Influence of Pyrolytic Ablative Nose-Tip Pattern on Hypersonic Boundary-Layer Instability

Hypersonic vehicle surface ablation morphology induced by aerodynamic heating significantly influences boundary-layer transition. Based on the ablation pattern of a lightweight, low-ablation carbonized pyrolytic composite typically used in thermal protection systems, a conical nose tip was designed and fabricated featuring coexisting distributed roughness elements and random pores via utilizing selective laser melting additive manufacturing. Experiments were conducted in a Mach 6 hypersonic Ludwieg tube tunnel to study boundary-layer instability over a sharp cone model. In this measurement campaign, we employed various instruments, including high-frequency pressure sensors, focused laser differential interferometry, and infrared thermography. Experimental results confirm that ablative surface morphology significantly advances the onset position of second-mode instability waves. Infrared thermography further demonstrates its transition-promoting influence. Based on the experimental results, the influence of the ablative nose–tip morphology on boundary–layer instability is attributed to nose-tip roughness elements. The roughness advances the onset of instability waves, increases the linear growth rate, and promotes an earlier transition to the nonlinear stage by modulating the receptivity process and distorting the mean flow. These combined effects ultimately lead to earlier transition onset.