Axis switching in low to moderate aspect ratio rectangular orifice synthetic jets

The dynamics of finite-span vortex rings expelled from a synthetic jet actuator is studied experimentally using stereoscopic particle image velocimetry as well as being modeled with an inviscid Biot-Savart velocity induction solver. Five rectangular orifices are tested with aspect ratios AR of 2, 4, 6, 13, and 19 at a single actuator stroke length. In the lower synthetic jet, vortex ring axis switching is the dominant factor influencing the jet's shape, while in the upper jet, viscous diffusion plays a more critical role. The variations in the jet's width driven by vortex ring axis switching becomes more extreme with increasing orifice AR. The height of the first axis switch also increases with AR, and for the three lowest AR values tested, the jet axis switches two to three times. However, at orifice AR of 13 and 19, the jet only axis switches once. The lack of additional axis switching is shown to be due to a collision of the vortex ring with itself after the first axis switch and a subsequent bifurcation of the vortex ring. The critical AR limit above which vortex ring bifurcation occurs is found to be consistent with prior work on isolated vortex rings. The axial profiles of centerline velocity for the AR = 4-19 jets exhibited two local peaks which become more prominent with increasing jet AR. These variations in centerline velocity are also predicted by the inviscid solver, indicating that they are most likely due to the dynamics of the primary vortex ring and not secondary structures as previously hypothesized.