Effect of freestream noise and boundary-layer transition on hypersonic shock-wave/boundary-layer interaction unsteadiness

The low-frequency shock and separation unsteadiness resulting from a three-dimensional transitional shock-wave/boundary-layer interaction is investigated for different freestream noise levels and incoming boundary-layer states in a quiet Mach-6 wind tunnel. In low-disturbance flow, the cone-slice-ramp geometry is studied with and without upstream boundary-layer trips, resulting in upstream-transitional and interaction-transitional SWBLI, respectively. When applied, the trips excite the second-mode instability, and the unsteady separation motion is studied for a range of second-mode magnitudes in the transitional upstream boundary layer. The effect of freestream noise is studied by operating the wind tunnel in both its low-disturbance and conventional-noise configurations. Unsteady separation motion is tracked by identifying the shear-layer location in schlieren visualization videos; pressure fluctuations are measured in the incoming boundary layer and inside the separation bubble. Exciting the second-mode instability in the boundary layer in quiet flow initially increases the magnitude of the unsteadiness and alters its frequency content by amplifying oscillations at SrL= Sr L = 0.025–0.05; however, as second-mode amplitude increases and separation length decreases, this trend reverses and the unsteadiness is suppressed. The comparison of upstream-transitional SWBLI in quiet and noisy flows suggests that the pressure fluctuations in the noisy freestream are interacting with the mechanism responsible for SWBLI and significantly amplifying the unsteadiness over all frequencies.