Bio-inspired propulsion

Phase-mediated interactions on collective locomotion of schooling fins (Chae & Jeong 2025, POF) POF: Physics of Fluids

• The effects of phase difference (ϕ/π) on the collective locomotion and schooling performance of two unconstrained self-propelled flexible fins, which are free to move in both the x- and y-directions, are numerically investigated. 

• The trends of flapping kinematics and propulsive performance of the upper fin are symmetric to those of the lower fin with respect to ϕ/π=1.0. 

• At specific ϕ/π values, the average efficiency of the two fins improves due to a significant reduction in the time-averaged input power of the following fin. This reduction is attributed to a combination of decreased peak-to-peak flapping amplitude, shorter energy-consuming stages and compensatory motion of the following fin induced by shared pressure fields.

Intermittent swimming of schooling fins (Jeong et al. 2023, JFM) JFM: Journal of Fluid Mechanics

• Similar to the continuous-tail-beating mode, equilibrium lateral gap distances between two fins with intermittent swimming mode exist.

• Although the cycle-averaged lateral force acting on two fins with continuous and intermittent swimming modes is mostly determined by an outward deflected jet and enhanced positive pressure between two fins, an added-mass lateral force related to an asymmetric flapping kinematics by passive flexibility also plays an important role in intermittent swimming mode to achieve a stable state with a lateral gap distance smaller than that in continuous swimming mode.

• When the cruising speed or the cycle-averaged input power is identical in a stable state, the cost of transport (COT) for two fins with intermittent swimming is smaller than that with continuous swimming due to not only a benefit from the intermittent swimming gait but also an enhanced schooling benefit with a small equilibrium lateral gap distance. The COT for two fins with continuous swimming is reduced further when the bending rigidity increases, whereas it is opposite with intermittent swimming.

Schooling fins near sidewalls (Jeong et al. 2021, JFM) JFM: Journal of Fluid Mechanics

• Contrary to the vortex interception for 0W, the follower employs spontaneously a mixed mode (i.e. a combination of the vortex interception mode and the slalom mode) for 1W and the slalom mode for 2W.

•  Although the lateral motion of the follower for 0W, 1W and 2W is synchronized with the induced lateral flow generated by the leader, the time-averaged input power of the follower for 1W and 2W is reduced significantly due to the enhanced lateral flow by the vortex–vortex interaction near the wall. 

• The jet-like flow opposite to the moving direction continuously hinders the movement of the follower for 0W, whereas the follower for 1W and 2W utilizes the negative horizontal flow when passing between the main vortex and the induced vortex near the wall, leading to a decrease of the thrust force acting on the follower allowing the follower to keep pace with the leader.