decrease in the acoustic amplitude associated with non-resonating flames. Thus, increasing the flame speed causes two competing effects to occur. Propane acoustic data are collapsed using scaling parameters that indicate that the burner temperature and the air mass flow effects behave independently. Copyright © 2012 by Patton M. Allison.

7. Jet Noise Receptivity to Nozzle-upstream Perturbations in Compressible Heated Jets

   Y. C. See, G. Amini, C. Koh, and M. Ihme

   18th AIAA/CEAS Aeroacoustics Conference (33rd AIAA Aeroacoustics Conference), 2012

   Abs DOI Open Access

   Peer Reviewed

8. Simulation of a Shear Coaxial GO2/GH2 Rocket Injector with DES and LES Using Flamelet Models

   S. Thakur, J. Wright, M. Ihme, and K. R. Tucker

   48th AIAA/ASME/SAE/ASEE Joint Propulsion Conference &amp; Exhibit, 2012

   Abs DOI Open Access

   HE analysis and design of rocket propulsion systems largely relies on full-scale prototype development and testing. Computational simulation techniques have the potential to assist in the design process if accurate physical models are available. The objective of this study is to address this issues by assessing the predictive capability of three different modeling approaches to simulate the reactive flow field structure of a shear-coaxial

9. Characterization of Mixing and Ignition Effects in Flow-Reactor Facilities Using a Particle Method

   S. Weiher, and M. Ihme

   50th AIAA Aerospace Sciences Meeting including the New Horizons Forum and Aerospace Exposition, 2012

   Abs DOI Open Access

   Stationary power plants utilizing integrated gasification combined cycle (IGCC) technologies are considered to be a viable option for clean electricity generation. Considerable design challenges still exist that are attributed to the IGCC-operating conditions and requirements for the stable combustion of syngas and high hydro-gen content (HHC) fuels. Recent investigations have indicated discrepancies be-tween measurements and simulations of the ignition delay for syngas-mixtures at high-pressure and low-temperature conditions. The objective of this study is to characterize effects of turbulence and flow-field inhomogeneities on the mixing and ignition-dynamics in flow-reactors. To this end, an idealized flow-reactor is consid-ered and a particle method is used to describe the mixing, induction, and subsequent ignition of the reactants. Utilizing this model, parametric studies are performed to quantify effects of flow-field and scalar inhomogeneities on the ignition-process for practically relevant temperature and pressure conditions. Results suggest that the unsteady velocity field affects the ignition location and residence time in the flow-reactor. Stochastic events, such as the formation of localized ignition spots upstream of the main ignition front could be observed in the simulation. Their impact on the global ignition dynamics needs to be evaluated in further research activities. I.

10. Linear Stability Analysis of a Non-premixed Buoyant Jet Flame

    Y. C. See, and M. Ihme

    50th AIAA Aerospace Sciences Meeting including the New Horizons Forum and Aerospace Exposition, 2012

    Abs DOI Open Access

    Peer Reviewed

11. Discontinuous Galerkin Method for Combustion

    Y. Lv, and M. Ihme

    Bulletin of the American Physical Society, 2012
12. A Priori Analysis of an Unsteady Flamelet Formulation for Reacting Jet in Cross Flows

    W. L. Chan, M. Ihme, H. Kolla, and J. Chen

    Bulletin of the American Physical Society, 2012
13. Numerical Investigation of a Piloted Premixed Jet Burner

    Y. Chen, and M. Ihme

    Bulletin of the American Physical Society, 2012
14. Detailed Simulations and Analysis of Shock Bifurcation

    Y. Sun, M. Ihme, and R. Deiterding

    Bulletin of the American Physical Society, 2012
15. Acoustic response of heated jets to nozzle-upstream perturbations

    M. Ihme, Y. C. See, and G. Amini

    Bulletin of the American Physical Society, 2012
16. Convective and Absolute Instability of Liquid Jets under Gravity Effects

