Jacqueline Chen - Profile on Academia.edu (original) (raw)
Papers by Jacqueline Chen
Next generation architectures necessitate a shift away from traditional workflows in which the si... more Next generation architectures necessitate a shift away from traditional workflows in which the simulation state is saved at prescribed frequencies for post-processing analysis. While the need to shift to in situ workflows has been acknowledged for some time, much of the current research is focused on static workflows, where the analysis that would have been done as a post-process is performed concurrently with the simulation at user-prescribed frequencies. Recently, research efforts are striving to enable adaptive workflows, in which the frequency, composition, and execution of computational and data manipulation steps dynamically depend on the state of the simulation. Adapting the workflow to the state of simulation in such a data-driven fashion puts extremely strict efficiency requirements on the analysis capabilities that are used to identify the transitions in the workflow. In this paper we build upon earlier work on trigger detection using sublinear techniques to drive adaptive workflows. Here we propose a methodology to detect the time when sudden heat release occurs in simulations of turbulent combustion. Our proposed method provides an alternative metric that can be used along with our former metric to increase the robustness of trigger detection. We show the effectiveness of our metric empirically for predicting heat release for two use cases.
KOSCO SYMPOSIUM 논문집, Jul 21, 2015
Journal of Fluid Mechanics, Aug 29, 2012
arXiv (Cornell University), Jul 25, 2022
In general, large datasets enable deep learning models to perform with good accuracy and generali... more 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 , for scientific machine learning.
Combustion Science and Technology, Jun 27, 2016
Dissipation spectra of velocity and reactive scalars-temperature and fuel mass fraction-in turbul... more Dissipation spectra of velocity and reactive scalars-temperature and fuel mass fraction-in turbulent premixed flames are studied using direct numerical simulation data of a temporally evolving lean hydrogen-air premixed planar jet (PTJ) flame and a statistically stationary planar lean methane-air (SP) flame. The equivalence ratio in both cases was 0.7, the pressure 1 atm while the unburned temperature was 700 K for the hydrogen-air PTJ case and 300 K for methane-air SP case, resulting in data sets with a density ratio of 3 and 5, respectively. The turbulent Reynolds numbers for the cases ranged from 200 to 428.4, the Damköhler number from 3.1 to 29.1, and the Karlovitz number from 0.1 to 4.5. The dissipation spectra collapse when normalized by the respective Favre-averaged dissipation rates. However, the normalized dissipation spectra in all the cases deviate noticeably from those predicted by classical scaling laws for constantdensity turbulent flows and bear a clear influence of the chemical reactions on the dissipative range of the energy cascade.
Journal of Computational Physics
Next-generation exascale machines with extreme levels of parallelism will provide massive computi... more Next-generation exascale machines with extreme levels of parallelism will provide massive computing resources for large scale numerical simulations of complex physical systems at unprecedented parameter ranges. However, novel numerical methods, scalable algorithms and re-design of current state-of-the art numerical solvers are required for scaling to these machines with minimal overheads. One such approach for partial differential equations based solvers involves computation of spatial derivatives with possibly delayed or asynchronous data using high-order asynchrony-tolerant (AT) schemes to facilitate mitigation of communication and synchronization bottlenecks without affecting the numerical accuracy. In the present study, an effective methodology of implementing temporal discretization using a multi-stage Runge-Kutta method with AT schemes is presented. Together these schemes are used to perform asynchronous simulations of canonical reacting flow problems, demonstrated in one-dimension including auto-ignition of a premixture, premixed flame propagation and non-premixed autoignition. Simulation results show that the AT schemes incur very small numerical errors in all key quantities of interest including stiff intermediate species despite delayed data at processing element (PE) boundaries. For simulations of supersonic flows, the degraded numerical accuracy of well-known shock-resolving WENO (weighted essentially non-oscillatory) schemes when used with relaxed synchronization is also discussed. To overcome this loss of accuracy, high-order AT-WENO schemes are derived and tested on linear and non-linear equations. Finally the novel AT-WENO schemes are demonstrated in the propagation of a detonation wave with delays at PE boundaries.
