Numerical investigation of a Mach 6 hypersonic laminar flow on two-dimensional cold-wall compression corners with controlled surface roughness

•Direct Numerical Simulations (DNS) of high-speed flow on 2D ramps.•Comparison between smooth and rough walls upstream of the ramp.•Effects of wall temperature and roughness on separation-bubble structure.•Impact of roughness elements on heat transfer at the wall. Numerical simulations are performed in order to investigate the physics of a Mach 6 hypersonic, laminar flow on different configurations of a two-dimensional compression corner. For this geometry, typically characterising control surfaces of re-entry vehicles, the interaction between the incoming boundary layer and the ramp-induced shock wave can lead to the formation of a separated-flow region. The numerical experiments, carried out under adiabatic and a cold-wall conditions, include the presence of a two-dimensional sinusoidal roughness patch, which is placed at various locations upstream of the ramp, in order to evaluate the effects of the surface waviness on the separation bubble. When the roughness elements are placed upstream of the smooth separation point, the recirculation bubble stretches towards the roughness position. This behaviour is induced by the thickening of the boundary layer downstream of the roughness patches, which cause a displacement of the laminar flow. No transition is observed downstream of the patches under the investigated geometrical and flow conditions. For a patch downstream of the smooth separation point, instead, no appreciable effects on the bubble length are noticed. However, the surface roughness inside the bubble can alter the recirculation pattern within the reverse-flow region. Hence, together with the length of the separated flow region, also the heat transfer at the wall is affected by the roughness because of the different recirculation patterns inside the bubble. In particular, the Stanton number highlights a redistribution of the heat-flux intensity between the main peak at reattachment and the secondary peaks inside the separated flow. This effect is more evident for higher ramp angles. These results highlight the importance of roughness-induced effects on the complex phenomenon that shock-wave/boundary-layer interaction is.