Measurements of Panel Vibration with DIC and LDV Imaged Through a Mach 5 Flow

Optical measurement techniques such as digital image correlation (DIC) and laser Doppler vibrometry (LDV) are advantageous to measure structural vibrations due to their non-contact nature. While these techniques are immune to electromagnetic interference, they can suffer from optical distortions due to index-of-refraction gradients associated with boundary layers, shock, shock and expansion waves, and combustion. When the optical distortion is due to unsteady processes, such as those associated with turbulence or other time-dependent flow phenomena, these effects cannot be easily accounted for and may result in erroneous measurements of the vibrations. An exploratory test campaign was performed to characterize these errors in a flow-structure interaction experiment conducted in a Mach 5 low-enthalpy blowdown wind tunnel. The flow-induced vibration of a compliant brass panel (0.25 mm thick) was measured using 3D-DIC and LDV, by imaging the panel through the flow. Additional optical distortions were generated by a 27.5 \({ }^{\circ }\) compression ramp that was installed on the floor of the tunnel and generated a shock-induced, turbulent separated flow. Results showed that the LDV data were not affected within the uncertainty of the measurement by flow-induced optical distortions. LDV and DIC results generally agreed well for frequencies between 250 Hz and 2000 Hz, which contained all dominant vibration modes. The shock unsteadiness was expected to generate dynamic distortions for all locations that were measured through the shock. No evidence of this was found, which indicates that such distortions were below the noise floor of the 3D-DIC setup used for this experiment. In one test, the local discrepancy between DIC and LDV differed more when imaged through the shock than for a case where both systems measured the vibration upstream of the shock.