Efficiency and vortex dynamics of orifice pulsed jet propulsion based on large eddy simulation

Pulsed water jets offer high transient thrust and energy efficiency, making them promising for small-scale propulsion systems and underwater vehicles. However, propulsion performance is strongly influenced by nozzle configuration and temporal control parameters, and the underlying mechanisms remain insufficiently understood, particularly for orifice-type pulsed jet propulsion, which has received considerably less attention than sharp-edged nozzle configurations. The objective of this study is to elucidate the intrinsic coupling mechanisms between vortex dynamics and propulsion efficiency in orifice-type pulsed jets. To this end, a three-dimensional large eddy simulation is employed to systematically investigate the effects of pulse waveform (0 < Φ < 1), stroke ratio (1.5 < L/D < 8), and duty cycle (0.4 < η < 0.9) on vortex-ring formation, instantaneous thrust response, and energy utilization efficiency. The results indicate that instantaneous thrust is primarily governed by axial velocity acceleration, while adverse acceleration phases can induce negative thrust. Variations in waveform shape significantly influence vortex development and power-to-thrust characteristics, with low-fullness waveforms promoting more coherent vortex structures. The propulsion efficiency exhibits a strong dependence on stroke ratio and duty cycle, reflecting the combined effects of vortex interactions and inter-cycle coupling. These findings provide new physical insight into the efficiency mechanisms of orifice-type pulsed jet propulsion and offer a quantitative basis for evaluating propulsion performance under different pulsed operating conditions.