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Title:Consistent treatment of transport properties of weakly-ionized gas mixtures in DSMC
Author(s):Swaminathan-Gopalan, Krishnan
Advisor(s):Stephani, Kelly Ann
Department / Program:Mechanical Science & Engineering
Discipline:Mechanical Engineering
Degree Granting Institution:University of Illinois at Urbana-Champaign
Subject(s):direct simulation Monte Carlo (DSMC) Parameter Calibration
Consistent Transport Properties
Collision Integrals
Ionized Gas Mixtures
Hypersonic Flows
Abstract:The direct simulation Monte Carlo (DSMC) is a probabilistic, particle-based computational technique, that is widely used for solving rarefied and highly non-equilibrium flows. The transport properties in DSMC are obtained as a result of the transport of mass, momentum and energy during binary collision processes between the simulated particles. The details of the collision processes, and therefore, the transport properties are dictated by the collision cross-section models employed in DSMC. In the DSMC method, phenomenological models are used for describing the collisional interaction of the simulated particles in the gas. Accurate modeling of transport properties is absolutely critical in any computational solver for obtaining physically realistic solutions of the flow field. In this work, a systematic approach for calibrating the DSMC collision model parameters to achieve consistency in the transport processes is presented. The DSMC collision cross-section model parameters are calibrated for high temperature conditions by matching the collision integrals from DSMC against physically accurate collision integrals that are currently employed in the Langley Aerothermodynamic Upwind Relaxation Algorithm (LAURA) and Data Parallel Line Relaxation (DPLR) high temperature CFD solvers. The DSMC parameter values are computed for the widely used Variable Hard Sphere (VHS) and Variable Soft Sphere (VSS) models using both the collision-averaged and collision-specific pairing approaches. An analysis on the applicability of the pairing approaches revealed that, each specific collision process needs to be treated independently (i.e., collision-specific pairing approach) in order to obtain physically accurate collision integrals and transport properties. In addition, the validity of the VHS and the VSS model to adequately capture the various phenomena occurring during the different types of collisional interactions in a weakly-ionized gas mixture is examined. Use of the VSS model, that could account for the anisotropic scattering of the collision process, was found to be necessary to achieve consistency in transport processes of ionized gases. The agreement of the VSS transport properties as determined by the ab initio based collision integral fits was found to be within 6% in the entire temperature range, regardless of the composition of the mixture. The recommended model parameter values can readily be applied to any gas mixture involving binary collisional interactions between the chemical species presented, for the specified temperature range. This general procedure is used for calibrating the collision model parameters for the interactions in some important gas systems and an extensive set of calibrated collision model parameters are presented. The recommended best-fit parameter values are provided for neutral gas mixtures over a temperature range of 1000-5000 K, and for ionized gas systems over a temperature range of 1000-20,000 K. Finally, the effect of the calibrated parameters are studied by comparing the flow field solutions computed using the calibrated parameters and Bird’s values for a neutral and ionized air mixture. Comparison of stagnation line heat flux values show significant differences (up to 40%) in the calibrated collision-specific VSS parameters with respect to Bird’s collision-averaged VHS model values.
Issue Date:2015-12-09
Rights Information:Copyright 2015 Krishnan Swaminathan-Gopalan
Date Available in IDEALS:2016-03-02
Date Deposited:2015-12

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