Quantitative focused laser differential interferometry with hypersonic turbulent boundary layers

Abstract

The effect of turbulent wind-tunnel-wall boundary layers on density change measurements obtained with focused laser differential interferometry (FLDI) was studied using a detailed direct numerical simulation (DNS) of the wall from the Boeing/AFOSR Mach-6 Quiet Tunnel run in its noisy configuration. The DNS was probed with an FLDI model that is capable of reading in three-dimensional time-varying density fields and computing the FLDI response. Simulated FLDI measurements smooth the boundary-layer root-mean-square (RMS) profile relative to true values obtained by directly extracting the data from the DNS. The peak of the density change RMS measured by the FLDI falls within 20% of the true density change RMS. A relationship between local spatial density change and temporal density fluctuations was determined and successfully used to estimate density fluctuations from the FLDI measurements. FLDI measurements of the freestream fluctuations are found to be dominated by the off-axis tunnel-wall boundary layers for lower frequencies despite spatial suppression provided by the technique. However, low-amplitude (0.05%–5% of the mean density) target signals placed along the tunnel centerline were successfully measured over the noise of the boundary layers (which have RMS values of about 12% of the mean). Overall, FLDI was shown to be a useful technique for making quantitative turbulence measurements and to measure finite-width sinusoidal signals through turbulent boundary layers, but may not provide enough off-focus suppression to provide accurate freestream noise measurements, particularly at lower frequencies.

Displacement of hypersonic boundary layer instability and turbulence through transpiration cooling

“…Several recent publications discussed the depth-of-field of FLDI in more detail with some particularly focusing on its ability to penetrate turbulent environments around an object of interest. 70,74,76,77 All those sources reveal dependencies of the depth-of-field on the frequency, wavelength, and amplitude of density disturbances within the probed flowfield. Therefore, additional post-processing steps need to added to investigate the performance of the Oxford FLDI system as it is not possible to simply compare the FWHM of the sensitivity at 300 kHz to the beam-wise width of the boundary layer.…”

“…4(a), as the FLDI's response depends on both parameters. 70,74,76,77 A 300 kHz frequency correspond to k % 1:1 mm sound wavelength at standard conditions, whereas the second mode wavelength is around 2.5 mm in the relevant tunnel flow, 42 rendering the FWHM from the Gaussian fit above imprecise. Ceruzzi and Cadou 74 present an overview of a transfer function for FLDI systems, by reproducing and combining work from Parziale et al 70,78 and Schmidt and Shepherd.…”

“…Benitez et al 76 used DNS to simulate a signal of interest, which was framed by turbulence. Their simulation of FLDI responses confirmed that relying on the axial diminution of its sensitivity was justified for high frequency measurements, but less acceptable for larger wavelengths and lower frequencies.…”

Wall-Normal Focused Laser Differential Interferometry

“…A comparison regarding the amplitude of the freestream disturbances is void, since the absolute value of the optical phase difference (Δs) depends on the individual measurement setup, and thus the prerequisite for an objective evaluation is missing. Although the laser beams are expanded as they traverse the nozzle boundary layer and thus primarily highfrequency signals are attenuated, studies [4,18,22] showed that the measurement signal may also contain shares of the low frequency density fluctuations from the nozzle boundary layer. (Fig.…”

Spatial Fluctuations of Optical Turbulence Strength in a Laboratory Turbulence Simulator

“…The characterization of RB convection provides essential insights to mitigate the effects of optical turbulence on laser beams in random media [18]. This is why convective RB turbulence simulators have received significant attention [10,11,[17][18][19][20]. Many experiments have focused on analyzing the light intensity and phase fluctuations of convective RB turbulence simulators with water and air as media through the path-integrated received light fields [14,18,20].…”