Measurement of hypersonic turbulent boundary layer on a flat plate using cylindrical focused laser differential interferometer

A modified Cylindrical-Focused Laser Differential Interferometer (CFLDI) system was used to measure the density fluctuations generated by a Mach 6 turbulent boundary layer on a flat plate. The amplitude, spectral statistics, and correlation scale of the density fluctuations were analyzed at different wall-normal heights throughout the boundary layer. Direct numerical simulation was performed under the similar condition to verify the experimental results. The results show that the CFLDI system herewith can accurately represent the statistical characteristics of density fluctuations with a bandwidth of 3–200 kHz in the hypersonic turbulent boundary layer. As the distance from the wall increases, the amplitude of density fluctuations keeps increasing until y/δ ≈ 0.8 and levels off after y/δ ≈ 1.5. As the measurement position moves into the freestream, the characteristic frequency of the density fluctuation decreases rapidly, while its integral timescale gradually increases. Similar phenomena were also observed for pressure fluctuations.