Direct Numerical Simulation of boundary-layer transitionunder consideration of chemical and thermal non-equilibrium at a generic capsule geometry with smooth and rough surfaces.

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In the proposed project, direct numerical simulations are used to investigate the laminar-turbulent transition evolution in a high-enthalpy flow under chemical equilibrium and non-equilibrium conditions. A generic geometry of the Apollo-capsule shape is used including high-temperature gas effects. The effect of dissociation and non-equilibrium effects on the disturbance development in the boundary layer subject to instabilities shall be investigated. Especially, roughness induced transition under chemical equilibrium and non-equilibrium conditions as for atmospheric re-entry missions is the main focus of the investigations. These investigations complement the ideal-gas investigations (cold wind-tunnel conditions at HLB) of the other PAK 742 partners. Disturbance amplification through the so-called transient growth mechanism for distributed roughnesses as well as secondary instability mechanisms for the discrete roughness wake flow are examined. Secondary instabilities are driven by local velocity and temperature gradients, which can be more pronounced in chemical (non-)equilibrium flows, leading to earlier transition in these cases. The simulations are then extended to incorporate the disturbance background in the wind tunnel ("wind-tunnel rebuilding") to better predict transition in experiments. Extrapolation of these results to high-enthalpy free-flight conditions are of particular interest.