Unsteady Separation Shock Dynamics in a Mach 4 Shock-Wave/Turbulent Boundary Layer Interaction

High-speed schlieren image processing is applied to characterise the unsteady separation shock dynamics of a step-induced axisymmetric shock-wave/turbulent boundary layer interaction at Mach ∼4, enabling shock motion to be tracked from frame to frame and subsequently analysed within the frequency domain. Two approaches are tested: a first one which assumes the separation shock to be linear and which relies on the Hough line transform for edge detection and a second one which tracks axial shock motions at selected wall-normal heights. Results yield clear qualitative detail into the formation of the shear layer upon boundary layer separation and go on to show that both schlieren-based methods capture well the low-frequency pulsations of the separation bubble, finding good agreement with fast-response wall pressure measurements near separation. Clear evidence on the influence of the separated shear layer on interaction unsteadiness is further provided, with both methods documenting the relative enhancement in higher-frequency motions associated to shear layer shedding (acoustic disturbances from shear layer eddies are clearly shown in the visualisations as well). Given the linearity assumptions within the first method – which intrinsically averages the shear layer jitter effects along the shock wave – the latter is found to perform better in this sense.