DSMC computations of separation over a ‘Tick’ model in hypersonic high enthalpy transitional flows

The shock-wave boundary layer interaction and separation characteristics concerning the flow over a leading edge separated flow configuration, also known as a `tick' configuration, is numerically studied in this paper. The flow domain spans localized areas from rarefled to continuum regimes, evolving from a combination of expansion and compression processes. The computations are performed using a Direct Simulation Monte Carlo code developed by Sandia Laboratories, called SPARTA. An extensive convergence study is taken to quantify the numerical error levels in the results and is found to be within 9% relative to a model with inflnitesimally small time step and cell-sizes. A theoretical sharp leading edge model is computed first to identity the distinct flow features and separation phenomena. An incipient separation behavior was observed for this case. Two collision schemes, one based on the traditional random selection of collision partners and the other based on a Near Neighbor approach, are evaluated for a gas mixture comprising species with significant mass differences. The relative deviation in velocity slip, between the two schemes, is found to be higher than the magnitude of error levels. Also, the effect of leading edge bluntness are evaluated at a radius of 0.01mm, 0.03mm and 0.1mm. The increase in bluntness is found to have significant effect on the surface quantities and separation size. The effect of limited equilibration of incident molecules to the surface temperature is evaluated by comparing accommodation coefficients of 1 and 0.75. The effect is more pronounced for the largest bluntness radius of 0.1mm, for which the separation and re-attachment locations are both displaced downstream by 5-6% for the accommodation coefficient of 0.75 relative to full accommodation.