No abstract

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, spectrum statistics and correlation scale of density fluctuations were analyzed at different wall-normal heights across the boundary layer. Direct Numerical Simulation (DNS) under similar conditions was also performed to verify the accuracy of the experimental results. The experiments show that the CFLDI system herewith can accurately represent the statistical characteristics of density fluctuations with a bandwidth of 3~200 kHz. As the distance from the wall increases, the amplitude of density fluctuations peaks at y/ δ ≈ 0.8 and levels off after y/ δ ≈ 1.5. As the measuring position moves into the freestream, the characteristic frequency of the density fluctuations decreases rapidly, while its integral time scale gradually increases. Similar phenomena were also found for pressure fluctuations.

In this work, we studied the bluntness effect on the hypersonic boundary-layer transition over a slender cone at Mach 6 with interchangeable tips in a noisy Ludwieg tube tunnel before the so-called “transition reversal” phenomenon occurs. The evolution of instability waves is characterized using surface flush-mounted pressure sensors deployed along the streamwise direction within unit Reynolds number from 4E+ 6/m ≤ Reunit ≤ 10E+ 6/m, and the bluntness of the cone nose ranges from 0.1 mm to 5 mm. Power spectral density (PSD) of pressure fluctuation indicates that small nose bluntness (ReR ≤ 2000) has little influence on the evolution of instability waves along the hypersonic boundary-layer, whereas with a moderate nose size (2000 ≤ ReR ≤ 5000), the hypersonic boundary layer transition is delayed monotonically as the nose radius increases before the boundary-layer turns into fully laminar without instability waves. The delaying effect can be attributed to the increased entropy-layer swallowing distance with a large tip radius. Instability wave characterization reveals that the second mode instability wave plays a dominant role before the transition reversal happens. The quadratic phase locking of second mode instabilities can be identified by bispectral analysis, and it attenuates as the nose tip radius increases.