Propagation of orifice- and nozzle-generated vortex rings in air

Abstract

The effect of the exit geometry of a vortex ring generator was studied experimentally. Two types of exit geometries were chosen: an orifice and a nozzle. Vortex rings were generated by pushing a solenoid-valve-controlled, pressurized-air jet through the circular opening of the orifice or nozzle. Experiments were performed over a wide range of initial Reynolds number (450 l Re l 4580) and length-to-diameter ratio (0.7 l L / D l 7.0) of the air jet. The exit geometry was found to significantly influence the entire course of propagation of the vortex ring. The orifice-generated vortex ring had superior characteristics to that produced by the nozzle under the same conditions. The vorticity generated along the wall in the orifice exit plane had a negligible effect on the circulation of the vortex ring within the specified range of Reynolds number. Compared to the nozzle-generated vortex ring, the orifice-generated ring showed reduced initial vorticity losses and less diffusive entrainment of ambient fluid. The vortex rings produced by the orifice attained more circulation, less entrainment of ambient fluid and hence rapidly propagated through longer distances in comparison to the nozzle-generated rings.