Hypersonic Boundary-Layer Instability on Flat Plates with Configured Distributed Roughness Elements

The hypersonic vehicles' thermal protection system often experiences severe ablation in flight due to aerodynamic heating, causing distributed roughness elements (DRE) that significantly affect the transition process. An experimental investigation was conducted to reveal the DRE’s effects on the transition process for a flat plate at a Mach 6 Ludwieg tube wind tunnel, using high-frequency pressure sensors and the schlieren technique. Nine roughness surfaces, with different DRE configurations (diamond, prism, and hemisphere) and heights, are designed and studied in this work. The streamwise instability is confirmed to be dominated by the Mack second-mode instability and promoted by the diamond DRE, especially at [Formula: see text]. The promotion effect exhibits the features of a narrower frequency band range, faster convective velocity, and smaller differences between the characteristic frequencies along streamwise stations. The spanwise instability of the flat plate is uneven by spectral analysis, eventually leading to asynchronous and irregular transition locations. Among the three configurations, the prism and hemisphere DRE have a larger effect on distorting the laminar boundary layer than the diamond DRE. Compared to the regularly arranged DRE, the results further reveal that an irregular DRE with a prism configuration is more stimulating on the boundary-layer instability than the diamond DRE case.