Evolution of impulse and thrust in the single-pulsed jet

It has been previously suggested that the unsteady jet propulsion and the associated vortex formation can increase propulsive efficiency relative to an equivalent steady jet. To reveal the physical connection between the propulsive characteristics and the vortex ring dynamics, the transient changes in impulse and thrust of single-pulsed jets are investigated numerically for cases with different velocity programs. The simulation quantitatively demonstrates that the unsteady thrust generation is highly sensitive to the jet velocity program. By exploiting the over-pressure effect at the jet initiation and mitigating the strength of the stopping vortex at the jet termination, the cases with fast acceleration and slow deceleration (FASD) velocity program have the maximum thrust augmentation over its steady counterparts. The pressure term of the jet impulse is found to be determined by two factors, i.e., the relative magnitude of jet acceleration to deceleration and the axial position of the leading vortex ring during the jet termination stage. Finally, a qualitative explanation on the unsteady propulsive characteristics is proposed according to the effects of formation and evolution of the leading vortex ring and the stopping vortex on the impulse provided by the jet.