Penetration of oblique shock wave through different thickness boundary layers

Abstract

Compression flows, characterized by high Reynolds numbers, typically sustain intense temperature gradients along the wall-normal near the surface due to high viscous dissipation, resulting in a thicker, more unstable developing turbulent boundary layer. In scenarios with large streamwise scales, such as scramjet inlets, where numerous shock waves are present, interactions between varying boundary layer thicknesses and oblique shocks are prevalent. This study investigates these interactions using a configuration involving two different plate turbulent boundary layers, generated by roughness elements arrayed at the leading edge of the plate, and a 37.5° oblique shock. Flow visualization, conducted using Nano-tracer Planar Laser Scattering (NPLS), shows that vortex heads are squeezed into small-scale structures by the discontinuous shock. Velocity fields, measured by Particle Image Velocimetry (PIV), combined with a novel density calibration scheme proposed in this study, are employed to quantitatively analyze the impact of shock waves on the two different thickness boundary layers. The separation region exhibits a similar wall-normal direction scale in both cases, while the streamwise scale extends to 3.4 mm in the thin boundary layer and 4.4 mm in the thicker one. Significant density fluctuations are observed in the reattachment region. Spectral analysis at four wall-normal positions highlights notable distinctions depending on boundary layer thickness. Due to the impingement of the strong oblique shock, the turbulent boundary layer downstream of separation region, originating from the reattachment region, becomes more unstable than the upstream boundary layer, with the power of the universal spectrum increasing by 5-10 times compared to upstream regions. These findings indicate that turbulent boundary layers downstream of multiple shock-wave boundary layer interaction regions exhibit more complex and unstable characteristics, distinct from naturally developing boundary layers.