Intermittent Locomotion of Two Self-Propelled Flexible Fins in a Side-by-Side Arrangement*

Inspired by the intermittent locomotion of fish school, numerical simulations are performed with two self-propelled flexible fins in a side-by-side arrangement with anti-phase oscillation. For an intermittent swimming gait, one type of half-tail-beating mode (HT mode) and two types of multiple-tail-beating modes coasting at the smallest (MTS mode) and largest (MTL mode) lateral gap distances are applied. Similar to continuous-tail-beating mode (CT mode), equilibrium lateral gap distances between two fins with HT and MTL modes exist, whereas two fins with MTS mode do not maintain an equilibrium state. Although the mean lateral force acting on two fins with the CT and MTL modes is mainly determined by an outward deflected jet and enhanced positive pressure between two fins, an outward lateral flow acceleration by unbalanced flapping motions for MTL mode also plays an important role to achieve an equilibrium lateral gap distance smaller than that for CT mode. When the mean input power is identical in a stable state, the cost of transport for two fins with MTL mode decreases compared to that with CT mode, stemming from thrust enhancement due to not only a benefit from intermittent swimming gait but also to an enhanced schooling benefit with a small equilibrium lateral gap distance.