Effect of the slot orientation angle on stabilization of a supersonic boundary layer

An experimental study is performed on the influence of surface slots (grooves) with a depth of 0.18 mm (the depth-based Reynolds number is Re$_h$ $\approx$ 1000) with orientation angles $\varphi$ = 0, 30, 45, 90$^\circ$ on the stability of a supersonic boundary layer on a flat plate. Experiments with natural disturbances at the free-stream Mach number M=2 are performed. It is found that the maximum rates of the spatial growth of disturbances decrease as the slot orientation angle decreases from $\varphi$ = 90$^\circ$ to zero. At $\varphi$ = 0, 30$^\circ$, they become smaller than the corresponding values for a smooth plate. The results obtained show that the instability of the first mode of the supersonic boundary layer, which determines the laminar-turbulent transition at M = 2, can be stabilized by small-depth slots with moderate angles of their orientation (0$^\circ$ $\le\varphi<$ 40$^\circ$).