Investigation of Diffusive Effects in High-speed Flows through Comparisons of DSMC and CFD

The present paper focuses on comparing state-of-the-art computational fluid dynamics (CFD) and direct simulation Monte Carlo (DSMC) codes developed at the University of Minnesota; specifically, we compare results obtained with US3D, a code routinely used for complex hypersonic CFD simulation, to those obtained using MGDS, a code capable of large- scale 3D DSMC simulations. In particular, we focus on conditions where commonly neglected diffusive effects, such as thermal diffusion (Soret effect) and pressure-gradient driven diffusion (baro-diffusion), play an important role. Comparisons between CFD and continuum DSMC simulations were conducted for several test cases using a high-speed cylinder setup with increasing underlying model complexity. These cases include a binary mixture of noble gases, a ternary mixture of noble gases, a mixture of molecular nitrogen and atomic oxygen, and a dissociated air mixture intended to replicate relevant flight conditions while avoiding gas-phase chemistry. Stagnation line extractions of flowfield properties and surface heat flux showed excellent agreement for all the conditions considered. However, some differences were observed that were either caused by slight under-resolution in the DSMC results (noble gas mixtures) or minor inconsistencies in the underlying CFD and DSMC modeling (dissociated air mixtures).