A combined experimental and numerical investigation of roughness induced supersonic boundary layer transition

• Roughness induced transition is studied using experimental and numerical methods.
• Good agreements are achieved between experimental results and numerical results.
• With increasing height of roughness, the transition tends to move forward.
• Two different types of transition mechanisms are found in this study.

The effect of surface roughness on boundary layer transition is of great importance to hypersonic vehicles. In this paper, both experimental and numerical methods are used to investigate the laminar-turbulent transition of a Mach 3 flat-plate boundary layer induced by isolated roughness element. Good agreements are achieved between experimental data and high-order numerical simulations. It is observed that, with increasing height of roughness, the transition tends to move forward. Two different types of transition mechanisms are found according to the height of the roughness elements. For the smallest roughness height of h=1 mm, the shear layer instability in the wake region appears to be the leading mechanism for transition to turbulence. For two larger roughness elements of h=2 mm and h=4 mm, strong unsteadiness is developed from the upstream separation zone and transition is immediately accomplished, which indicates that the absolute instability in upstream separation zone dominates the transition.