Implementation of Focused Laser Differential Interferometry for Measurements in Supersonic and Hypersonic Facilities at VKI.

This paper presents the implementation of focused laser differential interferometry (FLDI) in the S4 (Mach 3.5) and H3 (Mach 6) wind tunnels at the von Karman Institute for Fluid Dynamics (VKI). Two FLDI optical configurations were developed, each tailored to the geometrical constraints of the respective facility. Bench-top experiments using a turbulent jet were first conducted to evaluate the spatial response and sensitivity of both interferometer setups. FLDI measurements were then performed in the freestream of both tunnels, while additional probing targeted the nozzle boundary-layer region in S4 and the open-jet shear-layer region in H3. The spatiotemporal response of the systems was characterized using transfer functions, from which reliable frequency ranges were determined for estimating density-based turbulence intensity. Scaled RMS phase-change values are found to be proportional to the turbulence intensity based on density fluctuations, showing consistent trends with unit Reynolds number across all flow regions. Freestream turbulence intensity decreases with increasing unit Reynolds in both tunnels. In S4, the turbulence intensity ratio between boundary-layer and freestream regions is approximately 10:1, whereas in H3, the shear-layer-to-freestream ratio is about 6:1.