    G. Amini, M. Ihme, and A. Dolatabadi

    Bulletin of the American Physical Society, 2012

2011

1. Reduced-Order Modeling of Turbulent Reacting Flows with Application to Ramjets and Scramjets

   S. Torrez, J. F. Driscoll, M. Ihme, and M. Fotia

   Journal of Propulsion and Power, 2011

   Abs DOI Open Access

   Peer Reviewed

2. Construction of Optimal Artificial Neural Network Architectures for Application to Chemical Systems: Comparison of Generalized Pattern Search Method and Evolutionary Algorithm

   M. Ihme

   In InTech eBooks, 2011

   Abs DOI Open Access

   Construction of Optimal Artificial Neural Network Architectures for Application to Chemical Systems: Comparison of Generalized Pattern Search Method and Evolutionary Algorithm

3. LES Modeling of a Turbulent Lifted Flame in a Vitiated Co-flow Using an Unsteady Flamelet/Progress Variable Formulation

   M. Ihme, and Y. C. See

   In ERCOFTAC series, 2011

   DOI
4. Reconcile Discrepancies of Current Syngas Kinetics Models by Considering Turbulence Effects on Ignition Delay at Gas-Turbine Relevant Operating Conditions

   M. Ihme

   49th AIAA Aerospace Sciences Meeting including the New Horizons Forum and Aerospace Exposition, 2011

   Abs DOI

   With the increasing interest in utilizing syngas in gas turbine applications, the characterization of H2/CO reaction chemistry under high-pressure and moderate-temperature operating conditions has been the focus of recent investigations. Different chemical-kinetics and hydrodynamic processes have been identified as being responsible for the discrepancies between experimental measurements and kinetic predictions of syngas ignition delay times. This paper complements previous studies, and provides insight on the role of turbulence fluctuations and small-scale mixture inhomogeneities on the syngas combustion process. To this end, an analytic model has been developed that describes the turbulence amplification and combustion in the so-called adiabatic core region of a rapid compression machine (RCM). In this formulation, enhancement of turbulence and small-scale mixture fluctuations during the RCM compression phase are modeled using rapid distortion theory. The subsequent ignition and combustion processes are described using a Lagrangian Fokker-Planck model which considers turbulence/chemistry interaction and detailed reaction chemistry. The model was applied to different syngas mixtures and operating conditions, including pressures up to 20 atm and temperatures between 600 and 1300 K. Parametric studies showed that the model captures experimentally observed trends of reduced

5. Reconcile Discrepancies of Current Syngas Kinetics Models by Considering Turbulence Effects on Ignition Delay at Gas-Turbine Relevant Operating Conditions

   M. Ihme

   Deep Blue (University of Michigan), 2011

   Abs Open Access

   Peer Reviewed

6. Large-eddy simulation of a three-stream MILD combustion system

   J. Zhang, M. Ihme, and G. He

   Bulletin of the American Physical Society, 2011
7. Characterization of Mixing and Ignition Effects in Flow-Reactors Using a Particle Method

   S. Weiher, and M. Ihme

   Bulletin of the American Physical Society, 2011
8. Large Eddy Simulation Modeling and Flamelet Analysis of a Jet in Cross Flow

   W. L. Chan, H. Kolla, M. Ihme, and J. Chen

   Bulletin of the American Physical Society, 2011
9. Characterization of hydrodynamic instabilities in non- premixed flames by combining detailed simulations and linear stability analysis

   Y. C. See, and M. Ihme

   Bulletin of the American Physical Society, 2011
10. Detailed Simulations of Incident Shock Wave Development and Boundary Layer Growth in Shock Tube Facilities

    Y. Sun, M. Ihme, and R. Deiterding

    APS Division of Fluid Dynamics Meeting Abstracts, 2011

2010

1. Prediction of autoignition in a lifted methane/air flame using an unsteady flamelet/progress variable model

   M. Ihme, and Y. C. See

   Combustion and Flame, 2010

   DOI
2. LES flamelet modeling of a three-stream MILD combustor: Analysis of flame sensitivity to scalar inflow conditions

