Bulletin of the American Physical Society
69th Annual Meeting of the APS Division of Fluid Dynamics
Volume 61, Number 20
Sunday–Tuesday, November 20–22, 2016; Portland, Oregon
Session H17: Reacting Flows: LES |
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Chair: J.M. McDonough, University of Kentucky Room: D131 |
Monday, November 21, 2016 10:40AM - 10:53AM |
H17.00001: Wildfire simulation using LES with synthetic-velocity SGS models J. M. McDonough, Tingting Tang Wildland fires are becoming more prevalent and intense worldwide as climate change leads to warmer, drier conditions; and large-eddy simulation (LES) is receiving increasing attention for fire spread predictions as computing power continues to improve (see, e.g., Coen et al., {\it J. Appl. Meteor. Climatol.}, 2013; McGrattan, {\it NIST}, 2008). We report results from wildfire simulations over general terrain employing implicit LES for solution of the incompressible Navier--Stokes (N.--S.) and thermal energy equations with Boussinesq approximation, altered with Darcy, Forchheimer and Brinkman extensions, to represent forested regions as porous media with varying (in both space and time) porosity and permeability. We focus on subgrid-scale (SGS) behaviors computed with a synthetic-velocity model, a discrete dynamical system, based on the poor man's N.--S. equations (Tang et al., {\it Int. J. Bifur. Chaos}, 2016) and investigate the ability of this model to produce fire whirls (tornadoes of fire) at the (unresolved) SGS level. [Preview Abstract] |
Monday, November 21, 2016 10:53AM - 11:06AM |
H17.00002: LES Modeling of Supersonic Combustion at SCRAMJET Conditions Zachary Vane, Guilhem Lacaze, Joseph Oefelein Results from a series of large-eddy simulations (LES) of the Hypersonic International Flight Research Experiment (HIFiRE) are examined with emphasis placed on the coupled performance of the wall and combustion models. The test case of interest corresponds to the geometry and conditions found in the ground based experiments performed in the HIFiRE Direct Connect Rig (HDCR) in dual-mode operation. In these calculations, the turbulence and mixing characteristics of the high Reynolds number turbulent boundary layer with multi-species fuel injection are analyzed using a simplified chemical model and combustion closure to predict the heat release measured experimentally. These simulations are then used to identify different flame regimes in the combustor section. Concurrently, the performance of an equilibrium wall-model is evaluated in the vicinity of the fuel injectors and in the flame-holding cavity where regions of boundary layer and thermochemical non-equilibrium are present. [Preview Abstract] |
Monday, November 21, 2016 11:06AM - 11:19AM |
H17.00003: Hybrid Eulerian-Lagrangian Vortex Model for Turbulent Reacting Flows John Royero, Kareem Ahmed A hybrid Eulerian-Lagrangian model for three dimensional large eddy simulations of turbulent reacting flows is presented. The method utilizes a Eulerian grid to resolve large scale flow features and the Lagrangian vortex element method to capture smaller subgrid scale effects and carry out reactions which are then communicated back to the Eulerian grid after a set number of Lagrangian time steps. Lagrangian influences are localized in order to reduce computational cost. The Lagrangian vortex method which utilizes the Helmholtz decomposition of the velocity into potential, expansive, and solenoidal components allows the separation of the various mechanisms contributing to vorticity including gas expansion, diffusion, external body forces and baroclinic torque and is coupled with the Eulerian solver allowing easier implementation in arbitrary reacting flows at a reduced computational cost compared to a pure Lagrangian solver. [Preview Abstract] |
Monday, November 21, 2016 11:19AM - 11:32AM |
H17.00004: A Flamelet Modeling Approach for Multi-Modal Combustion with Inhomogeneous Inlets Bruce A. Perry, Michael E. Mueller Large eddy simulations (LES) of turbulent combustion often employ models that make assumptions about the underlying flame structure. For example, flamelet models based on both premixed and nonpremixed flame structures have been implemented successfully in a variety of contexts. While previous flamelet models have been developed to account for multi-modal combustion or complex inlet conditions, none have been developed that can account for both effects simultaneously. Here, a new approach is presented that extends a nonpremixed, two-mixture fraction approach for compositionally inhomogeneous inlet conditions to partially premixed combustion. The approach uses the second mixture fraction to indicate the locally dominant combustion mode based on flammability considerations and switch between premixed and nonpremixed combustion models as appropriate. To assess this approach, LES predictions for this and other flamelet-based models are compared to data from a turbulent piloted jet burner with compositionally inhomogeneous inlets, which has been shown experimentally to exhibit multi-modal combustion. [Preview Abstract] |
Monday, November 21, 2016 11:32AM - 11:45AM |
H17.00005: Three-dimensional CLEM-LES of irregular detonation propagation Brian Maxwell, Matei Radulescu Recently, thin-channel experiments and 2D simulations have been conducted in order to investigate the effect of turbulent mixing rates on the structure of irregular detonation wave propagation. Furthermore, the dependence of the observed cell pattern, and also the reaction zone thickness, on the mixing of burned products with pockets of unburned gases, was investigated. The current work now includes 3D simulations, which are conducted to provide further validation of, and insight into, the 2D results. All simulations have been conducted using the Compressible Linear Eddy Model for Large Eddy Simulation (CLEM-LES). To date, the 3D results are found to match closely the previous 2D results. The agrreement is partly due to sufficient resolution of the large scale fluid motions, which are observed experimentally to be predominant in only two directions. Furthermore, the CLEM-LES methodology incorporates 3D mixing effects at the subgrid level. Finally, it was found that turbulent fluctuations on the subgrid were found to give rise to statistically lower than average propagation velocities on the wave front. This lead to longer ignition delays for large amounts of gas passing through the wave, giving rise to the unburned pockets of gas observed experimentally. [Preview Abstract] |
