Roughness-induced crossflow-type instabilities in a hypersonic capsule boundary layer including nonequilibrium

The current study investigates the unsteady disturbance development in a three-dimensional, high-enthalpy boundary layer on a capsule-like hemispherical geometry with pseudo-random distributed roughness. Direct Numerical Simulations are conducted for a typical reentry scenario at M 20 where chemical dissociation takes place. Unsteady disturbances at various frequencies are introduced into the flow to analyze the instabilities developing in the wake of the roughness patch. The amplification of small disturbances is shown for various chemical models (i.e., chemical equilibrium, chemical nonequilibrium, and thermochemical nonequilibrium). The largest disturbance amplification is observed for a crossflow-type vortex developing in the wake of the highest skewed protuberance of the roughness patch. The modes of instability of this crossflow-type vortex are analyzed and compared for different frequencies. The influence of the different nonequilibrium effects on the steady base flow as well as on the disturbance development is quantified and compared. The study highlights the necessity to include nonequilibrium effects in transitional scenarios as the unsteady development of the instabilities are affected strongly by the chemical modeling.