   M. Ihme, and Y. C. See

   Proceedings of the Combustion Institute, 2010

   DOI
3. Turbulence-Chemistry Interaction and Heat Transfer Modeling of H2/O2 Gaseous Injector Flows

   E. Sozer, E. A. Hassan, S. Yun, S. Thakur, J. Wright, M. Ihme, and W. Shyy

   48th AIAA Aerospace Sciences Meeting Including the New Horizons Forum and Aerospace Exposition, 2010

   Abs DOI

   Reliable prediction of rocket injector flows introduces significant challenges associated with the complex physics involving recirculation, turbulence, scalar mixing, chemical reactions and wall heat transfer. This work is aimed at assessing the importance of turbulence-chemistry interaction and non-equilibrium effects in experimentally characterized single and multi-element injector flows. By examining the different chemistry models (laminar finite rate, assumed PDF with either flamelet or equilibrium assumption), it was found that for both cases investigated, chemical non- equilibrium is insignificant while substantial turbulence-chemistry interaction is observed. A zonal wall treatment was developed based on a blend of SST low-Re turbulence wall treatment and law- of-the-wall, showing improved predictive capability. A heat flux extraction method was also proposed to estimate heat flux results from adiabatic flamelet model under the consideration that wall heat loss is small compared to the overall energy generated by chemical reactions.

4. Turbulence-Chemistry Interaction and Heat Transfer Modeling of H 2/O 2 Gaseous Injector Flows

   E. Sozer, E. A. Hassan, S. Yun, S. Thakur, J. A. Wright, M. Ihme, and W. Shyy

   Deep Blue (University of Michigan), 2010

   Abs Open Access

   Reliable prediction of rocket injector flows introduces significant challenges associated with the complex physics involving recirculation, turbulence, scalar mixing, chemical reactions and wall heat transfer. This work is aimed at assessing the importance of turbulence-chemistry interaction and non-equilibrium effects in experimentally characterized single and multi-element injector flows. By examining the different chemistry models (laminar finite rate, assumed PDF with either flamelet or equilibrium assumption), it was found that for both cases investigated, chemical non-equilibrium is insignificant while substantial turbulence-chemistry interaction is observed. A zonal wall treatment was developed based on a blend of SST low-Re turbulence wall treatment and law-of-the-wall, showing improved predictive capability. A heat flux extraction method was also proposed to estimate heat flux results from adiabatic flamelet model under the consideration that wall heat loss is small compared to the overall energy generated by chemical reactions. Copyright © 2010 by Wei Shyy.

5. Effects of turbulence on syngas ignition in rapid compression machines

   M. Ihme, and A. Soimakallio

   Bulletin of the American Physical Society, 2010
6. Characterization and Sensitivity Analysis of a Turbulent Diffusion Flame in Diluted Hot Coflow

   Y. C. See, and M. Ihme

   48th AIAA Aerospace Sciences Meeting Including the New Horizons Forum and Aerospace Exposition, 2010

   DOI
7. Characterization and Sensitivity Analysis of a Turbulent Diffusion Flame in Diluted Hot Coflow

   Y. C. See, and M. Ihme

   Deep Blue (University of Michigan), 2010

   Abs Open Access

   Peer Reviewed

8. Linear stability analysis of planar reacting shear layers

   Y. C. See, and M. Ihme

   APS Division of Fluid Dynamics Meeting Abstracts, 2010

2009

1. Large-Eddy Simulation of a Turbulent Lifted Flame in a Vitiated Co-Flow

   M. Ihme, and Y. C. See

   47th AIAA Aerospace Sciences Meeting including The New Horizons Forum and Aerospace Exposition, 2009

   DOI

2008

1. Modeling of radiation and nitric oxide formation in turbulent nonpremixed flames using a flamelet/progress variable formulation

