Investigation of oblique breakdown in a supersonic boundary layer at Mach 2 using DNS

The early nonlinear stages of transition in a supersonic boundary layer at Mach 2 are investigated using spatial Direct Numerical Simulations (DNS). The computational setup matches earlier experimental studies by Kosinov et al., where transition was triggered by localized forcing leading to the development of a wedge-shaped wave packet. While the focus of these experiments has been on a new breakdown mechanism, called asymmetric subharmonic resonance, our interpretation of the experimental data indicates the presence of another, possibly competing mechanism, which exhibits the characteristics of an oblique breakdown. If confirmed, this would be the first experimental evidence of the oblique breakdown mechanism in a supersonic boundary layer. With the simulations presented here, the possible presence of this breakdown mechanism in the experiments is explored by deliberately suppressing subharmonic resonances in the DNS and by comparing the numerical results with the experimental data. The DNS results show excellent agreement with the experimental measurements for both linear and nonlinear transition stages. Most importantly, our results clearly show characteristic features of the oblique breakdown mechanism as observed in our earlier numerical investigations.