Repid growth of unsteady finite-amplitude perturbations in a supersonic boundary-layer flow

Spatial numerical simulations (SNS) are performed to study transition to turbulence in a supersonic flat plate boundary layer forced by finite amplitude perturbations, where the freestream Mach numbers is 2.5 with the isothermal wall condition. Three-dimensional isotropic disturbances are superimposed on the laminar profile for Reynolds number based on the displacement thickness δ^∗ of 1000 at the inlet plane of the computational domain. The effects of the peak location of the disturbance spectrum and the magnitude distribution in the wall normal direction are analyzed. Numerical results indicate that energy spectra with a peak located at around the wavenumber of kmax=1.0 play an important role to induce transition to turbulence. The disturbance at lower wavenumbers with concomitant turbulence in the free stream shows an early appearance of hairpin-like structure in the transition region. The vortical structures on set of transition to turbulence rapid evolving to a hairpin packet with fine secondary structures downstream are shown in this paper, which is an inherent structure induces transition to turbulence in the supersonic boundary layer.