IEEE Transactions on Visualization and Computer Graphics
Fig. . Left to right illustrates steps of our methodology. We decompose volume data by novel geod... more Fig. . Left to right illustrates steps of our methodology. We decompose volume data by novel geodesically based discrete centroidal Voronoi tessellations restricted by sequences of level sets. Then we aggregate statics within the resulting hierarchically structured segments. Finally, the data is reorganized according the hierarchical structure, and used for interactive spatial statistical analysis of large data at a reasonable cost.
The characteristics of turbulent lifted non-premixed hydrogen jet flames under various coflow con... more The characteristics of turbulent lifted non-premixed hydrogen jet flames under various coflow conditions have widely been investigated due to their relevance to practical applications. Three 3-D direct numerical simulations of turbulent lifted hydrogen/air jet flames in heated coflows near auto-ignition limit are performed to examine the stabilization mechanisms and flame structure of turbulent lifted jet flames. Chemical explosive mode analysis (CEMA) reveals the important variables and reactions for stabilizing the lifted flames.
Combustion and Flame, 2017
Transported probability density function (TPDF) methods are an attractive modeling approach for t... more Transported probability density function (TPDF) methods are an attractive modeling approach for turbulent flames as chemical reactions appear in closed form. However, molecular micro-mixing needs to be modeled and this modeling is considered a primary challenge for TPDF methods. In the present study, a new algebraic mixing rate model for TPDF simulations of turbulent premixed flames is proposed, which is a key ingredient in commonly used molecular mixing models. The new model aims to properly account for the transition in reactive scalar mixing rate behavior from the limit of turbulence-dominated mixing to molecular mixing behavior in flamelets. An a priori assessment of the new model is performed using direct numerical simulation (DNS) data of a lean premixed hydrogen-air jet flame. The new model accurately captures the mixing timescale behavior in the DNS and is found to be a significant improvement over the commonly used constant mechanical-to-scalar mixing timescale ratio model. An a posteriori TPDF study is then performed using the same DNS data as a numerical test bed. The DNS provides the initial conditions and time-varying input quantities, including the mean velocity, turbulent diffusion coefficient, and modeled scalar mixing rate for the TPDF simulations, thus allowing an exclusive focus on the mixing model. The new mixing timescale model is compared with the constant mechanical-to-scalar mixing timescale ratio coupled with the Euclidean Minimum Spanning Tree (EMST) mixing model, as well as a laminar flamelet closure by Pope and Anand (S.B. Pope, M.S. Anand, Proc. Combust. Inst. 20 (1984) 403-410). It is found that the laminar flamelet closure is unable to properly capture the mixing behavior in the thin reaction zones regime while the constant mechanical-to-scalar mixing timescale model under-predicts the flame speed. The EMST model coupled with the new mixing timescale model provides the best prediction of the flame structure and flame propagation among the models tested, as the dynamics of reactive scalar mixing across different flame regimes are appropriately accounted for.
Proceedings of the Combustion Institute, 2015
Direct numerical simulations of freely-propagating premixed flames in the turbulent boundary laye... more Direct numerical simulations of freely-propagating premixed flames in the turbulent boundary layer of fully-developed turbulent channel flows are used for a priori validation of a new model that aims to describe the mean shape of the turbulent flame brush during flashback. Comparison with the DNS datasets, for both fuel-lean and fuel-rich mixture conditions and for Damköhler numbers lower and larger than unity, shows that the model is able to capture the main features of the flame shape. Although further a priori and a posteriori validation is required, particularly at higher Reynolds numbers, this new simple model seems promising and can potentially have impact on the design process of industrial combustion equipment.
Combustion and Flame, 2015
This paper presents the results of DNS of a partially premixed turbulent syngas/air flame at atmo... more This paper presents the results of DNS of a partially premixed turbulent syngas/air flame at atmospheric pressure. The objective was to assess the importance and possible effects of molecular transport on flame behavior and structure. To this purpose DNS were performed at with two proprietary DNS codes and with three different molecular diffusion transport models: fully multi-component, mixture averaged, and imposing the Lewis number of all species to be unity. 1. At the Reynolds numbers of the simulations (Re turb = 600, Re = 8000) choice of molecular diffusion models affects significantly the temperature and concentration fields; 2. Assuming Le = 1 for all species predicts temperatures up to 250 K higher than the physically realistic multi-component model; 3. Faster molecular transport of lighter species changes the local concentration field and affects reaction pathways and chemical kinetics. A possible explanation for these observations is provided in terms of species diffusion velocity that is a strong function of gradients: thus, at sufficiently large Reynolds numbers, gradients and their effects tend to be large. The preliminary conclusion from these simulations seems to indicate molecular diffusion as the third important mechanism active in flames besides convective transport and kinetics. If confirmed by further DNS and measurements, molecular transport in high intensity turbulent flames will have to be realistically modeled to accurately predict emissions (gaseous and particulates) and other combustor performance metrics.
