Intermittent swimming of two self-propelled flapping plates in tandem configuration

Intermittent swimming and schooling individually offer the possibility of energy economy for fish. To study the role of the simultaneous use of both behaviors, the intermittent swimming of two self-propelled plates in tandem configuration is investigated numerically. Two intermittent swimming modes, i.e., the multiple-tail beat (MT) mode and the half-tail beat (HT) mode, are considered. For solitary plate swimming intermittently while maintaining fixed bout period, the propulsion velocity and energy consumption decrease monotonically as the duty cycle increases, consistent with the prediction of the reduced-order model. A pair of plates swimming with independent duty cycle can form orderly configurations, without separating or colliding, when their duty cycles are similar. For the MT mode, the asymmetric wake of the leader enhances the drag on the following plate by inducing an additional oncoming flow against it, making the follower harder to follow. For the HT mode, the symmetric wake of the leader reduces the drag on the follower, making it easier to follow. No significant difference was found in the propulsive performance of intermittent swimming between the leading plate in orderly configurations and solitary plate. The results of efficiency indicate that the HT mode is more economical than the MT mode for the follower. Usually, the smaller the equilibrium gap spacing is, the more efficient the follower is. The results of the cost of transport show that the follower achieves better energy economy for higher propulsion velocity. The results provide some insight into the collective intermittent swimming of fish and are helpful for bionic design.