Experiments on Fluid-Structure Coupling Under Impinging Shock Wave Loading

An impinging shock wave/turbulent boundary layer interaction over a compliant panel was experimentally examined in a Mach 4 Ludwieg Tube through simultaneous measurements of both the fluid and structure response. Synchronized measurements were obtained using colinear focused laser differential interferometry and high-speed schlieren together with single-camera stereo-photogrammetry. The static and dynamic behavior of the compliant panel itself was also characterized through response to pressurization, roving hammer, and panel flutter experiments. When the compliant panel was subjected to shock impingement at the mid-chord and leading-edge locations, the initial panel response was dominated by oscillations at the fundamental vibration mode and was primarily governed by one-way uncoupled fluid-structure behavior. However, under leading-edge impingement, oscillations at the fundamental vibration mode eventually dampened, and a transition to a two-way coupled fluid-structure response was observed. Under two-way coupling, the interaction was characterized by a reduction in oscillation amplitudes and an increase in relative strength of high-order vibration modes. Phase analysis at the fundamental vibration mode between the separation and reattachment shock motion revealed that the one-way uncoupled mid-chord impingement case exhibited a repeatable phase alignment, where the phase of the separation shock tended to lead that of the reattachment shock by approximately one quarter of a panel oscillation. However, consistent with the behavior in rigid wall shock wave/boundary layer interactions, the instantaneous reattachment motion was anti-correlated and led that of the separation shock in all compliant cases. The transition to a two-way coupled response strengthened the synchronization between the separation shock and panel motion compared to that of the one-way uncoupled response. Spectral analysis of the mid- and high-frequency content of density fluctuations in the separation bubble and boundary layer showed that the spectral distributions were not significantly modified due to surface compliance. Instead, the magnitude of the density fluctuations inside the separation bubble were attenuated under mid-chord impingement compared to that of leading edge impingement. Moreover, in all compliant cases, surface compliance amplified the spectral energy in the boundary layer downstream of flow reattachment by over a factor two compared to that of the rigid case.