Impact of a backward-facing step on oblique transition at Mach 6

The roughness element has the potential to control transition. The study investigates the impact of a backward-facing step on the oblique transition at Mach 6. The step's height is equal to the local boundary layer thickness. Complete transition processes are simulated by utilizing the implicit large eddy simulation method. The study compares and analyses the transition phenomenon of a backward-facing step to that without a step. In comparison to the oblique transition without a step, the flow with a backward-facing step results in a fundamental second mode transition. Modes decomposition results show that the backward-facing step reduces all instability modes, especially the oblique mode, which should have led to an oblique transition. All the harmonic modes are damped until a 2D disturbance dominates the hypersonic boundary layer flow. The delayed transition is attributed to the accelerated flow at the expansion region. The study indicates that a backward-facing step could be a promising tool for controlling hypersonic transition.