A direct simulation Monte Carlo study of hypersonic leading-edge separation with rarefaction effects

Hypersonic laminar flow separation preceded by a strong expansion can occur in practical situations such as the base flow of re-entry vehicles and flow over deflected control surfaces. A leading-edge separation configuration provides a case for studying fundamental aspects of such flow separation and reattachment in the absence of a pre-existing boundary layer. In hypersonic low-density flows, the onset of separation is complicated by the presence of rarefaction and thermal non-equilibrium. The reattachment process is also characterized by high compressibility. A computational study of the characteristics of flow separation and reattachment over such a configuration has been carried out using the direct simulation Monte Carlo code SPARTA. The salient features of separation are explained from a fluid dynamic perspective, and distinct characteristics in surface parameters are identified and discussed in detail. The physical mechanisms behind the formation as well as co-existence of primary and secondary vortices are described. The region close to the leading-edge is analyzed in detail, and the prevailing non-equilibrium aspects are presented. Various theoretical concepts, developed based on continuum flow separation, are applied to the present configuration to explore its applicability in the presence of slip, and the resulting relative variations are highlighted. The temporal evolution of flow structures at various wall temperatures is studied, and its strong dependence on the wall-to-stagnation temperature ratio is elucidated.