| Pulsatile jet propulsion can be accomplished using a fully-pulsed jet (i.e., a periodic series of starting jets or pulses), the unsteady nature of which engenders vortex ring formation. The propulsive significance of vortex ring formation in this setting is studied experimentally using a piston-cylinder mechanism to generate starting and fully-pulsed, round jets of water into water. Starting jets are considered separately since they are the limiting case of a fully-pulsed jet at zero pulsing frequency. Measurements of the total impulse per pulse (starting jets) and time-averaged thrust (fully-pulsed jets) are made using a force balance. Digital particle image velocimetry (DPIV) measurements provide information about the resulting jet flow.; Piston stroke to diameter ratios (L/D) between 2 and 8 are used to generate the pulses for both types of jets. This range brackets the transition between pulses that generate isolated vortex rings (small L/D) and pulses that yield a leading vortex ring that has pinched off from the generating jet, producing a trailing jet (large L/D).; Impulse measurements for the starting jets as a function of L/D indicate the leading vortex ring adds proportionately more impulse per pulse than a trailing jet. This leads to a maximum in the average thrust during a pulse at an L/D just before vortex ring pinch off is observed. The propulsive advantage of vortex ring formation over trailing-jet ejection is due to nozzle exit over-pressure. This over-pressure can be related to the acceleration of ambient fluid in the form of added and entrained mass during ring formation. A simple model is proposed to describe these effects at the initiation of an impulsive pulse.; The thrust measurements for fully-pulsed jets show a propulsive benefit from vortex ring formation (i.e., non-dimensional thrust, FIJ , > 1) for all non-dimensional pulsing frequencies, StL, accessible by a fully-pulsed jet. As with starting jets (StL = 0), vortex ring pinch off reduces FIJ at large L/D for StL > 0. As StL increases at a fixed L/D, FIJ decreases, first abruptly and then gradually. Both trends can be related to the effect of preceding pulses on ring formation in emerging pulses. |
