Bulletin of the American Physical Society
62nd Annual Meeting of the APS Division of Fluid Dynamics
Volume 54, Number 19
Sunday–Tuesday, November 22–24, 2009; Minneapolis, Minnesota
Session HG: Rarefied Gases and Direct Simulation Monte Carlo (DSMC) |
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Chair: Michael Gallis, Sandia National Laboratories Room: 101G |
Monday, November 23, 2009 10:30AM - 10:43AM |
HG.00001: DSMC Predictions of Chemical Reaction Rates between Atmospheric Species M.A. Gallis, R.B. Bond, J.R. Torczynski A recently proposed chemical reaction model based solely on molecular-level information is applied to calculate equilibrium and non-equilibrium chemical reaction rates for atmospheric reactions in hypersonic flows. The DSMC model is capable of reproducing measured equilibrium reaction rates without using any macroscopic reaction-rate information. Since it uses only molecular-level properties, the new model is inherently able to predict reaction rates for arbitrary non-equilibrium conditions. The DSMC-predicted chemical reaction rates are compared to theoretically calculated and experimentally measured reaction rates for non-equilibrium conditions. The observed agreement provides strong evidence that molecular-level modeling of chemical reactions provides an accurate method for predicting equilibrium and non-equilibrium chemical reaction rates. Sandia is a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the United States Department of Energy's National Nuclear Security Administration under contract DE-AC04-94AL85000. [Preview Abstract] |
Monday, November 23, 2009 10:43AM - 10:56AM |
HG.00002: Direct Simulation Monte Carlo Investigation of Noncontinuum Couette Flow J.R. Torczynski, M.A. Gallis The Direct Simulation Monte Carlo (DSMC) method of molecular gas dynamics is used to study noncontinuum effects in Couette flow. The walls have equal temperatures and equal accommodation coefficients but unequal tangential velocities. Simulations are performed for near-free-molecular to near-continuum gas pressures with accommodation coefficients of 0.25, 0.5, and 1. Ten gases are examined: argon, helium, nitrogen, sea-level air, and six Inverse-Power-Law (IPL) gases with viscosity temperature exponents of 0.5, 0.6, 0.7, 0.8, 0.9, and 1.0, as represented by the Variable Soft Sphere (VSS) interaction. In all cases, the wall shear stress is proportional to the slip velocity. The momentum transfer coefficient relating these two quantities can be accurately correlated in terms of the Knudsen number based on the wall separation. The two dimensionless parameters in the correlation are similar for all gases examined. Sandia is a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the United States Department of Energy's National Nuclear Security Administration under contract DE-AC04-94AL85000. [Preview Abstract] |
Monday, November 23, 2009 10:56AM - 11:09AM |
HG.00003: A multiscale, multiphysics simulation method for rarefied gas flows David Kessler, Elaine Oran, Carolyn Kaplan We present a coupled multiscale, multiphysics solution algorithm, CM$^3$, for rarefied gas flows. The method uses a general solution of the compressible fluid-dynamics equations that incorporates stresses and heat fluxes calculated directly using the Direct Simulation Monte Carlo (DSMC) method. The CM$^3$ is designed to solve transition-regime flows at a much lower computational cost than possible by directly solving the Boltzmann equation. The CM$^3$ is tested on a low-speed, Rayleigh flow and a thermal Fourier flow for several Knudsen numbers. Velocity, temperature, shear stress, and heat flux profiles compare well with DSMC solutions. We discuss the algorithmic details that are necessary to implement a true multiscale method, building upon the conceptual framework of E \& Engquist's (2003) heterogeneous multiscale methods. [Preview Abstract] |
Monday, November 23, 2009 11:09AM - 11:22AM |
HG.00004: Data Structures and Adaptive Mesh Refinement for a 3-Level Embedded Cartesian Mesh DSMC Implementation Chonglin Zhang, Da Gao, Tom Schwartzentruber The data structures and overall algorithms of a newly developed 3-D direct simulation Monte Carlo (DSMC) program are outlined. The code employs an embedded 3-level Cartesian mesh, accompanied by a cut-cell algorithm to incorporate triangulated surface geometry into the adaptively refined Cartesian mesh. Such an approach enables decoupling of the surface mesh from the flow field mesh, which is desirable for near-continuum flows, flows with large density variation, and also for adaptive mesh refinement (AMR). Two separate data structures are proposed in order to separate geometry data from cell and particle information, leading to high scalability and efficient AMR for parallel simulations. A simple and efficient AMR algorithm that maintains local cell size consistent with the local mean-free-path and therefore a constant number of particles in each cell will be detailed. The 3-level embedded Cartesian mesh combined with AMR allows increased flexibility for precise control of local mesh size and time-step, both vital for accurate and efficient DSMC simulation. Verification and validation of the code will be provided, and DSMC results for 3-D flows with large density variations will also be presented. [Preview Abstract] |
Monday, November 23, 2009 11:22AM - 11:35AM |
