Hypersonic boundary-layer instability characterization and transition downstream of distributed roughness

The ablation of thermal protection systems in hypersonic vehicles would generate randomly distributed surface roughness, which prominently influences laminar-turbulent boundary-layer transition. While discrete and regular-shaped roughness elements have been extensively studied, the effect of distributed roughness on hypersonic boundary-layer stability and transition is poorly understood. In the present study, experiments were performed on a 7-degree half-angle sharp cone with sandpaper-type roughness in a Mach 6 Ludwieg tube. The transition onset locations were obtained by infrared thermography. Single/two-point focused laser differential interferometer was utilized to characterize the instability waves over distributed roughness along both the streamwise and wall-normal directions within the laminar, transitional and turbulent regions. On distributed roughness, the transition onset location moves forward, and the second mode was still the dominant instability. It exhibits lower frequency, faster growth, and earlier decay compared with the smooth surface. Linear stability calculations were carried out to assist with experiments. Furthermore, bispectral analysis indicates complicated nonlinear phase-locked energy exchanges within the boundary layer. It is shown that the earlier saturation and faster decay of the second mode on distributed roughness is associated with intensified nonlinear interactions.