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S B Pope - One of the best experts on this subject based on the ideXlab platform.

  • large eddy simulation probability density function modeling of a non premixed co h2 temporally evolving jet flame
    Proceedings of the Combustion Institute, 2013
    Co-Authors: S B Pope, Yue Yang, Haifeng Wang, Jacqueline H Chen
    Abstract:

    Abstract We report a large-eddy simulation (LES)/probability density function (PDF) study of a non-premixed CO/H2 temporally-evolving turbulent planar jet flame, which has previously been studied using direct numerical simulations (DNS) with a skeletal chemical mechanism. The flame exhibits strong turbulence–chemistry interactions resulting in Local Extinction followed by re-ignition. In this study, the filtered velocity field in LES and the PDF transport equations with the interaction-by-exchange with the mean (IEM) mixing model (with molecular transport) are solved by the highly-scalable NGA/HPDF codes with second-order accuracy in space and time. The performance of the hybrid LES/PDF methodology is assessed through detailed a posteriori comparisons with DNS of the same flame. The comparison shows overall good agreement of the temporal evolution of the temperature and mass fractions of major chemical species, as well as the prediction of Local Extinction and re-ignition. The modeling of multi-scalar mixing is analyzed using the DNS and LES/PDF results. The DNS results exhibit an attracting manifold of the streamlines of the diffusion velocity in composition space, and the LES/PDF results show qualitative agreement on the manifold and joint PDFs of compositions.

  • calculations of bluff body stabilized flames using a joint probability density function model with detailed chemistry
    Combustion and Flame, 2005
    Co-Authors: S B Pope, David A Caughey
    Abstract:

    Joint probability density function (PDF) calculations are reported of the bluff-body stabilized flames (HM1, HM2, and HM3) and the results are compared with the available experimental data. The calculations are based on the modeled transport equation for the joint PDF of velocity, turbulence frequency, and composition (species mass fractions and enthalpy) using the interaction by exchange with the mean and Euclidean minimum spanning tree mixing models. The methane chemistry is described by a 19-species augmented reduced mechanism, and is implemented using in situ adaptive tabulation. The numerical accuracy of the calculations is carefully studied, and the associated errors are quantified. For flame HM1 (which has the least Local Extinction), there is generally good agreement between calculations and measurements, although (for all flames) the quality of the agreement deteriorates at downstream locations. The calculations correctly show essentially inert mixing in the shear layer between the recirculation zone and the coflow in flame HM1, but not in flames HM2 and HM3. In general, the calculations of flames HM2 and HM3 are not in good agreement with the experimental data and do not exhibit the observed Local Extinction. This deficiency is attributed to the inaccurate calculations of the mean mixture fraction in the recirculation zone (for flames HM2 and HM3). The sensitivity of the calculation to the mixing model constant is investigated, and the mean scalar dissipation is reported.

  • probability density function calculations of Local Extinction and no production in piloted jet turbulent methane air flames
    Proceedings of the Combustion Institute, 2000
    Co-Authors: Qing Tang, Jun Xu, S B Pope
    Abstract:

    The intense nonlinear interaction between turbulent fluctuations and finite-rate chemistry can cause Local Extinction and has a strong influence on NO production in non-premixed turbulent flames. Accurate predictions of Local Extinction and NO formation in turbulent flames require a rigorous means of representing such a strong coupling of turbulence and chemistry and hence are substantial challenges for turbulent combustion models. In this study, a self-contained joint velocity-composition-turbulence-frequency probability density function (PDF) method is used to make calculations of a series of piloted-jet nonpremixed flames of methane/air. The ingredients of the present model include the simplified Langevin model for velocity, a stochastic model of turbulence frequency, and the Euclidean minimum spanning tree (EMST) mixing model. An augmented reduced mechanism (ARM2) for methane oxidation, which involves 19 species and 15 reactions (including NO chemistry), is incorporated into the joint probability density function (JPDF) calculations using the in situ adaptive tabulation (ISAT) algorithm. The effects of radiative heat loss are studied using an optically thin limit model. The calculation results show good agreement with the experimental data, including the minor species NO and CO. The increase of Local Extinction with increasing jet velocity is accurately represented by the calculations.

