Impact of Beam Clipping and Size on Focused Laser Differential Interferometry in Hypersonic Disturbance Measurements

The sensitivity and spatial resolution of focused laser differential interferometry (FLDI) improve with a lower f-number, indicating that a larger beam diameter on the field lens is beneficial. However, when testing a wider flat plate model and probing within the boundary layer, the FLDI beam often gets clipped at the model's edge. This paper investigates the FLDI system’s response with a clipped beam, reduced beam diameter and modified beam shape through experiments conducted in the T4 Staker tube, measuring free stream disturbances and second-mode instabilities. Free stream results show a mere change in the signal-to-noise ratio and turbulent-based intensity values for the clipped and smaller diameter beam compared to an unclipped one, despite a change in the spectral slope. Nevertheless, information like second-mode frequencies is preserved and accurately measured for the unconventional FLDI systems. Validation against spectra from a modified cylindrical FLDI technique and higher energy mode images from modal analysis supports these findings. An FLDI system with a simple modification from spherical to elliptical beam would be helpful in future studies when a wider model is used instead of a complex cylindrical FLDI system.