Investigation of the unsteady surface pressure field under a Mach 2 compression-ramp shock/boundary-layer interaction

The shock/boundary-layer interaction induced by a 20° compression ramp in a Mach 2 flow was investigated using fast pressure-sensitive paint with a bandwidth of about 10 kHz. The mean separated flow length-scale is about two upstream boundary layer thicknesses, which indicates the interaction is weak. The primary analysis consists of cross-correlations, coherence, and time-domain filtering. Two different frequency bands were investigated: low-frequency (f < 2000 Hz; StL < 0.1) and mid-frequency (0.1 < StL < 0.26). The low-frequency band time sequences and coherence reveal the shock-foot motion is mainly correlated with the reattachment region, which is indicative of the well-established breathing motion of the separation bubble. The breathing motion is observed to occur locally and globally (spanwise-averaged). Furthermore, in the low-frequency band, fluctuations in the upstream boundary layer are moderately correlated with the reattachment region fluctuations, but show no correlation with the intermittent region fluctuations. In the mid-frequency band, the intermittent region, separation bubble and reattachment region all exhibit significant correlation with the upstream boundary layer fluctuations, with the upstream fluctuations leading. The time-sequences in this frequency band reveal broad regions of pressure fluctuations that sweep through the interaction and affect the entire interaction. There is no known turbulent source for such large-scale fluctuations and they are believed to be due to a wind tunnel phenomenon. It is concluded that the dominant low-frequency breathing motion follows an oscillator model, but there remain significant correlations to upstream fluctuations that are not tied to the dominant breathing motion and seem to follow an amplifier model.