Spanwise localized control for drag reduction in flow passing a cylinder

Active and passive controls for drag reduction in flow around a cylinder are obtained by computing the sensitivity of drag with respect to surface velocity perturbations and roughness, respectively. Both controls are concentrated around the separation line and localized in the spanwise direction, producing suction effects to the separating boundary layers. In the wake, the control induces localized vertical displacements and streamwise stretches of the upper and lower vorticity sheets, and subsequently delay the vortex shedding and push the local pressure minimum away from the cylinder. Instead of suppressing separation and recirculation as commonly observed in two-dimensional controls, the present three-dimensional control extends the recirculation zone to produce a virtual surface converting the bluff body flow to a streamlined body flow. Through this mechanism, the control reduces drag by 20A % at maximum control velocity 2A % of the free-stream velocity (or momentum coefficient) at Reynolds number. The control is much more efficient than the previously tested spanwise uniform suction or periodic suction/blowing, both requiring maximum control velocity above 8A % (or momentum coefficient above) to achieve similar drag reduction effects. The power savings ratio, defined as the ratio of the control-reduced drag power and the maximum input power, is above 20, up to, the highest Reynolds number considered in this work. This ratio reduces slightly to 17.8 when the control is simplified to spanwise localized suction around the separation lines in order to facilitate practical implementations.