The marriage of gas and dust

Abstract

Dust-gas mixtures are the simplest example of a two fluid mixture. We show that when simulating such mixtures with particles or with particles coupled to grids a problem arises due to the need to resolve a very small length scale when the coupling is strong. Since this is occurs in the limit when the fluids are well coupled, we show how the dust-gas equations can be reformulated to describe a single fluid mixture. The equations are similar to the usual fluid equations supplemented by a diffusion equation for the dust-to-gas ratio or alternatively the dust fraction. This solves a number of numerical problems as well as making the physics clear.

6 pages, 2 figures, proceedings of ASTRONUM-2014, 22nd-27th June 2014, Long Beach, CA, USA, eds. Pogorelov and Audit. v2: minor updates following referees comments