Direct Simulation Monte Carlo for astrophysical flows  I. Motivation and methodology
Abstract
We describe a hybrid Direct Simulation Monte Carlo (DSMC) code for simultaneously solving the collisional Boltzmann equation for gas and the collisional Boltzmann equation for stars and dark matter for problems important to galaxy evolution. This project is motivated by the need to understand the controlling dynamics at interfaces between gases of widely differing densities and temperature, i.e. multiphase media. While more expensive than hydrodynamics, the kinetic approach does not suffer from discontinuities and it applies when the continuum limit does not, such as in the collapse of galaxy clusters and at the interface between coronal halo gas and a thin neutral gas layer. Finally, the momentum flux is carried, selfconsistently, by particles and this approach explicitly resolves and thereby `captures' shocks. The DSMC method splits the solution into two pieces: (1) the evolution of the phasespace flow without collisions and (2) the evolution governed the collision term alone without phasespace flow. This splitting approach makes DSMC an ideal match to existing particlebased NBODY codes. If the meanfree path becomes very small compared to any scale of interest, the method abandons simulated particle collisions and simply adopts the relaxed solution in each interaction cell consistent with the overall energy and momentum fluxes. This is functionally equivalent to solving the NavierStokes equations on a mesh. Our implementation is tested using the Sod shocktube problem and the nonlinear development of a KelvinHelmholtz unstable shear layer.
 Publication:

Monthly Notices of the Royal Astronomical Society
 Pub Date:
 March 2014
 DOI:
 10.1093/mnras/stt2406
 arXiv:
 arXiv:1304.3941
 Bibcode:
 2014MNRAS.438.2995W
 Keywords:

 atomic processes;
 hydrodynamics;
 methods: numerical;
 ISM: evolution;
 ISM: structure;
 galaxies: ISM;
 Astrophysics  Instrumentation and Methods for Astrophysics;
 Astrophysics  Cosmology and Nongalactic Astrophysics
 EPrint:
 13 pages, 4 figures, submitted to MNRAS, revised figures, corrected typos, and incorporated comments