Thrust augmentation and vortex ring evolution in a fully pulsed jet

The time-averaged thrust of an incompressible fully pulsed jet containing a period of no flow between pulses is studied experimentally as a function of pulsing duty cycle Sr_L and the ratio of the ejected slug length (per pulse) to the jet diameter L/D. The parameter ranges investigated were 2≤L/D≤6 and 0.1≤Sr_L≤0.98. Significant thrust augmentation by pulsing was observed over the entire parameter range tested, both in terms of thrust compared to an equivalent steady jet with identical mass flux, denoted F_(SJ) >1, and in terms of thrust compared to an equivalent intermittent jet where vortex ring formation by pulsation was ignored, denoted F_(IJ) >1. F_(SJ) as high as 1.90 (90% thrust augmentation) was observed for the smaller L/D as Sr_L approached 1.0 (with larger F_(SJ) at lower Sr_L). The F_(IJ) results, which directly measured overpressure at the nozzle exit plane developed during vortex ring formation as the mechanism responsible for thrust augmentation, showed reduced augmentation at large L/D and Sr_L. The L/D dependence of F_(IJ) parallels single-pulse (Sr_L =0) results previously studied by the authors. The Sr_L dependence of F_(IJ) was linked to the interaction of forming vortex rings with vorticity from preceding pulses using digital particle image velocimetry (DPIV) measurements of the vorticity field. DPIV also revealed that the vortex rings tended to wander off axis and disintegrate as Sr_L became sufficiently large