Virtual and Physical Surface Modifications as Means for Flow Control

Abstract

This paper reviews recent work on several flow control actuators, as well as a new hybrid actuator, and their interaction with a laminar boundary layer over a flat plate. The focus is on 1) a finite-span synthetic jet, where the effects of the jet orifice aspect ratio and its skew and pitch angles relative to the flow are presented; 2) a finite-span low-aspect-ratio static or dynamic pin (with similar height to the local boundary layer), where the effect of the pin’s aspect ratio is discussed; and 3) a hybrid actuator, which incorporates the previous two actuators. First, the formation and evolution of flow structures associated with a synthetic jet issued into a laminar boundary layer, where the effect of the orifice’s aspect ratio, as well as its pitch and skew angles, are explored. Next, the effects of the pin’s aspect ratio and its dynamic motion are shown. Finally, the interaction of the hybrid actuator with the flow has shown that the combined effects of the synthetic jet and the pin resulted in alteration of the vortical structures with minimal penalty. This suggests that the hybrid actuator might be efficient for controlling different types of flowfields. An example of controlling separation over a flapped airfoil was also included, where an array of passive cylindrical pins yielded delayed or complete mitigation of separation. This was mostly due to the introduction of streamwise vortices, which redirected the flow toward the surface and energized the boundary layer. Moreover, the effect of a single dynamic pin on flow separation was larger than that of a single static pin.