Discrete-Roughness-Induced Laminar–Turbulent Transition in Ramp Flow at Mach 6

Roughness effects on a Mach 6 ramp-induced shock-wave/boundary-layer interaction are numerically investigated by injecting unsteady pressure disturbances into a steady baseflow. Employing direct numerical simulations, the paper investigates the effects of different roughness and flow-disturbance parameters. The vortical structure induced by the roughness placed upstream of the separation bubble breaks down into turbulence downstream of reattachment on the ramp. Such breakdown is caused by the amplification of the flow oscillations on the ramp driven by the strong wall-normal gradients of the streamwise velocity (“[Formula: see text]-modes”). The disturbance shape does not affect the growth rates on the ramp but rather the breakdown mode (symmetric or asymmetric for two- or three-dimensional waves, respectively), leading to different footprints of the high-temperature streaks forming at the wall. The breakdown-onset position depends on the amplification patterns upstream of the corner, which yield different initial amplitudes once the flow reattaches on the ramp. Injecting the disturbances directly on the ramp, therefore bypassing the amplification upstream of reattachment, leads to the same amplification rates on the ramp but with delayed transition. Similarly, the roughness-patch position influences the transition process via the baseflow effects (different boundary-layer separation and reattachment locations), having negligible effects on breakdown modes and growth rates on the ramp. Finally, increasing the inflow frequency from 100 to 250 kHz leads to a much longer and narrower breakdown process.