Influence of flat plate leading edge sweep and boundary layer state on unswept shock boundary layer interaction

An experimental investigation of spanwise variation in boundary layer development length acting on an unswept compression ramp shock boundary layer interaction (SBLI) has been conducted. A 26° unswept ramp was installed on a flat plate with leading edge sweep angles of ψ = 0° and 45° in Mach 3.9 free-stream flow. The SBLI structure has been observed for different boundary layer states (transitional and turbulent) for the same leading edge configurations. The incoming boundary layer at three spanwise locations is characterized from 2D PIV experiments as turbulent or transitional based on mean streamwise velocity profiles and the shape factor. An expected trend of decreasing fullness from root (H = 2.2) to tip (H = 2.0), consistent with decreasing Reynolds number, can be observed for the swept leading edge configuration with natural transition conditions. The incoming turbulent boundary layer is nominally 2D in all cases. Compared to the turbulent case, results indicate an increased streamwise interaction length with transitional incoming boundary layer conditions. This is despite the fact that the transitional boundary layer is significantly thinner than the turbulent one. Schlieren imaging, oil flow visualization, pressure fluctuations in the vicinity of the shock foot and PIV in a streamwise-spanwise plane show no clear evidence of 3D SBLI promoted by leading edge sweep. The low frequency motion near the start of the SBLI is approximately St_Lint = 0.09 for all cases. Results indicate that this particular transitional case is not too different from the fully turbulent case except that the scaled interaction is larger in comparison.