Dynamics of vortex rings in crossflow

Abstract

The spread and mixing of a fluid jet into an ambient stream occurs at a rate which deserves further study to improve efficient mixing. Mixing enhancement techniques, such as introduction of periodic disturbances into the jet flow, are used to increase mixing between a jet and the surrounding fluid. Pulsations were generated by the periodic closing and opening of a jet flow. The dynamics and trajectories of vortex rings, formed by the pulsation of the jet in a uniform crossflow, are studied. In particular, the effects of pulsation on the development of vortex rings and their penetration in a crossflow were investigated. Detailed measurements were made using flow visualization techniques including laser-induced fluorescence and hot-film anemometry. Vortex rings generated in a crossflow at one specific frequency (1 Hz) were measured using a hot-film probe. Measurements indicated that vortex rings were fully-formed at a distance of three times the jet exit diameter. To simulate the dynamics of vortex rings in crossflows, a numerical experiment was performed based on a Lagrangian, grid-free, three-dimensional vortex element method. At low frequencies, the fluid in the vortex rings penetrated into the crossflow to a height much greater than that for either high frequency pulsation or for a steady jet. At low frequencies, interaction between sequentially generated vortex rings was negligible; therefore, each ring behaved as a single discrete vortex ring. The vortex rings moved into the flow occasionally tilting up to about 30°, depending on the ring’s strength. Numerical simulation indicated that the tilting of the ring was due to the combined effects of viscosity and the crossflow. It is postulated that the increased penetration combined with the discretization of a jet into vortex rings results in a more efficient mixing rate.