Focused Laser Differential Interferometry with Contoured Tunnel Windows

Abstract

A unique focused laser differential interferometry (FLDI) configuration designed to work with contoured windows was developed for use with the Boeing/U.S. Air Force Office of Scientific Research Mach 6 Quiet Tunnel (BAM6QT) at Purdue University. The modified FLDI uses cylindrical lenses to help shape the beam profile together with the curved windows. The design was informed by a ray-tracing simulation of the apparatus. The spatial sensitivity of the BAM6QT FLDI was tested using density-fluctuation measurements of a small-diameter air jet and compared to a traditional FLDI. To validate the BAM6QT FLDI’s performance with the tunnel, second-mode fluctuation measurements were made on a 7 deg half-angle cone at a 0 deg angle of attack. Both conventional-noise and quiet-flow measurements were made with good peak frequency 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 surface pressure sensor signals resulted in an estimated disturbance velocity in the downstream direction, as well as the phase shift off the model surface.