Effect of surface imperfections on the transition of a Blasius boundary layer

The evolution to turbulence of a boundary-layer with surface imperfection was investigated. Three geometries were considered: backward and forward-facing steps (BFS and FFS), and a cavity. The step-sizes and cavity depth were a small fraction of the boundary-layer thickness. Transition was initiated using a narrow ribbon upstream of the surface imperfection, which generated small and monochromatic perturbations by periodic blowing and suction. A direct numerical simulation was performed. The transition was delayed for the small-step FFS case, whereas in all other cases it was accelerated. The BFS caused early transition, promoted by the instability of the separated shear layer. In the FFS a small region of local stabilisation was present immediately downstream of the step, where the Reynolds shear stress was negative. For the cavity, transiton was also accelerated, and the evolution of the flow features resembled that of the backward-facing step with a corresponding step height. Although the imperfections resulted, in most cases, in early transition, the features of a standard K-type transition were observed in all cases.