Proceedings of the 24th International Symposium on High-Performance Parallel and Distributed Computing, 2015
How do we recover after a Failure? • Current FT approach Coordinated PFS-based Checkpointing On f... more How do we recover after a Failure? • Current FT approach Coordinated PFS-based Checkpointing On failure, stop application and Restart Unfeasible at exascale! • Online recovery can dramatically reduce failure overhead • Global recovery involves all the cores in the recovery process -This can be done in a semi-transparent way, but... -Scalability issues! • Local recovery can further benefit certain classes of applications "Exploring Failure Recovery for Stencil-based Applications at Extreme Scales
The scalar mixing time scale, a key quantity in many turbulent combustion models, is investigated... more The scalar mixing time scale, a key quantity in many turbulent combustion models, is investigated for reactive scalars in premixed combustion. Direct numerical simulations of threedimensional, turbulent Bunsen flames with reduced methane-air chemistry have been analyzed in the thin reaction zones regime. Previous conclusions from single step chemistry studies are confirmed regarding the role of dilatation and turbulence-chemistry interactions on the progress variable dissipation rate. Compared to the progress variable, the mixing rates of intermediate species can be several times greater. The variation of species mixing rates are explained with reference to the structure of one-dimensional flamelets. According to this analysis, mixing rates are governed by the strong gradients which are imposed by flamelet structures at high Damköhler numbers. This suggests a modeling approach to estimate the mixing rate of individual species which can be applied, for example, in transported probability density function simulations. Flame turbulence interactions which modify the flamelet based representation are analyzed.
The influence of water vapor (H 2 O) and nitric oxide (NO) on the extinction and re-ignition of a... more The influence of water vapor (H 2 O) and nitric oxide (NO) on the extinction and re-ignition of a vortex-perturbed nonpremixed hydrogen-air flame is investigated. A steady nonpremixed flame is established in an axisymmetric counterflow configuration with a fuel stream of N 2 -diluted H 2 flowing against heated air containing small amounts of H 2 O and NO. Local flame extinction is induced by a fuel-side vortex, and the temporal evolution of the hydroxyl radical (OH) is measured during the extinction and re-ignition processes using planar laser-induced fluorescence (PLIF). It is well known that H 2 O behaves as an inhibitor in combustion due to its high specific heat and that NO can significantly enhance the ignition of hydrogen due to its catalytic effect. In the present study, we investigate the sensitivity of extinction and re-ignition processes to mixtures of H 2 O and NO. Direct numerical simulations are performed using a detailed H 2 -air mechanism and are compared with experiments.
Direct numerical simulations (DNSs) of the near field of threedimensional spatially-developing tu... more Direct numerical simulations (DNSs) of the near field of threedimensional spatially-developing turbulent hydrogen jet flames in heated coflows at the two intermediate temperatures of 750 and 950 K were performed with a detailed mechanism to elucidate the characteristics of the flame structures and to determine the stabilization mechanism. The DNSs were performed at a jet Reynolds number of 8,000 with 1.28 billion grid points. The results show that relatively-constant low flame speed stabilizes the lifted flame in the coflow of 750 K such that the oscillation of the flamebase is mainly attributed to the passage of large-scale flow structures of the fuel jet. However, for 950 K coflow case high flame speed in hot fuel-lean mixture at the flame base is the main source of the stabilization of the lifted jet flame. Chemical explosive mode analysis (CEMA) and Lagrangian tracking of the flamebase reveal the stabilization mechanisms of the two turbulent jet flames. details of the grid system and turbulence injection methods.
