Significance of the smaller scales for hypersonic turbulent boundary layers with focused laser differential interferometry

While designed to suppress disturbances located away from its beam foci, focused laser differential interferometry (FLDI) is known to have some integration of signals along its optical axis. This is especially true for longer-wavelength signals, although smaller-scale flow structures also have a non-infinitesimal sensitivity length. This study investigates the performance of FLDI in a hypersonic turbulent boundary layer to better understand its behavior. FLDI simulations were conducted using both full-scale direct numerical simulation (DNS) and spatially averaged DNS inputs to directly assess the influence of smaller flow structures on FLDI measurements. The full-scale FLDI results indicate that integration along the optical axis likely results in lower FLDI amplitudes than for true point measurements. Comparison with the spatially averaged FLDI simulations reveals the significance of small-scale structures in FLDI signal roll-off and root mean square amplitudes. Further, the influence of FLDI setup parameters on the response across the frequency spectrum are analyzed. Circular, Gaussian beams with smaller widths are verified to present increased performance relative to elliptical or uniform-intensity beams. Also, measurements using distinct differentiation directions suggest an experimental way to measure turbulence isotropy scales. These results have notable implications for understanding hypersonic turbulent boundary-layer dynamics and interpreting experimental data. Distribution Statement A: Approved for Public Release; Distribution is Unlimited. PA# AFRL-2024-4678; Cleared 08/23/2024.