Effect of orifice shape on three-dimensional vortex structure of a synthetic jet

Abstract

Synthetic jets are produced by periodic ejection and suction of fluid from an orifice induced by movement of a diaphragm inside a cavity, and a synthetic jet actuator is a useful tool for active flow control. The synthetic jet actuators are low operating power, zero-net-mass-flux and very compact devices which have demonstrated their capability in modifying the subsonic flow characteristics for boundary layer flow control. For the synthetic jet, the effect of the orifice shape on the vortex ring behavior has not been completely clarified yet. In the present study, the deformation process of the vortex ring in a quiescent fluid formed by a synthetic jet and the interaction between the vortex ring and a crossflow were investigated by using three types of orifice shape (a circular and two rectangular orifices). Furthermore, the three-dimensional vortex structure in the boundary layer was depicted by means of a scanning stereoscopic PIV measuring system. The breakdown of the vortex ring in quiescent flow occurs at a short distance from the orifice exit for the rectangular orifice. In the case of the synthetic jet into the crossflow, the jet deflects towards the crossflow direction with a shorter distance and vortices exist near the lower wall for the rectangular orifice with the long side of the orifice set in the spanwise direction in contrast to the other orifice shapes. 渦度 ωxが存在している領域の面積を考慮して算出した．つまり，領域内にある渦度とその領域の面積から循環を求めている．領域を決める際の渦度 ωxは，最大渦度の 70 %の値までを使用した．最大値の 70 %までを選んだ根拠は，領域の境界を明確に判断できることから，計算時の誤差を少なくするためである