Streak-Based Control for First-Mode Oblique Breakdown at Mach 4.5

Previous research demonstrated that streaks effectively suppress first-mode oblique breakdown in a Mach 2.0 supersonic boundary layer. However, whether streaks can be effective in delaying oblique breakdown in high-speed boundary layers is still unknown. The present study examines the control effect of streaks on the first-mode oblique breakdown in a high-speed boundary layer at Mach 4.5 using direct numerical simulation and stability analysis. The results show significant differences in control mechanisms. Firstly, the mean-flow distortion induced by streaks in the current study exerts a destabilizing effect on the oblique wave, in contrast to the suppression of the oblique mode reported in earlier work. Secondly, the control modes analyzed here, with a spanwise wavenumber twice that of the fundamental oblique wave, can delay the onset of oblique breakdown. When the spanwise wavenumber exceeds 2.0, the control mode results in a higher amplification rate of the fundamental oblique mode in the present study, whereas it was found to suppress the oblique wave in previous studies. Lastly, the streaks can suppress the receptivity process of the boundary layer to the oblique mode, which is the main reason for the transition delay.