Stabilization of a Two-Dimensional Hypersonic Boundary Layer Using a Shallow Cavity

Abstract

The stability of a two-dimensional hypersonic flat-plate boundary layer with a shallow cavity is investigated at Mach 6 using direct numerical simulations. The boundary layer is perturbed by a single-frequency wall blowing–suction actuator. The results indicate that the second mode is damped when the cavity is placed slightly downstream of the synchronization point (SP) of mode F and mode S, whereas the effect is reversed if it is located upstream or further downstream. The effect of the cavity depth is also studied. Strong damping of the second-mode disturbance is observed across the cavity. Energy budget analysis reveals that the presence of a shear layer bridging the leading and trailing edges of the cavity significantly reduces the work done by the Reynolds stress on the mean velocity gradient, which derives energy from the base flow and is the major contributor to the second-mode amplification near the flat-plate surface. It is suggested that a boundary layer dominated by second-mode instabilities can be efficiently stabilized by placement of a shallow cavity in the near-downstream region of the SP associated with the most amplified frequency. The damping effect depends on the cavity depth with a trend similar to that for hypersonic boundary layers over ultrasonically absorptive coatings, despite the fact that the flow structures can be very different.