STIRRING PROPERTIES OF VORTEX RINGS

Ring vortex evolution, from the initial roll-up phase through to the final turbulent phase, was experimentally studied to see the dependence of its stirring properties on both the initial (accelerating, constant, decelerating, slow, fast) piston motion as well as on the boundary (tube/hole geometry) conditions. Stirring between fluid initially upstream and that initially downstream of the nozzle plane is done more by convective entrainment at the beginning (roll-up and contraction phases), by diffusive entrainment during the laminar and wavy phases, and by mixed entrainment and ejection during the transition to turbulence and the turbulent phase itself. During vortex roll-up, it was found that tubes eject shorter streaklines than do holes, and that there is less Re dependence for this for tubes than for holes. During the contraction phase, entrainment ends, save for minimal entrainment due to axial inflow into the ring from along the cores of Goertler-type vortices. Generally, the rate of fluid ejected is largest during the transition from the wavy to the turbulent state. As far as the stability of the vortices is concerned, rings generated at holes are less stable than those generated at tubes. During the final turbulent phase, rings not only entrain fluid but eject it periodically into the wake: Between two and four hairpin vortices are generated and laid off in the wake during each ejection. The frequency at which such ejections takes place scales as a Strouhal number that takes on values of between 2 and 4.