Flow control over a circular cylinder using a slot and axially arranged holes

Circular cylinder flow is controlled using a slot and axially arranged holes (AAH) at a Reynolds number of 32000. When the slot and AAH are placed at small incidence angles from the free-stream direction, the passive jet generated on the leeward cylinder side pushes the wake vortex downstream, reducing the drag compared to the baseline cylinder. If the incidence angle is larger than the critical angle, the alternate blowing and suction induced by the slot and AAH promote a boundary-layer transition on one side and a shear-layer transition on the other. The earlier transition to turbulent flow substantially shortens the separated shear layers on both sides, shifting the wake formation region toward the base and leading to an increase in drag. The AAH has a higher critical incidence angle than the same-size slot, as the passive jet developed from the AAH undergoes a transition to alternate blowing and suction at a higher incidence angle than the slot. Just before the incidence angle of the AAH reaches the critical angle, the weak jet flow induced on the windward cylinder side delays the boundary-layer separation through early separation and laminar reattachment without shortening the separated shear layer, thereby allowing additional drag reduction. •Flow around a cylinder is experimentally controlled using a spanwise slot and axially arranged holes (AAH).•AAH reduces drag over a wider range of incidence angles than the slot of the same size.•Effects of the slot and AAH on the boundary layer, separated shear layer, and near wake are systematically investigated.•Mechanisms for drag variations are explained based on the flow modifications by the slot and AAH.