Monday, November 21, 2016 11:45AM - 11:58AM |
H17.00006: Comparison of Turbulence--Chemistry Interaction Models in the Large Eddy Simulation of High-Speed Combustion. Wenhai Li, Ken Alabi, Foluso Ladeinde, Zhipeng Lou In this study, three turbulence-chemistry interaction models: the flamelet, eddy-breakup (EBU), and laminar chemistry models, are compared in the large-eddy simulation (LES) of high speed combustion. It is the case that the simple models still find extensive applications, with fairly acceptable results in many instances. The standard flamelet model developed for low Mach number flows has been modified to account for compressibility effects in supersonic combustion. The comparison exercise has been based on the bluff-body flames that occur under high-speed conditions. [Preview Abstract] |
Monday, November 21, 2016 11:58AM - 12:11PM |
H17.00007: Evaluation of subgrid dispersion models for LES of spray flames Qing Wang, Xinyu Zhao, Lucas Esclapez, Pavan Govindaraju, Matthias Ihme Turbulent dispersion models for particle-laden turbulent flows have been studied extensively over the past few decades, and different modeling approaches have been proposed and tested. However, the significance of the subgrid dispersion model and its influence on the flame dynamics for spray combustion have not been examined. To evaluate the performance of dispersion models for spray combustion, direct numerical simulations (DNS) of three-dimensional counterflow spray flames are studied. The DNS configuration features a series of different droplet sizes to study effects of different Stokes numbers. An \textit{a priori} comparison of the statistics generated from three subgrid dispersion models is made, for both non-reacting and reacting conditions. Improved agreement with DNS is shown for the stochastic model and the regularized deconvolution model than a closure-free model. The effect of filter sizes in relation to droplet sizes are investigated for all models. Subsequently, \textit{a posteriori} modeling of the same configuration with different resolutions is performed to compare these models in the presence of other subgrid models. Finally, models for the subgrid closure of scalar transport for multiphase droplet combustion are proposed and evaluated. [Preview Abstract] |
Monday, November 21, 2016 12:11PM - 12:24PM |
H17.00008: Performance assessment of a pre-partitioned adaptive chemistry approach in large-eddy simulation of turbulent flames Perrine Pepiot, Youwen Liang, Ashish Newale, Stephen Pope A pre-partitioned adaptive chemistry (PPAC) approach recently developed and validated in the simplified framework of a partially-stirred reactor is applied to the simulation of turbulent flames using a LES/particle PDF framework. The PPAC approach was shown to simultaneously provide significant savings in CPU and memory requirements, two major limiting factors in LES/particle PDF. The savings are achieved by providing each particle in the PDF method with a specialized reduced representation and kinetic model adjusted to its changing composition. Both representation and model are identified efficiently from a pre-determined list using a low-dimensional binary-tree search algorithm, thereby keeping the run-time overhead associated with the adaptive strategy to a minimum. The Sandia D flame is used as benchmark to quantify the performance of the PPAC algorithm in a turbulent combustion setting. In particular, the CPU and memory benefits, the distribution of the various representations throughout the computational domain, and the relationship between the user-defined error tolerances used to derive the reduced representations and models and the actual errors observed in LES/PDF are characterized. [Preview Abstract] |
Monday, November 21, 2016 12:24PM - 12:37PM |
H17.00009: LES/PDF studies of joint statistics of mixture fraction and progress variable in piloted methane jet flames with inhomogeneous inlet flows Pei Zhang, Robert Barlow, Assaad Masri, Haifeng Wang The mixture fraction and progress variable are often used as independent variables for describing turbulent premixed and non-premixed flames. There is a growing interest in using these two variables for describing partially premixed flames. The joint statistical distribution of the mixture fraction and progress variable is of great interest in developing models for partially premixed flames. In this work, we conduct predictive studies of the joint statistics of mixture fraction and progress variable in a series of piloted methane jet flames with inhomogeneous inlet flows. The employed models combine large eddy simulations with the Monte Carlo probability density function (PDF) method. The joint PDFs and marginal PDFs are examined in detail by comparing the model predictions and the measurements. Different presumed shapes of the joint PDFs are also evaluated. [Preview Abstract] |
Monday, November 21, 2016 12:37PM - 12:50PM |
H17.00010: Scalar mixing in LES/PDF of a high-Ka premixed turbulent jet flame Jiaping You, Yue Yang We report a large-eddy simulation (LES)/probability density function (PDF) study of a high-Ka premixed turbulent flame in the Lund University Piloted Jet (LUPJ) flame series, which has been investigated using direct numerical simulation (DNS) and experiments. The target flame, featuring broadened preheat and reaction zones, is categorized into the broken reaction zone regime. In the present study, three widely used mixing modes, namely the Interaction by Exchange with the Mean (IEM), Modified Curl (MC), and Euclidean Minimum Spanning Tree (EMST) models are applied to assess their performance through detailed \emph{a posteriori} comparisons with DNS. A dynamic model for the time scale of scalar mixing is formulated to describe the turbulent mixing of scalars at small scales. Better quantitative agreement for the mean temperature and mean mass fractions of major and minor species are obtained with the MC and EMST models than with the IEM model. The multi-scalar mixing in composition space with the three models are analyzed to assess the modeling of the conditional molecular diffusion term. In addition, we demonstrate that the product of OH and CH$_2$O concentrations can be a good surrogate of the local heat release rate in this flame. [Preview Abstract] |
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