Proceedings of the Combustion Institute, 2015
This paper presents a conditional moment closure (CMC) model for ignition of a lean ethanol/air m... more This paper presents a conditional moment closure (CMC) model for ignition of a lean ethanol/air mixture under homogeneous charge compression ignition (HCCI) conditions. A set of direct numerical simulations (DNSs) presented by Bhagatwala et al. (2014) is used to evaluate the performance of the CMC model. The DNS data includes five cases with a mean temperature of 924 K and three different levels of thermal stratification. The effect of compression heating and expansion cooling is considered in the first three cases with T ′ = 15, 25 and 40 K. For this purpose, an inert mass source term is added to the governing equations. However, the other two cases with T ′ = 15 and 40 K do not consider compression heating and expansion cooling. The results show a better agreement between the CMC and DNS for cases in which compression heating and expansion cooling is considered. Further investigation of the DNS data shows that the contribution of the diffusion term in the CMC equations, representing the importance of deflagration mode, is only significant for the case which has the largest level of thermal stratification and does not involve compression heating and expansion cooling.
Combustion and Flame, 2012
Differential diffusion alters the balance of reaction and diffusion in turbulent premixed combust... more Differential diffusion alters the balance of reaction and diffusion in turbulent premixed combustion, affecting the performance and emissions of combustion devices. Modelling combustion devices with Probability or Filtered Density Function (PDF or FDF) methods provides an exact treatment for the change in composition due to chemical reaction, while molecular mixing has to be modelled. Previous PDF molecular mixing models do not account for differential diffusion in a manner which satisfies realizability requirements. A new approach for treating differential diffusion, which ensures realizability, is proposed for pairwise-exchange mixing models in general, and applied in the Interaction by Exchange with the Mean (IEM) model of Dopazo , and in the Euclidean Minimum Spanning Tree (EMST) model of Subramaniam and Pope . The new differential diffusion models are referred to as IEM-DD and EMST-DD respectively. Results from two and three-dimensional DNS of turbulent premixed methane-air combustion show that mixing rates and conditional statistics of species mass fractions depend on species diffusivities and the combustion regime. Zero-dimensional PDF model results obtained for the two-dimensional DNS case show that the EMST-DD model best reproduces the features that characterize differential diffusion in the DNS. The essential feature of the EMST-DD model, which accounts for its success in turbulent premixed combustion, is that differential mixing rates are imposed within a model which mixes locally in composition space.
Combustion and Flame, 2010
Reactive scalar mixing time scale have been investigated in direct numerical simulation data for ... more Reactive scalar mixing time scale have been investigated in direct numerical simulation data for turbulent premixed Bunsen flames with reduced methane-air chemistry. Previous conclusions from single step chemistry studies are confirmed regarding the role of dilatation and turbulence-chemistry interactions on the progress variable dissipation rate. Compared to the progress variable, the mixing rates of intermediate species can be several times greater. The variation of species mixing rates are explained with reference to the structure of one-dimensional flamelets. A new model is produced for the ratios of scalar mixing time scales which can be applied, for example, in transported probability density function simulations.
Combustion and Flame, 2006
The influence of thermal stratification on autoignition at constant volume and high pressure is s... more The influence of thermal stratification on autoignition at constant volume and high pressure is studied by direct numerical simulation (DNS) with detailed hydrogen/air chemistry with a view to providing better understanding and modeling of combustion processes in homogeneous charge compression-ignition engines. Numerical diagnostics are developed to analyze the mode of combustion and the dependence of overall ignition progress on initial mixture conditions. The roles of dissipation of heat and mass are divided conceptually into transport within ignition fronts and passive scalar dissipation, which modifies the statistics of the preignition temperature field. Transport within ignition fronts is analyzed by monitoring the propagation speed of ignition fronts using the displacement speed of a scalar that tracks the location of maximum heat release rate. The prevalence of deflagrative versus spontaneous ignition front propagation is found to depend on the local temperature gradient, and may be identified by the ratio of the instantaneous front speed to the laminar deflagration speed. The significance of passive scalar mixing is examined using a mixing timescale based on enthalpy fluctuations. Finally, the predictions of the multizone modeling strategy are compared with the DNS, and the results are explained using the diagnostics developed.
