Effects of upstream boundary layer on the unsteadiness of shock induced separation

The relationship between the upstream boundary layer and the unsteadiness of the separated flow in a Mach 2 compression ramp interaction is investigated by performing wide-field PIV and PLS measurements in streamwise-spanwise planes. Measurements in the upstream boundary layer indicate the presence of spanwise strips of elongated regions of uniform momentum with lengths greater than $40\delta$. These long coherent structures have been observed in a Mach 2 supersonic boundary layer and they exhibit strong similarities to those that have been found in incompressible boundary layers. At a wall-normal location of $y/\delta = 0.2$, the upstream envelope of the separation region is found to oscillate between $x/\delta =$ -3 and -1 (where $x/\delta = 0$ is the ramp corner). The instantaneous spanwise separation line is found to respond to the elongated regions of uniform momentum. It is shown that high- and low-momentum regions are correlated with smaller and larger scale for the separation region, respectively. Furthermore, the instantaneous separation line exhibits large-scale undulations that conform to the low- and high-speed regions in the upstream boundary layer. The low frequency unsteadiness of the separation region/shock foot observed in numerous previous studies can be explained by a turbulent mechanism that includes elongated regions of uniform momentum.