Influence of Small Gap Ratios on Wake-Induced Boundary Layer Transition*

Wake-induced boundary layer transition due to a cylinder placed near a wall at Re_D=3900 and Re_θ=75 is investigated using direct numerical simulations. The ratio between the gap and the diameter is G=0.9. Thus, direct interaction between the vortices shed in the wake of the cylinder and the boundary layer results in flow transition. The transition mechanism is fundamentally different from that observed at high gap ratios (G>2.5), where the velocity forcing induced by the wake results in the standard bypass-transition scenarios. For G=0.9, a secondary coherent 2-D vortex forms on the wall due to the no-slip condition, which generates vorticity of the opposite sign at the wall, subsequently lifted by advection from the shed vortex. This vortex undergoes spanwise wavy deformations with lobes and troughs and eventually breaks down as it continues interacting with the wake vortices traveling close to the wall. This breakdown leads to the generation of hairpin vortices, resulting in the onset of turbulence. The wake-vortex dynamics vary significantly from one shedding cycle to the next, leading to different transition processes cycle-to-cycle. In some cases, the primary vortices in the wake directly interact with the secondary vortex, causing a faster transition to turbulence.