Evolution of the elliptical synthetic jet at low aspect ratio and formation length

Synthetic jets have emerged as a versatile tool for flow control of boundary layers, propulsion, etc. Non-circular synthetic jets offer more complex flow phenomena than circular synthetic jets, thus making them more suitable for flow control applications. The experiments to characterize the flow in a low aspect ratio (AR = 2) elliptical synthetic jet (ESJ) issued in a quiescent environment are conducted for two formation lengths (Lo = 1.4 and 2.6) with respective Reynolds numbers of 280 and 520, utilizing planar time-resolved particle image velocimetry (TRPIV) and laser-induced fluorescence (LIF) flow visualization. The evolution of the synthetic jet is studied using phase-averaged flow structures and vortex core locations, which reveal that at Lo = 1.4, the phenomenon of vortex ring bifurcation occurs, which is generally assumed to be a high aspect ratio phenomenon in the literature. A complete axis-switching is observed for Lo = 2.6. The statistical flow characteristics reveal that the flow fluctuations in the elliptical synthetic jet (ESJ, AR = 2) are more complex than the circular synthetic jet (CSJ, AR = 1). The vortices are more stretched in the case of ESJ than CSJ due to enhanced vortex interaction in the former case, which enables higher momentum exchange and increased flow fluctuations. The vortex strength of ESJ during axis-switching (Lo = 2.6) is higher than the vortex bifurcation case (Lo = 1.4). The fluctuating kinetic energy budget of the elliptical synthetic jet (ESJ) obtained in both minor axis plane (X-Y plane) and major axis plane (X-Z plane) is compared with that of the circular synthetic jet (CSJ). The production term is found to be the most significant one in both minor and major planes. The transport of the energy due to flow fluctuations acts as a source term in the shear layer region of the vortex rings, while it behaves as a sink in the vortex core regions. The two surrogates of dissipation rate obtained based on resolved velocity gradients are compared, and it is found that the axisymmetric dissipation rate resembles the flow better. The present study shows that the elliptical synthetic jet (ESJ) has more complex flow physics than the circular synthetic jet (CSJ) and thus can be useful for various heat transfer and flow control applications.