Effects of surface imperfections on the boundary layer stability in a high speed flow

The effect of surface imperfections on the second mode instability in a Mach 4.5 flow is investigated using direct numerical simulation (DNS) and stability analysis by linear stability theory (LST). Two types of nonuniformities are considered, including backward-facing steps (BFS) and microgrooves located at different streamwise positions. A controlled Klebanoff-type transition is initiated by imposed unstable second modes and oblique waves on the domain inlet. The mean flow results indicate that BFS cause an abrupt expansion at the step corner, while microgrooves have a less drastic effect on the boundary layer over the cavities. Compared with the transition behaviours over a flat-plate, it is evident that surface imperfections can notably delay the second-mode dominant transition, with BFS showing superior delay capabilities. Further analysis of DNS and LST results reveals that the amplification of disturbances at 90 kHz is significantly inhibited by surface irregularities. Specifically, such imperfections, positioned downstream of the synchronization point for the leading unstable second mode, emerges as an effective strategy to stabilize the high-speed boundary layers.