Second-mode instability plays an important role in the transition of a hypersonic boundary layer. The second-mode wave on a flared cone at Mach 6 has been investigated experimentally through the Nanoparticle-tracer based Planar Laser Scattering (NPLS) technique in this paper. Different behaviors of the second-mode wave ranging from linear growth to deformation are visualized under different unit Reynolds numbers. The propagation velocity of the second-mode wave is quantitatively calculated, and the spectral properties of the second-mode wave have been analyzed by the power spectrum density. The characteristic frequency of the second-mode wave calculated from the NPLS images agrees well with the results measured by the transducers. Evolution of the wall-normal distribution of disturbance shows that high frequency harmonics are generated during the growth process of the second-mode wave and its normal distribution range shrinks with the increase in its frequency. The bispectral analysis indicates that the high degree of phase coupling may be the reason for the deformation of the second-mode wave. The nonlinear interaction between the disturbance waves is strong during the deformation stage of the second-mode waves.
The flowfield structure, heat flux distribution, and pressure fluctuations of the wall-mounted cylinder-induced hypersonic boundary-layer transition are investigated at a 10 deg angle of attack. Experiments are conducted in a Mach 6 low-noise wind tunnel using the nanotracer-based planar laser scattering (NPLS) technique, temperature-sensitive paints (TSP), and high-frequency pressure sensors. First, the streamwise and spanwise NPLS images, TSP results, and power spectral density results of isolated cylinders at different heights show that with the increase of the cylinder height [Formula: see text], the size of the separated region and the spanwise width of the horseshoe vortex increase, and the transition moves forward. Second, the flowfield structure and wall heat flux distribution around the streamwise cylinder arrays are investigated. The results demonstrate that the downstream cylinder will destroy the development of the hairpin vortex in the upstream cylinder wake but will expand the horseshoe vortex to both sides, increasing the influence area of the cylinder.
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