   M. Ihme, and H. Pitsch

   Physics of Fluids, 2008

   Abs DOI

   A model for the prediction of the nitric oxide (NO) formation in turbulent nonpremixed flames is proposed. Since the NO formation has a strong temperature sensitivity, the accurate prediction of the flame temperature under the consideration of radiative heat losses is required. The first part of the paper addresses the extension of a flamelet-based combustion model to account for radiative heat loss effects by introducing enthalpy as an additional parameter. A transport equation for enthalpy is solved, and the radiative sink term in this equation is obtained from unsteady flamelet solutions. The model is applied to a large-eddy simulation (LES) of Sandia flame D, and the importance of the interaction between turbulence and radiation on temperature and mixture fraction is investigated. Based on the radiative flamelet formulation, a consistent model for the prediction of NO formation is developed in the second part of the paper. In this model, an additional transport equation for the NO mass fraction is solved, and the chemical source term is obtained from a flamelet library. Since the consumption rate is dependent on the NO mass fraction, this term requires modeling, which is discussed in this paper. By employing a scale similarity argument, a closure model for application in LES is presented. After the analysis of the proposed model for the thermal, nitrous oxide, and prompt pathways for NO formation, the NO model is integrated into the extended flamelet/progress variable model and applied in LES of Sandia flame D and a Pratt & Whitney aircraft engine combustor configuration.

2. Prediction of extinction and reignition in nonpremixed turbulent flames using a flamelet/progress variable model

   M. Ihme, and H. Pitsch

   Combustion and Flame, 2008

   DOI
3. Prediction of extinction and reignition in nonpremixed turbulent flames using a flamelet/progress variable model

   M. Ihme, and H. Pitsch

   Combustion and Flame, 2008

   DOI
4. Optimal artificial neural networks and tabulation methods for chemistry representation in LES of a bluff-body swirl-stabilized flame

   M. Ihme, C. Schmitt, and H. Pitsch

   Proceedings of the Combustion Institute, 2008

   DOI
5. Radiation of noise in turbulent non-premixed flames

   M. Ihme, H. Pitsch, and D. J. Bodony

   Proceedings of the Combustion Institute, 2008

   DOI
6. Large-eddy simulation of turbulent reacting flows

   H. Pitsch, O. Desjardins, G. Balarac, and M. Ihme

   Progress in Aerospace Sciences, 2008

   DOI
7. Large-Eddy Simulation of Turbulent Reacting Flows

   H. Pitsch, O. Desjardins, G. Balarac, and M. Ihme

   46th AIAA Aerospace Sciences Meeting and Exhibit, 2008

   Abs DOI

   In this paper, a few advances and challenges towards predictive large-eddy simulations of turbulent reacting flow are discussed in the context of numerical simulations of aircraft engines, which require an adequate description of liquid fuel injection, liquid fuel atomization, drop breakup, drop dynamics, and evaporation, large-scale turbulent fuel air mixing, small scale molecular fuel air mixing, chemical reactions, and turbulence/chemistry interactions. We have identified three of the most important and most challenging modeling problems in this process, namely primary atomization, sub-filter scalar mixing, and pollutant formation. Some recent progress on all three topics is presented.

8. Large-eddy simulation of turbulent reacting flows

   H. Pitsch, O. Desjardins, G. Balarac, and M. Ihme

   Deep Blue (University of Michigan), 2008

   Abs Open Access

   Peer Reviewed

9. Analysis of Different Sound Source Formulations to Simulate Combustion Generated Noise Using a Hybrid LES/APE-RF Method

