The trade-off between locomotion speed and hydromechanical efficiency determines the optimal orifice ratio of a salp-inspired swimmer*

Different from most marine invertebrates that swim by rear single-jet propulsion (such as jellyfish and squid), salps take water in at the front and expel water out at the rear during locomotion. Inspired by this unique dual-jet propulsion mode, we develop a two-dimensional numerical model to investigate the hydrodynamic performance of a salp-inspired swimmer. Our results show that higher orifice ratio can improve the hydrodynamic efficiency of the salp-inspired swimmer, but at the cost of its locomotion speed. Moreover, while smaller duty cycle is benefit for both locomotion speed and hydrodynamic efficiency, the lower bound of duty cycle is restricted by energy consumption coefficient. In particular, the salp-inspired swimmer reaches a trade-off between the locomotion speed and the hydromechanical efficiency when the orifice ratio is around 0.4, a value in agreement with the case of real salps in nature. We also found that its locomotion speed depends on the jet-based Reynolds number with a simple 4/5 scaling law, which can be explained by the force balance during locomotion.