Experimental investigation on the impingement of elliptic synthetic jet vortex rings onto a porous wall: Effect of orifice aspect ratio

Time-resolved stereoscopic particle image velocimetry is employed to investigate elliptic synthetic jet vortex rings impinging onto a porous wall. The three-dimensional flow evolutions from vortex generation to impingement and subsequent transmission through the porous wall are captured by the phase-locked reconstruction method. Experiments are conducted under a constant orifice-to-wall distance (H₀/D₀ = 5) and wall porosity (ϕ = 50%), while four orifice aspect ratios (AR = 1, 3, 5, and 7) are selected to examine geometric effects. All elliptic vortex rings undergo axis switching before impingement, with switching intensity increasing with AR. For AR = 7, pronounced axis switching triggers vortex-ring bifurcation into two sub-vortex rings. During their interaction with the porous wall, a vortex bridge forms between the sub-rings and promotes their merging into a single vortex ring, establishing a vortex bridge–mediated merging mechanism. The porous wall significantly attenuates the transmitted vortices, with circulation and momentum flux losses increasing with AR. Enhanced axis switching and bifurcation strengthen upstream entrainment, resulting in increased radial momentum transport and up to a 35% elevation in peak streamwise mass flow rate compared with the circular case. However, higher AR accelerates downstream vortex decay and reduces momentum transport persistence due to enhanced dissipation and momentum flux loss. These findings demonstrate that aspect ratio fundamentally controls vortex topology, entrainment, and momentum transport, and reveal a trade-off between enhanced upstream mixing and reduced downstream momentum delivery, providing guidance for the design of applications involving elliptic synthetic jet vortex rings impinging on a porous wall.