The effect of localized roughness on a Ma=4.8 boundary-layer and its instability is investigated by means of unsteady direct numerical simulations. The occurrence of special flow features, such as shocks, boundary-layer separation zones, streamwise vortices and streamwise streaks behind a three-dimensional square-shaped roughness element are discussed. In order to investigate the convective instability of the boundary layer downstream of the roughness, the steady-state flow field with roughness element has been perturbed with a small-amplitude perturbation of fixed frequency upstream of the element. The transformation of a first-mode instability wave upstream of the roughness into a strongly growing localized perturbation downstream of the square-shaped roughness is observed. Preliminary results for an oblique roughness are compared to results obtained for the square-shaped roughness.
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