Effects of multi-jet coupling on propulsive performance in underwater pulsed jets

Despite the importance of pulsed jets for underwater propulsion, the effect of multiple-jet interactions remains poorly understood. We experimentally investigate how interactions between neighboring jets in a pulsed-jet thruster affect the thruster's propulsive performance. Using high-speed fluorescence imaging, we investigate the mutual influence of two pulsed jets under conditions relevant to low-speed maneuvering in a vehicle ($Re\approx350,$ $L/D\leq2$). Thrust production and propulsive efficiency are evaluated for different nozzle spacings using a new force estimation technique based on the fluorescence data. This analysis reveals that, compared to non-interacting jets, the efficiency and thrust generated by the pair of interacting jets can fall by as much as 10\% when the jets are brought into close proximity. Empirically, the thrust $T$ falls off with the non-dimensional jet spacing $\widetilde{\Delta}$ as $T=T_\infty(1-Co \widetilde{\Delta}^{-6})$ for a thrust coupling coefficient $Co=2.04 \pm 0.11.$ Finally, we predict this leading order dependence of thrust on spacing using a model that relates the thrust and efficiency drop to streamline curvature and vortex induction at the nozzles.