Next generation architectures necessitate a shift away from traditional workflows in which the si... more Next generation architectures necessitate a shift away from traditional workflows in which the simulation state is saved at prescribed frequencies for post-processing analysis. While the need to shift to in situ workflows has been acknowledged for some time, much of the current research is focused on static workflows, where the analysis that would have been done as a post-process is performed concurrently with the simulation at user-prescribed frequencies. Recently, research efforts are striving to enable adaptive workflows, in which the frequency, composition, and execution of computational and data manipulation steps dynamically depend on the state of the simulation. Adapting the workflow to the state of simulation in such a data-driven fashion puts extremely strict efficiency requirements on the analysis capabilities that are used to identify the transitions in the workflow. In this paper we build upon earlier work on trigger detection using sublinear techniques to drive adaptive workflows. Here we propose a methodology to detect the time when sudden heat release occurs in simulations of turbulent combustion. Our proposed method provides an alternative metric that can be used along with our former metric to increase the robustness of trigger detection. We show the effectiveness of our metric empirically for predicting heat release for two use cases.
KOSCO SYMPOSIUM 논문집, Jul 21, 2015
Journal of Fluid Mechanics, Aug 29, 2012
arXiv (Cornell University), Jul 25, 2022
In general, large datasets enable deep learning models to perform with good accuracy and generali... more 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 , for scientific machine learning.
Combustion Science and Technology, Jun 27, 2016
Dissipation spectra of velocity and reactive scalars-temperature and fuel mass fraction-in turbul... more Dissipation spectra of velocity and reactive scalars-temperature and fuel mass fraction-in turbulent premixed flames are studied using direct numerical simulation data of a temporally evolving lean hydrogen-air premixed planar jet (PTJ) flame and a statistically stationary planar lean methane-air (SP) flame. The equivalence ratio in both cases was 0.7, the pressure 1 atm while the unburned temperature was 700 K for the hydrogen-air PTJ case and 300 K for methane-air SP case, resulting in data sets with a density ratio of 3 and 5, respectively. The turbulent Reynolds numbers for the cases ranged from 200 to 428.4, the Damköhler number from 3.1 to 29.1, and the Karlovitz number from 0.1 to 4.5. The dissipation spectra collapse when normalized by the respective Favre-averaged dissipation rates. However, the normalized dissipation spectra in all the cases deviate noticeably from those predicted by classical scaling laws for constantdensity turbulent flows and bear a clear influence of the chemical reactions on the dissipative range of the energy cascade.
Journal of Computational Physics
Next-generation exascale machines with extreme levels of parallelism will provide massive computi... more Next-generation exascale machines with extreme levels of parallelism will provide massive computing resources for large scale numerical simulations of complex physical systems at unprecedented parameter ranges. However, novel numerical methods, scalable algorithms and re-design of current state-of-the art numerical solvers are required for scaling to these machines with minimal overheads. One such approach for partial differential equations based solvers involves computation of spatial derivatives with possibly delayed or asynchronous data using high-order asynchrony-tolerant (AT) schemes to facilitate mitigation of communication and synchronization bottlenecks without affecting the numerical accuracy. In the present study, an effective methodology of implementing temporal discretization using a multi-stage Runge-Kutta method with AT schemes is presented. Together these schemes are used to perform asynchronous simulations of canonical reacting flow problems, demonstrated in one-dimension including auto-ignition of a premixture, premixed flame propagation and non-premixed autoignition. Simulation results show that the AT schemes incur very small numerical errors in all key quantities of interest including stiff intermediate species despite delayed data at processing element (PE) boundaries. For simulations of supersonic flows, the degraded numerical accuracy of well-known shock-resolving WENO (weighted essentially non-oscillatory) schemes when used with relaxed synchronization is also discussed. To overcome this loss of accuracy, high-order AT-WENO schemes are derived and tested on linear and non-linear equations. Finally the novel AT-WENO schemes are demonstrated in the propagation of a detonation wave with delays at PE boundaries.
