Experimental study on influences of steps on hypersonic boundary layer transition at different angles-of-attack

The influences of the forward-facing step (FFS) and backward-facing step (BFS) on the 7° half-angle conical boundary layer instability and transition are investigated at different angles-of-attack (AoAs) in a hypersonic quiet wind tunnel using the Nano-tracer-based Planar Laser Scattering techniques, Temperature-Sensitive Paints, and high-frequency pressure sensors. The results show that the FFS stabilizes the second mode instability but strongly destabilizes the crossflow instability. Conversely, the BFS destabilizes both the second mode and crossflow instabilities, yet its impact on crossflow instability is weaker compared to that of an FFS with an identical height. At a small AoA (AoA < 3° in this paper), the boundary layer transition is dominated by the second mode instability. For sharp cones, the transition is delayed on the windward side but promoted on the leeward side, resulting in a monotonically inclined transition front. In contrast, blunt cones exhibit localized depressions in the transition front on the leeward side. At large AoA, the transition process is dominated by the crossflow instability, resulting in heat flux stripes on the leeward side, making the transition front distribution more complex. Therefore, at small AoAs, the promotion effect of the BFS on the conical boundary layer transition is stronger than that of the FFS with the same height. Moreover, the FFS with a small height even exhibits a suppressive effect on the transition. However, as the AoA increases, low-frequency instability modes, such as crossflow instability, gradually become dominant. Consequently, the promoting effect of the FFS surpasses that of the BFS.