Nonlinear interactions in a hypersonic boundary layer

Nonlinear interactions between different disturbances in a hypersonic boundary layer are investigated by bispectral analysis. Experiments are performed on a flared cone at Mach 6 using fast-response pressure sensors and Rayleigh-scattering flow visualization. At the modal growth stage, the phase-coupled sum interactions have profound effects on the growth of the disturbances. Then, the difference interactions between different disturbances contribute to the spectral broadening. At the final stage, the nonlinear interactions between the second mode and low-frequency waves result in the stretching and deformation of the second mode, which breaks down to smaller structures rapidly and the flow transitions to turbulence.