Effect of Slip Flow on Vortex Ring Formation*

Formation of vortex rings by transient jet ejection from a nozzle is governed by the formation of the boundary layer within the nozzle and its subsequent separation and rollup at the nozzle exit.  Apparent slip flow on the nozzle surface may provide a significant impact on this process, particularly for lower Reynolds number flows where vorticity is more diffuse.  In the present work, a novel approach to entrap air near the nozzle inner surface in order to provide apparent slip in an aqueous flow is used to experimentally investigate the influence of slip flow on vortex ring formation in comparison to the case with no-slip boundaries.  Transient jets with ratios of the ejected jet slug length to jet diameter (L/D) in the range of 1 – 6 and jet Reynolds numbers of 350 – 400 were investigated using a nozzle with apparent slip from entrapped air on one side and a no-slip solid surface on the other.  Apparent slip resulted in small but measurable effects on the resulting vortex, with generally higher peak vorticity and impulse than the vorticity associated with the no-slip surface.  These results indicate the potential for manipulating vortex formation and propulsion at lower Reynolds numbers.