On the use of Sanderson prisms for variable beam separation FLDI applications

The effect of variable beam separation on the performance of focused laser differential interferometry (FLDI) in detecting second-mode instabilities is investigated experimentally using what we believe to be a novel co-linear Sanderson prism-based FLDI and schlieren system. Experiments are conducted in a Mach-6 Ludwieg Tube on a 7° cone with a sharp nose tip. Beam separations of 100, 150, 250, and 400 µm are tested. The 250 µm case provides the clearest spectral peaks, highest coherence, and highest in-band to out-of-band root-mean-square (RMS) amplitude ratio. Overall, diagnostic performance improves with increasing beam separation, consistent with trends predicted by previous FLDI simulations. The reduced performance for the 400 µm separation is attributed to its lower position within the boundary layer. These results illustrate the importance of matching beam separation to the relevant flow structure scales.