Hypersonic Boundary-Layer Instability Response to Nose-Tip Distributed Roughness Elements

Severe aerodynamic heating on hypersonic vehicle nose tips generates ablation-induced distributed roughness elements (DRE), critically impacting aerodynamic performance. This work aims to reveal the effects of nose-tip DRE on the nonlinear evolution of hypersonic boundary-layer instability and elucidate the associated transition mechanism. The DRE nose tips fabricated using additive manufacturing were applied on a standard [Formula: see text] half-angle cone, and various experimental techniques were employed. The results show that nose-tip DRE can significantly amplify boundary-layer disturbance levels compared to smooth cases. Detailed analysis revealed that while nose bluntness exerts a stabilizing effect, density fluctuation eigenfunctions indicate substantial distortion of near-wall base flow by the DRE. Bispectral analysis further identified accelerated nonlinear evolution of modulated high-frequency instability waves due to DRE. In the competition between bluntness-induced stabilization and DRE-driven destabilization mechanisms, the latter appears to dominate, resulting in a significant upstream shift of transition onset with irregular serrated features in the transitional region. This work uncovered the coexistence of high-frequency instability waves and streamwise vortex structures. The modulated Mack second mode does not govern the transition process. Instead, transition under DRE cases primarily arises from the downstream breakdown of periodic streamwise vortex structures induced by larger aft-located circumferential roughness elements.