An overlap of two heating peaks in a Mach 6 hypersonic boundary layer over a flared cone with a small nose bluntness at a freestream Reynolds number of $10.0\times {10}^6{\mathrm{m}}^{-1}$ is experimentally observed. It is found that a small bluntness can only delay the evolution of the second mode, which reduces but does not eliminate the dilatation-induced heating peak before the shear-induced heating peak. The approach of the saturation location of the second mode and the onset location of transition indicated by the broadening disturbance spectrum leads to shorter hot streaks and low-temperature valleys over the blunt cone. The simulations using the random forcing replicate the measured instability spectra qualitatively. A heating decomposition is conducted to confirm that the initial peak over both cones is due to the second mode.

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• Compressible boundary layers
• Compressible flows
• Flow instability
• High-speed flow
• Transition to turbulence
• Direct numerical simulations
• Infrared techniques
• Optical interferometry