Transition initiated by a localized disturbance in a hypersonic flat-plate boundary layer

Direct Numerical Simulations (DNS) were performed to investigate transition initiated by a localized disturbance in a hypersonic flat-plate boundary layer. In order to model a natural transition scenario, the boundary-layer was forced by a short duration (localized) pulse through a hole on the flat-plate. The pulse disturbance developed into a three- dimensional wave packet which consisted of a wide range of disturbance frequencies and wave numbers. First, the linear development of the wave packet was studied by pulsing the flow with a low amplitude (0:001% of the freestream velocity). The dominant waves within the resulting wave packet were identified as two-dimensional second mode disturbance waves. Hence the wall{pressure disturbance spectrum exhibited a maximum at the spanwise mode number κ = 0. The spectrum broadened in downstream direction and the lower frequency first mode oblique waves were also identified in the spectrum. However, the peak amplitude remained at κ = 0 which shifted to lower frequencies in the downstream direction. In order to investigate the nonlinear transition regime, the °ow was pulsed with a higher amplitude disturbance (5% of the freestream velocity). The developing wave packet grows linearly at first before reaching the nonlinear regime. The wall pressure disturbance spectrum confirmed that the wave packet developed linearly at first. The response of the flow to the high amplitude pulse disturbance indicated the presence of a fundamental resonance mechanism. Lower amplitude secondary peaks were also identified in the disturbance wave spectrum at approximately half the frequency of the high amplitude frequency band, which would be an indication of a subharmonic resonance mechanism. The disturbance spectrum indicates however, that fundamental resonance is much stronger than subharmonic resonance.