Hypersonic Boundary-Layer Transition on Cold-Wall Canonical Geometries with Quantified Distributed Roughness Elements

Given the challenges of obtaining a natural turbulent boundary layer on common test model geometries in a shock tunnel, this work aims to investigate the influence of roughness elements on the boundary layer with respect to transition to turbulence. The experiments were conducted in the High Enthalpy Shock Tunnel Göttingen at the German Aerospace Center on a 1100-mm-long, 7°-half-angle cone and a 602-mm-long flat plate at Mach 7.4. Roughness elements were applied on the nosetip of the cone and near the leading edge of the flat plate. The roughness elements were scanned with a laser profilometer, allowing their specification in terms of a roughness Reynolds number based on the 70th-percentile element height and an exceedance probability distribution. Transition was examined for the cone geometry using streamwise-aligned coaxial thermocouples on the 0° meridian. This assisted sizing the roughness elements required for transition to occur as far upstream as detectable. Breakdown of roughness-induced vortical structures generated by the roughness elements with a similar roughness Reynolds number was then examined using the flat plate geometry with temperature-sensitive paint applied downstream of the roughness elements. It was found that roughness-induced vortices required a finite distance (persistence length) to break down into turbulent structures. The persistence length was successfully reduced by interspersing roughness elements with smaller ones