Vortex ring pinchoff in the presence of simultaneously initiated uniform background co-flow

Vortex rings were formed with a piston-cylinder mechanism in the presence of uniform background co-flow supplied through a shroud surrounding the cylinder. The jet and co-flow were started simultaneously. Ratios of the co-flow to jet velocity (R-v) in the range 0-1 were considered. The formation number (F) as a function of R-v was determined using the procedure of Gharib [J. Fluid Mech. 360, 121 (1998)] and a generalized definition of formation time. The results show a sharp decrease in F as R-v increases from 0.5-0.75, suggesting possible performance limitations for pulsed-jet propulsion. (C) 2003 American Institute of Physics.