A Macroscopic Chemistry Method for the Direct Simulation of Gas Flows

Lilley, Charles R. and Macrossan, Michael N. (2004) A Macroscopic Chemistry Method for the Direct Simulation of Gas Flows. Physics of Fluids, 16 6: 2054-2066. doi:10.1063/1.1712973

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Author Lilley, Charles R.
Macrossan, Michael N.
Title A Macroscopic Chemistry Method for the Direct Simulation of Gas Flows
Journal name Physics of Fluids   Check publisher's open access policy
ISSN 1070-6631
Publication date 2004-06-01
Sub-type Article (original research)
DOI 10.1063/1.1712973
Open Access Status File (Author Post-print)
Volume 16
Issue 6
Start page 2054
End page 2066
Total pages 13
Editor John Kim
Publisher American Institute of Physics
Language eng
Subject 240502 Fluid Physics
250404 Flow Analysis
250699 Theoretical and Computational Chemistry not elsewhere classified
290299 Aerospace Engineering not elsewhere classified
780102 Physical sciences
Abstract In most chemistry methods developed for the direct simulation Monte Carlo (DSMC) technique, chemical reactions are computed as an integral part of the collision simulation routine. In the macroscopic chemistry method developed here, the simulation of collisions and the simulation of reactions are decoupled; reactions are computed independently, after the collision routine. The number of reaction events to perform in each cell is calculated using the macroscopic reaction rates k+, k- and equilibrium constant K*, calculated from the local macroscopic flow conditions. The macroscopic method is developed here for the symmetrical diatomic dissociating gas. For each dissociation event, a single diatomic simulator particle is selected with a probability based on its internal energy, and is replaced by two atomic particles. For each recombination event, two atomic particles are selected at random, and are replaced by a single diatomic particle. The dissociation energy is accounted for by adjusting the translational thermal energies of all particles in the cell. The macroscopic method gives density profiles in agreement with experimental data for the chemical relaxation region downstream of a strong shock in nitrogen. In the non-equilibrium regions within the shock, and along the stagnation streamline of a blunt cylinder in rarefied flow, the macroscopic method gives results in excellent agreement with those obtained using the most common conventional DSMC chemistry method in which reactions are calculated during the collision routine. The number of particles per computational cell has a minimal effect on the results provided by the macroscopic method. Unlike most DSMC chemistry methods, the macroscopic method is not limited to simple forms of k+, k- and K*. Any forms may be used, and these may be any function of the macroscopic conditions. This is demonstrated by using a two-temperature rate model, and a form of K* with a number density dependence. With the two-temperature model, the macroscopic method gives densities in the post-shock chemical relaxation region that also agree with the experimental data. For a form of K^* with a number density dependence, the macroscopic method can accurately reproduce chemical recombination behavior. In a primarily dissociative flow, the number density dependence of K* has very little effect on the flow. The macroscopic method requires slightly less computing time than the most common DSMC chemistry method.
Keyword DSMC
two-temperature rate
collision model
vibration-dissociation coupling
any reaction rate
any equilibrium constant
Q-Index Code C1

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Created: Wed, 23 Feb 2005, 10:00:00 EST by Michael N Macrossan on behalf of School of Engineering