Optimal jetting velocity and nozzle considerations for a cephalopod inspired underwater thruster

This paper examines a new type of thruster which, similar to squid and jellyfish, generates propulsive forces by cyclically ingesting and expelling jets of water. The thrust output of this device depends greatly on the nature of the expelled jet, with greater output occurring for jets which form a single coherent vortex ring rather than a vortex ring with a trailing wake. The average thrust output as well as the jetting efficiency are maximized with respect to the thruster's nozzle diameter and jet velocity program. The appropriate constraints of the problem are defined including a dynamic constraint to ensure that the propulsive jet forms a single vortex ring. It is shown that the average thrust output under these constraints reaches a maximum when the nozzle radius is set to a critical value, and that using an orifice nozzle decreases propulsive efficiency at low Reynolds numbers.