Cylindrical Focused Laser Differential Interferometer

Abstract

A novel configuration of focused laser differential interferometry (FLDI) was developed, simulated, and tested. The new configuration is referred to as cylindrical FLDI or CFLDI. Using cylindrical lenses in place of standard lenses changes the three-dimensional shape of the focusing beam from a cone shape for FLDI to a triangular sheet for CFLDI, allowing for probing of flat-plate boundary layers closer to the wall. The beam propagation and interference pattern were modeled to determine the transfer functions for one-dimensional sinusoidal disturbances of varying wavelength () and varying spanwise thickness () of the tested area. CFLDI was employed to measure the density perturbations in supersonic blowdown wind tunnel SBR-50 at the University of Notre Dame with the test section width of 0.076.2 m. It was determined that the CFLDI system sufficiently filtered the sidewall boundary layers to measure the spectra of density fluctuations in a plane wall boundary layer. However, accurate measurements of the density disturbances in the freestream flow must account for the contribution of the sidewall boundary layers. This serves as an initial validation of the CFLDI maintaining spatial filtering similar to a regular FLDI.