Investigation of impulse enhancement of a train of vortex rings via the vortex nozzle effect

The vortex nozzle effect, wherein the impulse of a vortex ring is enhanced via passive interaction with a wall with co-axial aperture, has been previously demonstrated for an isolated ring. For ring-to-aperture radius ratios near one, the hydrodynamic impulse of the ring is enhanced due to fluid entrainment during the collision increasing ring volume and minimized circulation loss due to the low-velocity core region of the ring enveloping the aperture tip, which reduces opposite-sign vorticity production. The vortex nozzle was shown to increase impulse up to 11% compared to a freely advecting ring for a ring Reynolds number of 3000. What remains unknown is whether the same impulse enhancement arises for a train of vortex rings, wherein inter-ring interactions may alter the dynamics near the aperture. In this study, the dynamics of a train of vortex rings advecting towards a wall with a co-axially aligned aperture is explored numerically and experimentally for source Reynolds numbers from 1000 to 3000 (based on generating orifice diameter and time averaged orifice exit velocity) and various inter-ring spacings. Ring geometry, circulation, and impulse are examined to assess the viability of the vortex nozzle phenomenon as an impulse enhancement method for synthetic jets.