   T. P. Bui, M. Ihme, W. Schröder, and H. Pitsch

   International Journal of Aeroacoustics, 2008

   Abs DOI

   Combustion noise and sound source mechanisms of the DLR-A flame are investigated. A hybrid large-eddy simulation/computational aeracoustics (LES/CAA) approach is employed. In the first step of the hybrid analysis the flamelet/progress variable (FPV) model is employed as combustion model followed by the acoustic simulation in the second step using the acoustic perturbation equations for reacting flows (APE-RF). The flamelet/progress variable database has been extended in terms of acoustic source terms. The analysis of the acoustic field of low MACH number reacting flows induced by the thermoacoustic sources such as the unsteady heat release leads to a very stiff problem formulation, since the related sources require highly resolved regions in the source area, which restricts the possible time step. To simulate combustion generated noise using such a hybrid approach, a suitable source description has to be used, which preferably satisfies two requirements, i.e, to efficiently and accurately predict the generated sound field, while the source term can be easily evaluated from the LES. Using the source term, which is expressed via the scaled partial time derivative of the density, the acoustic field can be reproduced best up to a maximum Strouhal number of St D = 2. However, this source formulation requires a rigorous constraint at the interface of the hybrid approach to avoid spurious noise due to artificial acceleration caused by interpolation. To be more precise, the convection speed of density inhomogeneities has to be preserved during interpolation. A compromise between efficiency and accuracy can be achieved using the source formulation expressed via the scaled material derivative of the density, since by definition this formulation does not describe the convection of density inhomogeneities.

10. Numerical Applicability of Different Sound Source Formulations to Compute Combustion Noise Using Acoustic Perturbation Equations for Reacting Flows

    T. P. Bui, M. Ihme, M. Meinke, W. Schröeder, and H. Pitsch

    14th AIAA/CEAS Aeroacoustics Conference (29th AIAA Aeroacoustics Conference), 2008

    Abs DOI

    Combustion noise analyses in a hybrid computational aeroacoustics (CAA) context are presented. Acoustic perturbation equations for reacting flows (APE-RF) describe the wave propagation, while unsteady results from a variable density incompressible reactive large-eddy simulation (LES) are used to evaluate the acoustic source terms. To simulate combustion generated noise via such a hybrid approach, an appropriate source description has to be taken, which preferably matches two requirements, i.e., on the one hand, to efficiently and accurately predict the generated acoustic field, and on the other hand, to easily determine the source term from the LES. In this study, three source formulations for the APE-RF system are examined on two acoustic meshes with different resolutions in the source region to predict the acoustic field of an open turbulent nonpremixed flame. Using the source term, which is expressed via the scaled partial time derivative of the density, the acoustic field can be reproduced best up to a maximum Strouhal number of StD = 2 on the fine mesh. However, for this source formulation spurious noise can be observed depending on the CAA resolution. It will be shown in this study that this observation can be related to the “artificial interpolation induced acceleration” effect. A compromise between efficiency and accuracy represents the source formulation expressed via the scaled material derivative of the density.

11. Numerical Applicability of Different Sound Source Formulations to Compute Combustion Noise Using Acoustic Perturbation Equations for Reacting Flows

    T. Bui, M. Ihme, M. Meinke, W. Schroeder, and H. Pitsch

    Deep Blue (University of Michigan), 2008

    Abs Open Access

    Peer Reviewed

12. High-frequency Analysis of Acoustic Refraction Effects in Turbulent Flames

    R. Reusser, and M. Ihme

    Bulletin of the American Physical Society, 2008
13. Pollutant Formation and Noise Emission in TurbulentDiffusion Flames

    M. Ihme

    2008

    Abs

    The accurate prediction of combustion processes in turbulent reacting flows is essential for the design of advanced combustion technologies. Numerical simulation techniques become viable tools to address such design challenges if adequate models are available. In this work, computational models are developed for application to large-eddy simulation (LES). In particular, three critical modeling aspects are considered, namely local flame extinction and reignition, nitrogen oxide formation, and noise-generation by combustion. These models are integrated into the flamelet/progress variable formulation, thus allowing for a consistent treatment of different combustion-physical effects in a unified framework. The computational models are analyzed in a priori studies and applied to LES for different flame configurations, including turbulent jet flames and a realistic aircraft engine combustor. All LES models are validated using experimental data, and the underlying physical phenomena are discussed in great detail.

