An experimental study on the formation of negatively-buoyant vortex rings

Experiments to examine the formation of dense saline vortex rings projected vertically upwards into a quiescent freshwater environment were conducted. The setup was designed to dispense a cylindrical column of source fluid with aspect ratio $L/D$ (the length $L$ of dispensed saline column to the nozzle diameter $D$) over a pre-set time interval. In an effort to execute an impulsive start and finish, a controlled flow circulation driven by a gear pump was developed to approximate a top-hat profile of source exit velocity versus time. Our measurements focus on describing the evolving morphology of the vortex rings with time and with source conditions ($L/D$ and source Froude number). Our results reveal distinct formation regimes and our estimates of time required for formation as a function of density difference confirm predictions from previously published numerical simulations. The volume-based approach we adopt provides potentially a new angle for investigating the physics of these flows.