Optimisation of a bio-inspired jet propulsor*

Bio-inspired underwater vehicles imitate the kinematics of biological organisms for locomotion. One mechanism exploited by marine organisms is pulsatile jet propulsion, the periodic ejection of vortex rings for thrust generation. Here, we present the design of a jellyfish-inspired device that produces vortices by compressing a bulb and ejecting fluid. The size of the orifice through which vortices are ejected can be varied in time to mimic medusae that alter their velar diameter to generate optimal vortex rings. Vortices are studied through time-resolved velocity field measurements and thrust measurements using strain gages. A multi-objective optimization is implemented to determine the time-varying compression and orifice-diameter profiles that are most energy-efficient for steady-state cruising and rapid escaping conditions. The effect of these kinematics on the vortex formation process, vortex ring circulation, impulse, and non-dimensional energy is studied. We also expand upon the specific role of secondary vortices in thrust generation. These findings enable us to optimally harness vortex rings for the energy-efficient locomotion of bio-inspired vehicles.