Investigations of Cavity Depth on Boundary-Layer Transition over a Sharp Cone

The influences of a variable-depth, two-dimensional cavity (cavity for short) on boundary-layer instabilities and transition are investigated over a 500-mm-long sharp cone with a 7 deg half-angle. Experiments are conducted in a Mach 6 Ludwieg tube (SKLA-TT1) using Rayleigh-scattering flow visualization, fast-response pressure sensors, and infrared thermography at two unit Reynolds numbers. The cavity is located at [Formula: see text] from the tip of the cone with a fixed streamwise length of 20 mm and depths of [Formula: see text], [Formula: see text], and [Formula: see text] ([Formula: see text] is the boundary-layer thickness at [Formula: see text] of [Formula: see text], where the cavity starts). Flow visualization reveals that, for the [Formula: see text]-deep cavity, both the onsets of the Mack second-mode waves and the subsequent boundary-layer transition are shifted downstream. Quantitative pressure measurements confirm that the [Formula: see text]-deep cavity attenuates the Mack second-mode waves and consequently delays the onset of boundary-layer transition. For deeper cavity cases ([Formula: see text] and [Formula: see text]), the Mack second-mode waves remain suppressed in the upstream region, yet boundary-layer transition is slightly advanced at the unit Reynolds number of [Formula: see text]. The shallow cavity ([Formula: see text]) is shown to delay transition, thereby demonstrating a viable passive-control approach for high-speed boundary layers.