Knudsen-number dependence of two-dimensional single-mode Rayleigh-Taylor fluid instabilities.

We present a study of single-mode Rayleigh-Taylor instabilities with a modified direct simulation Monte Carlo (MDSMC) code in two dimensions. The MDSMC code is aimed to capture the dynamics of matter for a large range of Knudsen numbers within one approach. Our method combines the traditional Monte Carlo technique to efficiently propagate particles and the point-of-closest-approach method for high spatial resolution. Simulations are performed using different particle mean free paths and we compare the results to linear theory predictions for the growth rate including diffusion and viscosity. We find good agreement between theoretical predictions and simulations and, at late times, observe the development of secondary instabilities, similar to hydrodynamic simulations and experiments. Large mean free paths favor particle diffusion, reduce the occurrence of secondary instabilities, and approach the noninteracting gas limit.