2007

1. Generation of Optimal Artificial Neural Networks Using a Pattern Search Algorithm: Application to Approximation of Chemical Systems

   M. Ihme, A. L. Marsden, and H. Pitsch

   Neural Computation, 2007

   Abs DOI

   A pattern search optimization method is applied to the generation of optimal artificial neural networks (ANNs). Optimization is performed using a mixed variable extension to the generalized pattern search method. This method offers the advantage that categorical variables, such as neural transfer functions and nodal connectivities, can be used as parameters in optimization. When used together with a surrogate, the resulting algorithm is highly efficient for expensive objective functions. Results demonstrate the effectiveness of this method in optimizing an ANN for the number of neurons, the type of transfer function, and the connectivity among neurons. The optimization method is applied to a chemistry approximation of practical relevance. In this application, temperature and a chemical source term are approximated as functions of two independent parameters using optimal ANNs. Comparison of the performance of optimal ANNs with conventional tabulation methods demonstrates equivalent accuracy by considerable savings in memory storage. The architecture of the optimal ANN for the approximation of the chemical source term consists of a fully connected feedforward network having four nonlinear hidden layers and 117 synaptic weights. An equivalent representation of the chemical source term using tabulation techniques would require a 500 x 500 grid point discretization of the parameter space.

2. Numerical Investigation of Combustion Noise and Sound Source Mechanisms in a Non-Premixed Flame Using LES and APE-RF

   T. P. Bui, M. Ihme, M. Meinke, W. Schröeder, and H. Pitsch

   13th AIAA/CEAS Aeroacoustics Conference (28th AIAA Aeroacoustics Conference), 2007

   Abs DOI

   In this study, combustion noise and sound source mechanisms of an unconfined turbulent non-premixed flame, i.e., the DLR-A flame is investigated. A hybrid LES/CAA approach is thereby employed in which a low Mach number variable density large-eddy simulation (LES) is combined with the acoustic perturbation equations for reacting flows (APE-RF). In the first step of the hybrid analysis the flamelet/progress variable (FPV) model is employed as combustion model followed by the acoustic simulation in the second step using the acoustic perturbation equations for reacting flows (APE-RF). In the acoustic analysis, special emphasis is placed on the impact of the thermoacoustic source contributions within the pressure-density relation of the APE-RF system on the radiated acoustic field and the applicability of these source formulations in terms of a hybrid CFD/CAA approach. The flamelet/progress variable database has been extended in terms of acoustic source terms. The unsteady heat release rate, the source describing the effect of non-isomolar combustion, and the species diffusion term are described by two independent parameters, i.e., the mixture fraction and the progress variable. From the findings in the present study, the analysis of the acoustic field of low Mach number reacting flows induced by the thermoacoustic sources such as the unsteady heat release leads to a very stiff problem formulation, since the related sources require highly resolved regions in the source area, which restricts the possible time step during temporal integration of the equations. The numerical bottleneck is not so restrictive when a source term formulation based on the density distribution is used. Spectra obtained from the simulated acoustic field, using two different source term formulations involving derivatives of the density are in good agreement with the experimental data even in the higher frequency range.

3. Enhanced Hybrid LES/CAA Approach for Combustion-Induced Noise in Complex Geometries

   M. Kaltenbacher, M. Ihme, and H. Pitsch

   13th AIAA/CEAS Aeroacoustics Conference (28th AIAA Aeroacoustics Conference), 2007

   DOI
4. EFFECTS OF HEAT RELEASE ON TURBULENT JET FLOWS

   M. Ihme, and H. Pitsch

   Proceeding of Fifth International Symposium on Turbulence and Shear Flow Phenomena, 2007

   Abs DOI

   Large-eddy simulations of isothermal and reactive turbulent round jets at a jet Reynolds number of 14,720 have been performed to study the effects of heat release on jet dynamics and entrainment. Results for flow field statistics from the simulations are in good agreement with available experimental data. Effects of buoyancy and temperature-dependent molecular properties on the instantaneous flow field and statistical flow field quantities are investigated in separate numerical simulations. Mass flux and jet entrainment rates have been computed from the simulations. It is shown that the fluctuating mass flux, which is difficult to determine experimentally, amounts to as much as 15% of the total mass flux.

