Investigation of Thermal Nonequilibrium on Hypersonic Boundary-Layer Transition by DNS

High-temperature gas effects are known as the physical processes which result in a deviation of the behavior of the calorically perfect gas in the hypersonicflow regime. These effects may have a significant impact on laminar-turbulent transition of the boundary layer, and thus affect the heat loads of hypersonic vehicles. In the present paper we deal with thermal effects, i.e. rotational and vibrational energy relaxation. A fundamental-breakdown scenario of a Mach-6.8 flat-plate boundary layer at flight conditions is simulated by high-order DNS using a calorically perfect gas, or thermal equilibrium, or a nonequilibrium model. A similar behavior is found for the calorically perfect gas and the thermal equilibrium case. In contrast, a stabilizing effect is observed in the thermal nonequilibrium case, leading to a fall off of fundamental breakdown.