To investigate hypersonic boundary layer transition and the effect of nosetip bluntness on transition, this study carries out experiments in both FD-14 and FD-14A shock tunnels. Integrated measurements including heat flux measurement, pressure disturbances measurement, and high-speed schlieren visualization are conducted. Results show that the transition Reynolds number as a function of Reynolds number based on nosetip bluntness exhibits a trend of transition-reversal. Power Spectral Density (PSD) analysis of pressure disturbances in the boundary layer is shown in the form of discrete contour maps in a streamwise direction. Development of second-mode waves, generation of turbulence and effect of the entropy layer on the boundary layer structure are visualized in images of high-speed schlieren. The entropy layer with an intense entropy gradient derived from large nosetip bluntness shows significant influence on the frequency spectral of pressure disturbances and flow structures of the boundary layer, playing an important role in the mechanism of transition-reversal. In addition, the effect of the Mach number should not be ignored.
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