Structural characteristics of a supersonic turbulent boundary layer subject to convex-concave curvature coupling

With an inflow Mach number of 3.0, the structural characteristics of a supersonic turbulent boundary layer subject to a convex-concave coupled curvature are experimentally investigated by Nano-tracer Planar Laser Scattering (NPLS) technology. The NPLS images reveal intricate flow features, including multi-scale vortices, near-wall streaks, and oblique shock waves. Wall pressure measurements indicate a feedback mechanism between the turbulent structures and wall pressure within the curvature coupling region, resulting in the upstream shift of the minimum pressure point into the convex wall. Additionally, the oblique shock generated by the concave curvature leads to an increased wall pressure in the downstream flat plate. By utilizing fractal dimension analysis and two-point spatial correlation analysis, the complexity and morphology of the turbulent vortices are quantitatively described. It is found that no significant changes in fractal dimension near the curvature coupling point due to the hysteresis effect. Furthermore, the coherent structures in the inner boundary layer shrink before and after the curvature coupling point, also attributed to hysteresis. The turbulence structures in the middle layer show lower sensitivity to the convex curvature, indicating distinct responses of turbulence structures at different normal positions to the wall's geometric features. Statistical analysis of the vortex structures, including maximum Feret diameter, aspect ratio, and inclination angles, further confirms the enhanced vortex breaking up caused by the concave wall and the increased size disparities of vortex structures due to curvature variations.