  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.
