Control of flow around a circular cylinder by bleed near the separation points

The flow around a circular cylinder under bleed control is experimentally investigated in a water tunnel. The bleed jets are issued from the narrow slots directed from the front stagnation point to the positions near the upper and lower separation points. The Reynolds numbers based on the cylinder diameter are Re = 400, 780, and 1470, while the widths of the bleed slot are selected as w/D = 0.05, 0.1, and 0.2. The bleed jet interacts with the boundary layer, postponing the separation point to be near the downstream edge of the slot. It further modifies the wake shear layer, which moves away from the centerline. Thus, the recirculation bubble downstream of the circular cylinder is enlarged and the near wake width are increased, resulting in an increased vortex formation length and a decreased vortex shedding frequency. Such changes of the flow field increase with the bleed width. The vortex dynamics is also analyzed, showing that the wake pattern could be modified by the bleed control. The wake vortex is converted into a bistable mode at w/D = 0.1 and 0.2, Re = 780 and 1470, where the symmetric and asymmetric modes are both observed, while all the control cases at Re = 400 still show the asymmetric mode.