IEEE Transactions on Visualization and Computer Graphics
Fig. . Left to right illustrates steps of our methodology. We decompose volume data by novel geod... more Fig. . Left to right illustrates steps of our methodology. We decompose volume data by novel geodesically based discrete centroidal Voronoi tessellations restricted by sequences of level sets. Then we aggregate statics within the resulting hierarchically structured segments. Finally, the data is reorganized according the hierarchical structure, and used for interactive spatial statistical analysis of large data at a reasonable cost.
The characteristics of turbulent lifted non-premixed hydrogen jet flames under various coflow con... more The characteristics of turbulent lifted non-premixed hydrogen jet flames under various coflow conditions have widely been investigated due to their relevance to practical applications. Three 3-D direct numerical simulations of turbulent lifted hydrogen/air jet flames in heated coflows near auto-ignition limit are performed to examine the stabilization mechanisms and flame structure of turbulent lifted jet flames. Chemical explosive mode analysis (CEMA) reveals the important variables and reactions for stabilizing the lifted flames.
Combustion and Flame, 2017
Transported probability density function (TPDF) methods are an attractive modeling approach for t... more Transported probability density function (TPDF) methods are an attractive modeling approach for turbulent flames as chemical reactions appear in closed form. However, molecular micro-mixing needs to be modeled and this modeling is considered a primary challenge for TPDF methods. In the present study, a new algebraic mixing rate model for TPDF simulations of turbulent premixed flames is proposed, which is a key ingredient in commonly used molecular mixing models. The new model aims to properly account for the transition in reactive scalar mixing rate behavior from the limit of turbulence-dominated mixing to molecular mixing behavior in flamelets. An a priori assessment of the new model is performed using direct numerical simulation (DNS) data of a lean premixed hydrogen-air jet flame. The new model accurately captures the mixing timescale behavior in the DNS and is found to be a significant improvement over the commonly used constant mechanical-to-scalar mixing timescale ratio model. An a posteriori TPDF study is then performed using the same DNS data as a numerical test bed. The DNS provides the initial conditions and time-varying input quantities, including the mean velocity, turbulent diffusion coefficient, and modeled scalar mixing rate for the TPDF simulations, thus allowing an exclusive focus on the mixing model. The new mixing timescale model is compared with the constant mechanical-to-scalar mixing timescale ratio coupled with the Euclidean Minimum Spanning Tree (EMST) mixing model, as well as a laminar flamelet closure by Pope and Anand (S.B. Pope, M.S. Anand, Proc. Combust. Inst. 20 (1984) 403-410). It is found that the laminar flamelet closure is unable to properly capture the mixing behavior in the thin reaction zones regime while the constant mechanical-to-scalar mixing timescale model under-predicts the flame speed. The EMST model coupled with the new mixing timescale model provides the best prediction of the flame structure and flame propagation among the models tested, as the dynamics of reactive scalar mixing across different flame regimes are appropriately accounted for.
Proceedings of the Combustion Institute, 2015
Direct numerical simulations of freely-propagating premixed flames in the turbulent boundary laye... more Direct numerical simulations of freely-propagating premixed flames in the turbulent boundary layer of fully-developed turbulent channel flows are used for a priori validation of a new model that aims to describe the mean shape of the turbulent flame brush during flashback. Comparison with the DNS datasets, for both fuel-lean and fuel-rich mixture conditions and for Damköhler numbers lower and larger than unity, shows that the model is able to capture the main features of the flame shape. Although further a priori and a posteriori validation is required, particularly at higher Reynolds numbers, this new simple model seems promising and can potentially have impact on the design process of industrial combustion equipment.
Combustion and Flame, 2015
This paper presents the results of DNS of a partially premixed turbulent syngas/air flame at atmo... more This paper presents the results of DNS of a partially premixed turbulent syngas/air flame at atmospheric pressure. The objective was to assess the importance and possible effects of molecular transport on flame behavior and structure. To this purpose DNS were performed at with two proprietary DNS codes and with three different molecular diffusion transport models: fully multi-component, mixture averaged, and imposing the Lewis number of all species to be unity. 1. At the Reynolds numbers of the simulations (Re turb = 600, Re = 8000) choice of molecular diffusion models affects significantly the temperature and concentration fields; 2. Assuming Le = 1 for all species predicts temperatures up to 250 K higher than the physically realistic multi-component model; 3. Faster molecular transport of lighter species changes the local concentration field and affects reaction pathways and chemical kinetics. A possible explanation for these observations is provided in terms of species diffusion velocity that is a strong function of gradients: thus, at sufficiently large Reynolds numbers, gradients and their effects tend to be large. The preliminary conclusion from these simulations seems to indicate molecular diffusion as the third important mechanism active in flames besides convective transport and kinetics. If confirmed by further DNS and measurements, molecular transport in high intensity turbulent flames will have to be realistically modeled to accurately predict emissions (gaseous and particulates) and other combustor performance metrics.
