Modification of flow behind a circular cylinder by steady and time-periodic blowing

We apply steady and time-periodic blowing, respectively, to flow over a circular cylinder at a Reynolds number of 21 000 for the modification of flow in the wake. The blowing is applied near the separation point on the upper and lower surfaces from either a slit (uniform blowing) or eight holes (discrete blowing) along the spanwise direction at each surface. For steady blowing, the blowing momentum coefficient ranges from 0.008 to 0.269, and, for time-periodic blowing, it does from 0.013 to 0.162. The changes in the drag are estimated by a momentum theorem with the velocity measurements in the wake. The steady discrete blowing is more effective and efficient for drag reduction than the steady uniform blowing, providing a maximum drag reduction of 38%. Counter-rotating vortices induced by the optimal discrete blowing change the Kármán vortex cores into three-dimensional vortices and weaken their strength. On the other hand, steady uniform blowing significantly increases the wake width and leads to drag increase. With time-periodic blowing, the maximum drag reductions by uniform and discrete blowing are 68% and 28%, respectively, indicating that the time-periodic uniform blowing is more effective than the steady discrete blowing. For the time-periodic blowing, two different vortex lock-on phenomena (symmetric and asymmetric vortex shedding in the wake) are found, and they lead to a significant drag decrease and increase, respectively.