Investigation of vortical structures in the wake of pseudo-random roughness surfaces in hypersonic reacting boundary-layer flows

Abstract

The influence of roughness on the laminar-turbulent transition of hypersonic flows is investigated with Direct Numerical Simulations (DNS) in a re-entry scenario of an Apollo-like space capsule. A Mach 20 flow regime is simulated for conditions at an altitude of 57.5 km. This study uses two different generating shapes (sinusoidal and triangular shaped waves) for random distributed patches. Stronger streamwise vorticity is observed for most configurations with the triangular base functions. The simulation includes the chemical equilibrium gas model. Vortical structures in the wake of both roughness patch types interact with an acoustic perturbation derived from local one-dimensional inviscid (LODI) relations. A breakdown of the cross-flow-like vortex in the wake of the roughness patch is observed for both roughness patches. The growth of modal instabilities is studied with a spatio-temporal Fourier analysis. Both y- and z-type instabilities, which are present in cross-flow vortices at low speeds, are observed for the sinusoidal patch. In the triangular configuration, the y-type instability is the dominating instability which is in line with the stronger wall-normal velocity gradients caused by the increase in spanwise slope of the patch.