Proceedings of the 24th International Symposium on High-Performance Parallel and Distributed Computing, 2015
How do we recover after a Failure? • Current FT approach Coordinated PFS-based Checkpointing On f... more How do we recover after a Failure? • Current FT approach Coordinated PFS-based Checkpointing On failure, stop application and Restart Unfeasible at exascale! • Online recovery can dramatically reduce failure overhead • Global recovery involves all the cores in the recovery process -This can be done in a semi-transparent way, but... -Scalability issues! • Local recovery can further benefit certain classes of applications "Exploring Failure Recovery for Stencil-based Applications at Extreme Scales
The scalar mixing time scale, a key quantity in many turbulent combustion models, is investigated... more The scalar mixing time scale, a key quantity in many turbulent combustion models, is investigated for reactive scalars in premixed combustion. Direct numerical simulations of threedimensional, turbulent Bunsen flames with reduced methane-air chemistry have been analyzed in the thin reaction zones regime. Previous conclusions from single step chemistry studies are confirmed regarding the role of dilatation and turbulence-chemistry interactions on the progress variable dissipation rate. Compared to the progress variable, the mixing rates of intermediate species can be several times greater. The variation of species mixing rates are explained with reference to the structure of one-dimensional flamelets. According to this analysis, mixing rates are governed by the strong gradients which are imposed by flamelet structures at high Damköhler numbers. This suggests a modeling approach to estimate the mixing rate of individual species which can be applied, for example, in transported probability density function simulations. Flame turbulence interactions which modify the flamelet based representation are analyzed.
The influence of water vapor (H 2 O) and nitric oxide (NO) on the extinction and re-ignition of a... more The influence of water vapor (H 2 O) and nitric oxide (NO) on the extinction and re-ignition of a vortex-perturbed nonpremixed hydrogen-air flame is investigated. A steady nonpremixed flame is established in an axisymmetric counterflow configuration with a fuel stream of N 2 -diluted H 2 flowing against heated air containing small amounts of H 2 O and NO. Local flame extinction is induced by a fuel-side vortex, and the temporal evolution of the hydroxyl radical (OH) is measured during the extinction and re-ignition processes using planar laser-induced fluorescence (PLIF). It is well known that H 2 O behaves as an inhibitor in combustion due to its high specific heat and that NO can significantly enhance the ignition of hydrogen due to its catalytic effect. In the present study, we investigate the sensitivity of extinction and re-ignition processes to mixtures of H 2 O and NO. Direct numerical simulations are performed using a detailed H 2 -air mechanism and are compared with experiments.
Direct numerical simulations (DNSs) of the near field of threedimensional spatially-developing tu... more Direct numerical simulations (DNSs) of the near field of threedimensional spatially-developing turbulent hydrogen jet flames in heated coflows at the two intermediate temperatures of 750 and 950 K were performed with a detailed mechanism to elucidate the characteristics of the flame structures and to determine the stabilization mechanism. The DNSs were performed at a jet Reynolds number of 8,000 with 1.28 billion grid points. The results show that relatively-constant low flame speed stabilizes the lifted flame in the coflow of 750 K such that the oscillation of the flamebase is mainly attributed to the passage of large-scale flow structures of the fuel jet. However, for 950 K coflow case high flame speed in hot fuel-lean mixture at the flame base is the main source of the stabilization of the lifted jet flame. Chemical explosive mode analysis (CEMA) and Lagrangian tracking of the flamebase reveal the stabilization mechanisms of the two turbulent jet flames. details of the grid system and turbulence injection methods.
