Investigation of Focused Laser Differential Interferometry (FLDI) Sensitivity Function

Abstract

A method for modeling the sensitivity function of Focused Laser Differential Interferometry (FLDI) is evaluated. FLDI is an optical technique that aims to measure density disturbances in wind tunnel core flow while rejecting disturbances from the tunnel's sidewall boundary layers. The extent to which FLDI can accomplish this rejection is investigated by conducting multiple experiments where FLDI beams are translated through turbulent flows, simulating disturbances away from the instrument focus. The first set of experiments are conducted on a round, 3.2mm turbulent air jet using a bench scale FLDI. The next set of experiments are conducted on a 6.4cm wide Mach 3 tunnel using a large scale FLDI constructed for use in Arnold Engineering Development Complex Hypervelocity Wind Tunnel No. 9 (T9). These experiments validate a model for understanding the FLDI response function and provide a practical method for interpreting the FLDI signal.