On Approximating the Translational Velocity of Vortex Rings

A method is presented whereby the translational velocity of a vortex ring can be approximated from the total circulation, impulse, and kinetic energy of the vortex system. Assuming a uniform vorticity density, these bulk quantities define a unique stable vortex ring configuration, and the translational velocity can be inferred from this configuration and the system scaling. Here, the accuracy of this approximation is presented for vortex rings formed from starting jets, and the translational velocity is also characterized as it relates to the driving parameters. The translational velocity is well approximated for a wide range of experimentally generated vortex rings. It is observed that starting jets with a converging radial velocity create vortex rings with a significantly higher translational velocity. The converging radial velocity was observed to increase translational velocity by as much as 30% over parallel jet flows with identical volume flux and nozzle diameter, but the exact increase is specific to the nozzle arrangement and driving conditions. [DOI: 10.1115/1.4025287]