Detailed comparison of diffusive transport phenomena between cfd and dsmc

The present paper focuses on comparing state-of-the-art CFD and DSMC codes developed at the University of Minnesota; we compare results obtained with US3D, a code routinely used for complex hypersonic CFD simulations with those obtained using MGDS, a code capable of large-scale 3D DSMC simulations. This work is the first part of a long term effort to eventually perform coupled hybrid CFD-DSMC simulations using these codes. First, we compare the codes for canonical 1-D problems using a single species monoatomic gas. The Fourier heat transfer and Couette flow problems are chosen, where a varying Knudsen number is used to investigate non-continuum effects. The variable hard sphere (VHS) cross section model is used in MGDS and the consistent expressions for viscosity and thermal conductivity are used in US3D. For the lowest Knudsen number case (0.001) we observe excellent agreement between US3D and MGDS for both 1-D test cases. As the Knudsen number is increased in MGDS, the behavior gradually departs from the continuum solution. In order to investigate a more relevant 2-dimensional flow field with strong diffusive transport, we compare a shear flow generated by two parallel supersonic jets with different velocities using US3D and MGDS. Although this flow field has a discontinuity at the inflow that creates two oblique shocks, having the entire flow field remain supersonic allows for more consistent treatment of boundary conditions between DSMC and CFD. Comparisons are made at several locations, where the disagreement between peak values of heat flux and shear stress decreases as the distance from the inflow increases. Although the agreement is reasonable between US3D and MGDS, the differences motivate a more consistent comparison involving one-way coupling from CFD to DSMC as future work.