Separated laminar shear layer transition over a two-dimensional bump

The laminar boundary layer separation over a two-dimensional bump is experimentally investigated in a water channel with hydrogen-bubble visualization and particle image velocimetry (PIV) techniques, focusing on the characteristics of separated shear layer, the evolution of flow structure and the development of unstable disturbances. It is shown that the separation point, to a large extent, depends on the geometry of the bump, while the reattachment point is mainly affected by Reynolds number. A quite long and narrow separation bubble with its vortex centre located very close to the reattachment point is observed when the laminar boundary layer separates over the bump. The shear layer transition begins as soon as the laminar boundary layer flow is separated, with the flow field perturbation increasing rapidly to a maximum value and then decreasing slowly along the streamwise direction. The formation and shedding of two-dimensional spanwise vortex structures are observed along the shear layer due to Kelvin-Helmholtz instability. These spanwise vortices gradually lose their two-dimensionality and evolve into hairpin vortices downstream.