Direct numerical simulation of oblique-wave transition in concave boundary layer

Investigation of transition in a concave boundary layer is conducted via three-dimensional direct numerical simulation at Mach 3. The model consists of a flat plate and a concave plate, connected smoothly. The development of the boundary layer in the unperturbed flow is computed initially. It is found that the boundary layer thickness rapidly increases due to the separation bubble, caused by an adverse pressure gradient. Subsequently, spanwise vortices are generated by the Kelvin-Helmholtz instability, which develops within the strong shear layer. Then, a pair of oblique waves is introduced at the inlet of the computational domain through suction and blowing slot to examine the impact of oblique waves on transition and separation of the concave plate boundary layer. The investigation reveals that oblique waves significantly reduce the separation bubble and the boundary layer thickness and weaken the Kelvin-Helmholtz instability. Oblique waves generate streamwise vortices, while high-amplitude oblique waves lead to a three-dimensional checkerboard structure and staggered Lambda vortices. The findings demonstrate that oblique breakdown can advance to a fully developed turbulent boundary layer, hence operating as a relevant mechanism for transition in supersonic concave boundary layers.