Proper Orthogonal Decomposition Analysis of Swept-Ramp Shock-Wave/Boundary-Layer Unsteadiness at Mach 2

This study investigates the large-scale unsteadiness of the 3D shock-wave/boundary-layer interaction generated by a 22.5 degrees compression ramp with 30 degrees sweep in a Mach 2 flow. This study extends previous work to examine the possible unsteadiness mechanisms of the separation line in differing frequency bands. This is achieved through ensemble averaging and lower-order proper orthogonal decomposition reconstructions that are targeted to extract certain physical processes. Unsteadiness is examined within three frequency bands: low frequency (St<0.01), midfrequency (0.01 < St < 0.1), and high frequency (St > 0.1). Each frequency band contains a very similar degree of kinetic energy (similar to 30%) and likely contributes equally to the movement of the separation line. The low- and midfrequency unsteadiness of the separation line continues to show a strong correlation with the inflowing boundary-layer velocity, with boundary-layer superstructures driving the midfrequency unsteadiness. The high-frequency unsteadiness appears related to coherent structures within the separation region, which form in alternating pairs of high- and low-velocity deficit and appear to cause the separation line to ripple as they travel with the cross-flow. The frequency of this type of unsteadiness is dependent on the cross-flow velocities and the size of the separation structures, a finding that agrees with the literature.