Direct numerical simulations of roughness-induced transition in the boundary layer of a hypersonic spherical forebody under consideration of high-temperature gas effects

The current study investigates the unsteady, non-linear 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 showing the linear disturbance amplification and a non-linear breakdown scenario for various chemical models (e.g., chemical equilibrium, chemical non-equilibrium and thermochemical non-equilibrium). Unsteady disturbances at various frequencies are introduced into the flow to analyze the instabilities developing in the wake of the roughness patch. Simulations are conducted for a typical re-entry scenario at M = 20 where chemical dissociation takes place. The influence of the different non-equilibrium effects on the steady base flow as well as on the disturbance development is quantified and compared. The study highlights the necessity to include non-equilibrium effects in transitional scenarios as the unsteady, non-linear development of the instabilities are affected strongly by the chemical modeling.