HG.00005: Parallel Performance Optimization of the Direct Simulation Monte Carlo Method Da Gao, Chonglin Zhang, Thomas Schwartzentruber Although the direct simulation Monte Carlo (DSMC) particle method is more computationally intensive compared to continuum methods, it is accurate for conditions ranging from continuum to free-molecular, accurate in highly non-equilibrium flow regions, and holds potential for incorporating advanced molecular-based models for gas-phase and gas-surface interactions. As available computer resources continue their rapid growth, the DSMC method is continually being applied to increasingly complex flow problems. Although processor clock speed continues to increase, a trend of increasing multi-core-per-node parallel architectures is emerging. To effectively utilize such current and future parallel computing systems, a combined shared/distributed memory parallel implementation (using both Open Multi-Processing (OpenMP) and Message Passing Interface (MPI)) of the DSMC method is under development. The parallel implementation of a new state-of-the-art 3D DSMC code employing an embedded 3-level Cartesian mesh will be outlined. The presentation will focus on performance optimization strategies for DSMC, which includes, but is not limited to, modified algorithm designs, practical code-tuning techniques, and parallel performance optimization. Specifically, key issues important to the DSMC shared memory (OpenMP) parallel performance are identified as (1) granularity (2) load balancing (3) locality and (4) synchronization. Challenges and solutions associated with these issues as they pertain to the DSMC method will be discussed. [Preview Abstract] |
Monday, November 23, 2009 11:35AM - 11:48AM |
HG.00006: Molecular Dynamics Simulations of Nanoscale Gas Flows Murat Barisik, BoHung Kim, Ali Beskok Three-dimensional molecular dynamics (MD) simulations of rarefied gas flows confined within nano-scale channels are investigated by introduction of a smart wall model that drastically reduces the memory requirements of MD simulations for gas flows. The smart wall molecular dynamics (SWMD) represents three-dimensional FCC walls using only 74 wall molecules. Linear Couette flow of argon at Knudsen number 10 is investigated using the SWMD utilizing Lennard-Jones potential interactions. Presence of the walls creates an additional length scale based on the Lennard-Jones force field near the walls. This is typically 3 molecular diameters ($\sigma)$ in the parametric regime studied here. Therefore 3$\sigma$ region near the walls becomes a critical length scale that admits deviations from the kinetic theory. Our results have shown increase in the gas density and sudden change of the velocity profiles within this region. Kinetic theory solutions based on the Boltzmann equation neglect the wall force field effects, and hence, cannot predict this density increase and the change in the velocity profile. We have shown that the velocity profile within the interface region exhibits self similar behavior regardless of the channel height (provided that $H >$ 6$\sigma)$. Overall, the slip velocity is over predicted using kinetic theory solutions. [Preview Abstract] |
Monday, November 23, 2009 11:48AM - 12:01PM |
HG.00007: Catalytic Mechanism Modeling of Oxygen/Platinum Systems using ReaxFF MD Simulation Paolo Valentini, Thomas Schwartzentruber, Ioana Cozmuta ReaxFF Molecular Dynamics simulations are performed to study some of the fundamental surface mechanisms that characterize the catalytic behavior of a Pt(111) surface exposed to oxygen. The use of the reactive empirical potential ReaxFF allows the simulation of chemical bond breaking/formation, essential to describe the detailed processes at the surface, while maintaining computational feasibility for rather massive systems (thousands of atoms). The ReaxFF potential was initially trained using a set of Quantum Chemistry energy data relevant for the system of interest, including molecular/atomic adsorption enthalpies at various surface sites. Subsequently, the sticking coefficients were determined for oxygen molecules impinging onto the surface. The MD simulations well reproduce some of the experimental trends observed for the adsorption process in molecular-oxygen/platinum systems. At very low incident energies (less than 0.1 eV), the adsorption is determined by a weakly-bound physisorption state, but is rapidly suppressed by additional rotational energy (due to steric hindrance) or by increasing the substrate temperature. At higher incident energies, the sticking probability levels off due to dynamic trapping as observed experimentally. Because ReaxFF is solely based on Quantum Chemistry, the objective of this study is to extend the approach proposed here to other gas/surface systems for which good experimental evidence on the detailed catalytic surface processes is currently not available. [Preview Abstract] |
Monday, November 23, 2009 12:01PM - 12:14PM |
HG.00008: A robust semi-discrete quadrature-based moment method for solution of Boltzmann equation Prakash Vedula We present an efficient and robust semi-discrete quadrature-based moment method (sDQMOM) for solution of the Boltzmann equation with the full collision operator, for prediction of nonequilibrium flows in the rarefied thru continuum regimes. In the sDQMOM formulation, the distribution function is represented by a set of delta functions with associated weights and locations, whose evolution is determined by semi-discrete equations obtained from evolution of macroscopic moment equations in conservative form, where spatial fluxes of moments are evaluated using appropriate flux limiters. Based on this representation of the distribution function, the generalized moment production terms due to full collision operators can be evaluated analytically using multinomial expansions that ensure preservation collision invariants and correct rates of evolution of selected low order moments. It appears that sDQMOM also addresses some previously encountered limitations of quadrature-based moment methods arising from discontinuities and the ill-posed problem of moment inversion. To ensure realizability and robustness, an automatic and adaptive moment constraint selection algorithm for sDQMOM is also proposed based on orthogonal polynomials with a discrete support. Prediction capabilities of sDQMOM are demonstrated via good agreement between sDQMOM and other approaches for Poiseulle flow and shock tube problem. [Preview Abstract] |
Monday, November 23, 2009 12:14PM - 12:27PM |
HG.00009: ABSTRACT WITHDRAWN |
Monday, November 23, 2009 12:27PM - 12:40PM |
HG.00010: ABSTRACT HAS BEEN MOVED TO PU.00005 |
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