5. Artificial neural networks for chemistry representation. Part 1: Generation of optimal ANNs using a pattern search algorithm

   M. Ihme, C. Schmitt, and H. Pitsch

   Bulletin of the American Physical Society, 2007
6. Artificial neural networks for chemistry representation. Part 2: Application in LES of turbulent reacting flows

   C. Schmitt, M. Ihme, and H. Pitsch

   Bulletin of the American Physical Society, 2007

   Abs DOI

   The sensitizing properties of the fragrances eugenol and isoeugenol have been investigated experimentally. The potential of these materials to induce sensitization of the respiratory tract was examined using the mouse IgE test, a novel but as yet unvalidated method for the predictive identification of chemical respiratory allergens. Comparisons were made with the activity of eugenol and isoeugenol in two predictive tests for contact sensitization potential: the murine local lymph node assay and the guinea pig maximization test. Both chemicals elicited positive responses in these tests, isoeugenol exhibiting a greater potential for contact sensitization than eugenol. Isoeugenol was negative at all concentrations examined in the mouse IgE test. In the same assay, exposure to eugenol was associated with a statistically significant increase in serum IgE concentrations when initial application concentrations of 2.5% were used. However, at higher test concentrations eugenol was negative in the mouse IgE test. It is concluded that neither eugenol nor isoeugenol have a significant potential to cause sensitization of the respiratory tract, a conclusion that is apparently consistent with the lack of evidence for occupational respiratory allergy associated with exposure to these chemicals. The evidence for isoeugenol lacking respiratory sensitization activity is particularly strong and it is proposed that this chemical may be of value as a negative control in the development and validation of new predictive test methods for the identification of chemical respiratory allergens.

2006

1. Modeling of high-pressure mixing and combustion in liquid rocket injectors

   L. Cutroney, M. Ihme, and M. Herrmann

   2006
2. Prediction of combustion-generated noise in non-premixed turbulent jet flames using large-eddy simulation

   M. Ihme, D. J. Bodony, and H. Pitsch

   AIAA/CEAS Aeroacoustics Conference, 2006
3. Prediction of Combustion-Generated Noise in Non-Premixed Turbulent Jet Flames Using LES

   M. Ihme, D. J. Bodony, and H. Pitsch

   12th AIAA/CEAS Aeroacoustics Conference (27th AIAA Aeroacoustics Conference), 2006

   DOI
4. Numerical Prediction of Combustion-induced Noise using a hybrid LES/CAA approach

   M. Ihme, H. Pitsch, and M. Kaltenbacher

   APS Division of Fluid Dynamics Meeting Abstracts, 2006

2005

1. Prediction of local extinction and re-ignition effects in non-premixed turbulent combustion using a flamelet/progress variable approach

   M. Ihme, C. M. Cha, and H. Pitsch

   Proceedings of the Combustion Institute, 2005

   DOI
2. An Unsteady/Flamelet Progress Variable Method for LES of Nonpremixed Turbulent Combustion

   H. Pitsch, and M. Ihme

   43rd AIAA Aerospace Sciences Meeting and Exhibit, 2005

   Abs DOI

   An unsteady flamelet/progress variable model has been developed and formulated as an extension of the steady flamelet/progress variable model. For this model, a large number of unsteady laminar flamelet simulations is performed for various conditions, and solutions are recorded as function of time. From this, a flamelet library is generated, which provides the filtered quantities of all scalar values as function of the filtered mixture fraction, the mixture fraction sub-filter variance, the filtered reaction progress variable, and the filtered scalar dissipation rate. The model has been implemented in an LES code. Simulations have been performed for a confined swirl burner using the unsteady flamelet/progress variable model. The results are compared with experimental data for velocities and velocity fluctuations, temperature, CO2, and CO mole fractions. The results agree reasonably well with the experiments for all quantities. In particular, CO is predicted with good accuracy.

