Focused Laser Differential Interferometry for Hypersonic Flow Instability Measurements with Contoured Tunnel Windows

Abstract

A focused laser differential interferometer (FLDI) was developed for use with the Boeing/AFOSR Mach 6 Quiet Tunnel (BAM6QT) at Purdue University. To test the initial FLDI, density-fluctuation measurements were made in a shock tube. The modified FLDI design was then made to work with the contoured windows of the tunnel, and its spatial sensitivity was tested using density-fluctuation measurements of a small-diameter air jet. Second mode fluctuation measurements were made on a 7 degree half-angle cone at 0 degrees angle of attack. Both conventional-noise and quiet-flow measurements were made with good agreement between the FLDI and associated surface-pressure measurements. The density-fluctuation amplitudes increase to a peak and then decrease with increasing distance from the model surface, agreeing with computational predictions. Cross-correlation of the FLDI and PCB signals resulted in an estimated disturbance velocity in the downstream direction, as well as the phase shift off the model surface. A balanced photoreceiver results in increased signal-to-noise ratio as compared to a single photodiode receiver.