  • probability density function calculations of Local Extinction and no production in piloted jet turbulent methane air flames
    Proceedings of the Combustion Institute, 2000
    Co-Authors: Qing Tang, S B Pope
    Abstract:

    The intense nonlinear interaction between turbulent fluctuations and finite-rate chemistry can cause Local Extinction and has a strong influence on NO production in non-premixed turbulent flames. Accurate predictions of Local Extinction and NO formation in turbulent flames require a rigorous means of representing such a strong coupling of turbulence and chemistry and hence are substantial challenges for turbulent combustion models. In this study, a self-contained joint velocity-composition-turbulence-frequency probability density function (PDF) method is used to make calculations of a series of piloted-jet nonpremixed flames of methane/air. The ingredients of the present model include the simplified Langevin model for velocity, a stochastic model of turbulence frequency, and the Euclidean minimum spanning tree (EMST) mixing model. An augmented reduced mechanism (ARM2) for methane oxidation, which involves 19 species and 15 reactions (including NO chemistry), is incorporated into the joint probability density function (JPDF) calculations using the in situ adaptive tabulation (ISAT) algorithm. The effects of radiative heat loss are studied using an optically thin limit model. The calculation results show good agreement with the experimental data, including the minor species NO and CO. The increase of Local Extinction with increasing jet velocity is accurately represented by the calculations.

Qing Tang - One of the best experts on this subject based on the ideXlab platform.

  • probability density function calculations of Local Extinction and no production in piloted jet turbulent methane air flames
    Proceedings of the Combustion Institute, 2000
    Co-Authors: Qing Tang, Jun Xu, S B Pope
    Abstract:

    The intense nonlinear interaction between turbulent fluctuations and finite-rate chemistry can cause Local Extinction and has a strong influence on NO production in non-premixed turbulent flames. Accurate predictions of Local Extinction and NO formation in turbulent flames require a rigorous means of representing such a strong coupling of turbulence and chemistry and hence are substantial challenges for turbulent combustion models. In this study, a self-contained joint velocity-composition-turbulence-frequency probability density function (PDF) method is used to make calculations of a series of piloted-jet nonpremixed flames of methane/air. The ingredients of the present model include the simplified Langevin model for velocity, a stochastic model of turbulence frequency, and the Euclidean minimum spanning tree (EMST) mixing model. An augmented reduced mechanism (ARM2) for methane oxidation, which involves 19 species and 15 reactions (including NO chemistry), is incorporated into the joint probability density function (JPDF) calculations using the in situ adaptive tabulation (ISAT) algorithm. The effects of radiative heat loss are studied using an optically thin limit model. The calculation results show good agreement with the experimental data, including the minor species NO and CO. The increase of Local Extinction with increasing jet velocity is accurately represented by the calculations.

  • probability density function calculations of Local Extinction and no production in piloted jet turbulent methane air flames
    Proceedings of the Combustion Institute, 2000
    Co-Authors: Qing Tang, S B Pope
    Abstract:

    The intense nonlinear interaction between turbulent fluctuations and finite-rate chemistry can cause Local Extinction and has a strong influence on NO production in non-premixed turbulent flames. Accurate predictions of Local Extinction and NO formation in turbulent flames require a rigorous means of representing such a strong coupling of turbulence and chemistry and hence are substantial challenges for turbulent combustion models. In this study, a self-contained joint velocity-composition-turbulence-frequency probability density function (PDF) method is used to make calculations of a series of piloted-jet nonpremixed flames of methane/air. The ingredients of the present model include the simplified Langevin model for velocity, a stochastic model of turbulence frequency, and the Euclidean minimum spanning tree (EMST) mixing model. An augmented reduced mechanism (ARM2) for methane oxidation, which involves 19 species and 15 reactions (including NO chemistry), is incorporated into the joint probability density function (JPDF) calculations using the in situ adaptive tabulation (ISAT) algorithm. The effects of radiative heat loss are studied using an optically thin limit model. The calculation results show good agreement with the experimental data, including the minor species NO and CO. The increase of Local Extinction with increasing jet velocity is accurately represented by the calculations.