3. LES of a Non-Premixed Flame Using an Extended Flamelet/Progress Variable Model

   M. Ihme, and H. Pitsch

   43rd AIAA Aerospace Sciences Meeting and Exhibit, 2005

   Abs DOI

   In previous a priori studies we have examined the flamelet/progress variable model proposed by Pierce and Moin and identified the most important areas for model improvements. The flamelet/progress variable approach is a conserved scalar based model for non-premixed combustion and uses a reactive scalar as an additional parameter in a table lookup. One of the main assumptions in this model is the use of a delta function as a presumed probability density function (PDF) for this reactive scalar. We have previously shown that the use of the beta function provides some improvement. Here, the statistically most-likely distribution is proposed as a presumed PDF for the flamelet parameter. This provides two advantages. First of all, the function depends on a chemical time-scale, and in addition, an arbitrary number of moments can be enforced. The model is applied to flames D and E of the Sandia piloted turbulent jet flame series and results are compared with experimental data.

4. Stochastic mixing model with power law decay of variance

   S. Fedotov, M. Ihme, and H. Pitsch

   Physical Review E, 2005

   Abs DOI Open Access

   A stochastic mixing model based on the law of large numbers is presented that describes the decay of the variance of a conserved scalar in decaying turbulence as a power law, sigma2(c) proportional t(-alpha). A general Lagrangian mixing process is modeled by a stochastic difference equation where the mixing frequency and the ambient concentration are random processes. The mixing parameter lambda is introduced as a coefficient in the mixing frequency in order to account for initial length-scale ratio of the velocity and scalar field and other physical dependencies. We derive a nonlinear integral equation for the probability density function (pdf) of a conserved scalar that describes the relaxation of an arbitrary initial distribution to a delta-function. Numerical studies of this equation are conducted, and it is shown that lambda has a distinct influence on the decay rate of the scalar. Results obtained from the model for the evolution of the pdf are in a good agreement with direct numerical simulation (DNS) data.

5. Numerical Prediction of Nitrogen Oxide Emission Using Flamelet/Progress Variable Model

   M. Ihme, and H. Pitsch

   Bulletin of the American Physical Society, 2005

2004

1. An Extended Flamelet/Progress Variable Method for LES of Nonpremixed Turbulent Combustion

   M. Ihme, and H. Pitsch

   APS Division of Fluid Dynamics Meeting Abstracts, 2004

2003

1. Stochastic Mixing Model with Power Law Decay of Variance

   S. Fedotov, M. Ihme, and H. Pitsch

   Defense Technical Information Center (DTIC), 2003

   Abs

   The mixing of a conserved scalar c = c(t, x), advected by a turbulent flow, remains a problem of both fundamental and practical interest. One of the basic characteristics of the mixing process is the rate at which the scalar variance sigma(exp 2)(sub c)(t) = ((c - mu)(exp 2) decays with time. Here mu is the mean value, and the angular brackets (.) denote an averaging procedure. One of the simplest and widely used mixing models is the interaction by exchange with the mean (IEM) (Villermaux & Devillon 1972) or linear mean square estimate model (LMSE) (Dopazo & O’Brien 1974, Sabel’nikov & Gorokhovski 2001). In this model, the scalar relaxes toward its mean mu according to the equation.

2. A Priory Analysis of a Flamelet/Progress-Variable Approach

   M. Ihme, and H. Pitsch

   APS Division of Fluid Dynamics Meeting Abstracts, 2003