Proceedings of the Combustion Institute, 2015
This paper presents a conditional moment closure (CMC) model for ignition of a lean ethanol/air m... more This paper presents a conditional moment closure (CMC) model for ignition of a lean ethanol/air mixture under homogeneous charge compression ignition (HCCI) conditions. A set of direct numerical simulations (DNSs) presented by Bhagatwala et al. (2014) is used to evaluate the performance of the CMC model. The DNS data includes five cases with a mean temperature of 924 K and three different levels of thermal stratification. The effect of compression heating and expansion cooling is considered in the first three cases with T ′ = 15, 25 and 40 K. For this purpose, an inert mass source term is added to the governing equations. However, the other two cases with T ′ = 15 and 40 K do not consider compression heating and expansion cooling. The results show a better agreement between the CMC and DNS for cases in which compression heating and expansion cooling is considered. Further investigation of the DNS data shows that the contribution of the diffusion term in the CMC equations, representing the importance of deflagration mode, is only significant for the case which has the largest level of thermal stratification and does not involve compression heating and expansion cooling.
Combustion and Flame, 2012
Differential diffusion alters the balance of reaction and diffusion in turbulent premixed combust... more Differential diffusion alters the balance of reaction and diffusion in turbulent premixed combustion, affecting the performance and emissions of combustion devices. Modelling combustion devices with Probability or Filtered Density Function (PDF or FDF) methods provides an exact treatment for the change in composition due to chemical reaction, while molecular mixing has to be modelled. Previous PDF molecular mixing models do not account for differential diffusion in a manner which satisfies realizability requirements. A new approach for treating differential diffusion, which ensures realizability, is proposed for pairwise-exchange mixing models in general, and applied in the Interaction by Exchange with the Mean (IEM) model of Dopazo , and in the Euclidean Minimum Spanning Tree (EMST) model of Subramaniam and Pope . The new differential diffusion models are referred to as IEM-DD and EMST-DD respectively. Results from two and three-dimensional DNS of turbulent premixed methane-air combustion show that mixing rates and conditional statistics of species mass fractions depend on species diffusivities and the combustion regime. Zero-dimensional PDF model results obtained for the two-dimensional DNS case show that the EMST-DD model best reproduces the features that characterize differential diffusion in the DNS. The essential feature of the EMST-DD model, which accounts for its success in turbulent premixed combustion, is that differential mixing rates are imposed within a model which mixes locally in composition space.
Combustion and Flame, 2010
Reactive scalar mixing time scale have been investigated in direct numerical simulation data for ... more Reactive scalar mixing time scale have been investigated in direct numerical simulation data for turbulent premixed Bunsen flames with reduced methane-air chemistry. Previous conclusions from single step chemistry studies are confirmed regarding the role of dilatation and turbulence-chemistry interactions on the progress variable dissipation rate. Compared to the progress variable, the mixing rates of intermediate species can be several times greater. The variation of species mixing rates are explained with reference to the structure of one-dimensional flamelets. A new model is produced for the ratios of scalar mixing time scales which can be applied, for example, in transported probability density function simulations.
Combustion and Flame, 2006
The influence of thermal stratification on autoignition at constant volume and high pressure is s... more The influence of thermal stratification on autoignition at constant volume and high pressure is studied by direct numerical simulation (DNS) with detailed hydrogen/air chemistry with a view to providing better understanding and modeling of combustion processes in homogeneous charge compression-ignition engines. Numerical diagnostics are developed to analyze the mode of combustion and the dependence of overall ignition progress on initial mixture conditions. The roles of dissipation of heat and mass are divided conceptually into transport within ignition fronts and passive scalar dissipation, which modifies the statistics of the preignition temperature field. Transport within ignition fronts is analyzed by monitoring the propagation speed of ignition fronts using the displacement speed of a scalar that tracks the location of maximum heat release rate. The prevalence of deflagrative versus spontaneous ignition front propagation is found to depend on the local temperature gradient, and may be identified by the ratio of the instantaneous front speed to the laminar deflagration speed. The significance of passive scalar mixing is examined using a mixing timescale based on enthalpy fluctuations. Finally, the predictions of the multizone modeling strategy are compared with the DNS, and the results are explained using the diagnostics developed.