Independent roles of leading-edge and downstream distributed roughness elements in modulating hypersonic boundary layer transition

Aerodynamic heating-induced ablation generates distributed roughness elements (DRE) on the surfaces of hypersonic vehicles, significantly influencing boundary layer transition and aerodynamic performance. However, location-dependent DRE modulation mechanisms remain unresolved. This study employs surface-mounted PCB® pressure sensor arrays in a Mach 6 wind tunnel to elucidate the critical role of regularly spaced DRE starting locations in modulating hypersonic boundary layer transition over a flat plate. Experimental results reveal that leading-edge DRE starting at X = 0 mm induces significant second-mode instability wave suppression. In comparison, DRE starting at X = 1 mm (with the DRE height-to-local-boundary-layer-thickness ratio k/δref = 2.83) provokes immediate bypass transition to turbulence, while downstream DRE starting at X = 120 mm (k/δref = 0.26) minimally alters natural transition. Addressing the spanwise instability non-uniformity phenomenon in the flat-plate boundary layer, proper orthogonal decomposition analyses were performed and demonstrate that it exhibits fundamentally distinct organization mechanisms across different DRE cases, with mode 3 dominating under leading-edge DRE cases, whereas mode 1 prevails in smooth and downstream DRE cases. Importantly, DRE width variations exert negligible effects on the downstream boundary layer instability across all cases, whereas the k/δref is identified as the critical scaling parameter controlling transition behavior. These findings resolve location-dependent disparities in DRE physics and establish k/δref as a predictive metric for ablation-induced DRE effects, thereby providing critical insights for the design of thermal protection systems on hypersonic vehicles.