Focused Laser Differential Interferometry Performance Through Wind Tunnel Boundary Layers

Abstract

A focused laser differential interferometer (FLDI) model that is capable of simulating time-varying signals was used to study how an FLDI responds to a realistic wind tunnel boundary layer. Initial tests using a sinusoidal approximation for test-section-wall boundary layers showed the FLDI strongly favored the signal of interest at the focal point to signals present along the wall. Direct numerical simulation (DNS) was then used to model the turbulent boundary layer along the nozzle wall of the Boeing/AFOSR Mach-6 Quiet Tunnel (BAM6QT) as run in its noisy configuration. The simulated FLDI probed the boundary layer, and its results were compared to the true density fluctuations. The FLDI values slightly underestimated the true peak density fluctuations and also smoothed out the RMS peak. When used to simulate the FLDI traversing the test section through, the signal from the turbulent boundary layers was shown to persist despite FLDI suppression. Low-amplitude sinusoidal signals were still capable of being measured by the simulated FLDI over the interference of these boundary layers.