The effect of the wake from a circular cylinder on boundary-layer transition

Results are given of systematic experiments which aim to clarify the relation between the behavior of the wake from a circular cylinder within a laminar boundary layer and the boundary layer transition. The behavior of the wake is found to be different according to the gap width between a circular cylinder and a flat plate, the boundary layer thickness, and the distance from the origin of boundary layer to a circular cylinder, and it is classified into three regions. In region I, no vortex shedding from a circular cylinder is found, and the process of boundary layer transition is substantially the same as the process caused by a roughness element attached to the plate. In region II, the periodic laminar vortex shedding is observed to decay in propagating downstream and the transition process is similar to that in region I. In region III, the regular vortex shedding develops rapidly into a turbulent wake and causes boundary layer transition.