Jeffrey W Streicher - One of the best experts on this subject based on the ideXlab platform.

  • climate change Extinction and sky island biogeography in a montane lizard
    Molecular Ecology, 2019
    Co-Authors: John J Wiens, Agustin Camacho, Aaron Goldberg, Tereza Jezkova, Matthew E Kaplan, Shea M Lambert, Elizabeth C Miller, Jeffrey W Streicher
    Abstract:

    Around the world, many species are confined to "Sky Islands," with different populations in isolated patches of montane habitat. How does this pattern arise? One scenario is that montane species were widespread in lowlands when climates were cooler, and were isolated by Local Extinction caused by warming conditions. This scenario implies that many montane species may be highly susceptible to anthropogenic warming. Here, we test this scenario in a montane lizard (Sceloporus jarrovii) from the Madrean Sky Islands of southeastern Arizona. We combined data from field surveys, climate, population genomics, and physiology. Overall, our results support the hypothesis that this species' current distribution is explained by Local Extinction caused by past climate change. However, our results for this species differ from simple expectations in several ways: (a) their absence at lower elevations is related to warm winter temperatures, not hot summer temperatures; (b) they appear to exclude a low-elevation congener from higher elevations, not the converse; (c) they are apparently absent from many climatically suitable but low mountain ranges, seemingly "pushed off the top" by climates even warmer than those today; (d) despite the potential for dispersal among ranges during recent glacial periods (~18,000 years ago), populations in different ranges diverged ~4.5-0.5 million years ago and remained largely distinct; and (e) body temperatures are inversely related to climatic temperatures among sites. These results may have implications for many other Sky Island systems. More broadly, we suggest that Sky Island species may be relevant for predicting responses to future warming.

P Koutmos - One of the best experts on this subject based on the ideXlab platform.

Peter R Lindstedt - One of the best experts on this subject based on the ideXlab platform.

  • impact of molecular mixing and scalar dissipation rate closures on turbulent bluff body flames with increasing Local Extinction
    Combustion and Flame, 2019
    Co-Authors: Lu Tian, Peter R Lindstedt
    Abstract:

    Abstract Bluff-body turbulent CH4: H2 (1:1) flames at 50% (HM1), 75% (HM2) and 91% (HM3) of the blow-off velocity (235 m s − 1 ) were studied experimentally by Masri and co-workers and found to exhibit gradually increasing periodic and shear layer instabilities. The latter are coupled with increasing levels of Local Extinction with subsequent re-ignition further downstream. This study provides a systematic evaluation of the sensitivity of predictions to molecular mixing and scalar dissipation rate closures. The latter include extended forms of the Euclidean Minimum Spanning Tree (EMST) and modified Curl’s (MC) models, applicable to premixed turbulent flames via a closure that accounts for Local Damkohler number effects (EEMST and EMC), and a conceptually related blended scalar time-scale approach (BEMST and BMC). Computations are performed using a hybrid finite volume (FV) – transported Joint Probability Density Function (JPDF) algorithm featuring stochastic Lagrangian particles, a comprehensive 48-scalar systematically reduced C/H/N/O mechanism, and a second moment method based on the Generalised Langevin Model that provides a partial resolution of the unsteady fluid motion. The sensitivity to solution parameters affecting the temporal resolution is quantified using Fourier transforms of the time histories of velocity and scalar traces. Radial profiles, conditional means and scatter plots are compared to the experimental data along with burning indices based on the conditional mean temperature. Vortex related instabilities  ∼ 1 kHz in the outer shear layer appear for all closures with EMC showing periodic Local Extinction and re-ignition in the neck region for HM3 and flame turbules (i.e., discrete pockets of hot gas) separating periodically at frequencies  ∼ 85 Hz. Results are similar to well–resolved JPDF/LES simulations for HM1. It is shown that the EMC and (E)EMST models essentially enclose the experimental data for HM2 and HM3. For HM3, emissions of NO are controlled by Local Extinction events that become increasingly sensitive to the molecular mixing closure as blow-off is approached.