Effects of Nozzle Geometry on the Hydrodynamics of a Squid-Inspired Pulsed Jet Propulsion System

This study presents a numerical investigation of a squid-inspired pulsed jet propulsion system with three nozzle geometries: converging, tubular, and diverging. The cavity contraction of three-dimensional models with different nozzle geometries was simulated under various maximum stroke ratios, and analyzed based on thrust decomposition, flow field visualization, and wake structure identification. Results show that nozzle geometry significantly influences propulsion performance and energy efficiency. The converging nozzle enhances momentum transfer and over-pressure due to its smaller exit and stronger radial compression, which produces the highest thrust. However, this configuration requires higher internal pressure and generates disordered trailing vortices, leading to greater energy loss and reduced efficiency. The tubular nozzle exhibits stable jetting and well-defined vortex rings, resulting in superior energy efficiency, especially at low stroke ratios. The diverging nozzle provides lower thrust at small stroke ratios but surpasses the tubular case at higher ratios, driven by the prolonged initial vortex ring and stepwise momentum increase. The work highlights the coupled effects of nozzle geometry and stroke ratio on jet dynamics. The findings offer theoretical guidance for the design and optimization of efficient bioinspired jet propulsion systems.