Effect of an Exit-Wedge Angle on Pinch-off and Mass Entrainment of Vortex Rings in Air

Abstract

The effects of exit-wedge angle on evolution, formation, pinch-off, propagation and diffusive mass entrainment of vortex rings in air were studied using digital particle image velocimetry. Vortex rings were generated by passing a solenoid-valve-controlled air jet through a cylindrical nozzle. Experiments were performed over a wide range of exit-wedge angles (10A degrees a parts per thousand currency signaEuro parts per thousand alpha a parts per thousand currency sign 90A degrees) of the cylindrical nozzle, initial Reynolds numbers (450 a parts per thousand currency sign Re a parts per thousand currency sign 4,580) and length-to-diameter ratios (0.9 a parts per thousand currency sign L/D a parts per thousand currency sign 11) of the air jet. For sharp edges (alpha a parts per thousand currency sign 10A degrees), a secondary ring may emerge at high Reynolds numbers, which tended to distort the vortex ring if ingested into it. For blunt edges (alpha a parts per thousand yen 45A degrees), by contrast, stable vortex rings were produced. The formation phase of a vortex ring was found to be closely related to its evolution pattern. An exit-wedge angle of 45A degrees was found to be optimal for rapid pinch-off and faster propagation and better stability of a vortex ring. Diffusive mass entrainment was found to be between 35% and 40% in the early stages of a vortex ring propagation and it gradually increased throughout the course of vortex ring propagation. Entrainment fraction was found to be sensitive to the L/D ratio of the initial jet and decreases when the L/D ratio is increased.