School cohesion, speed, and efficiency are modulated by the swimmers flapping motion

TL;DR

This study uses a vortex sheet model to analyze how different flapping motions in fish schools influence group stability, speed, and energy efficiency, revealing that flow interactions passively promote cohesion and are affected by the mode of flapping.

Contribution

It introduces a minimal vortex sheet model to explore how flapping modes affect fish school dynamics, highlighting the passive stabilization and efficiency benefits of pitching motions.

Findings

Follower stabilizes at discrete wake locations

Pitching motions enhance group cohesion and efficiency

Heaving motions slightly increase swimming speed

Abstract

Fish schools are ubiquitous in marine life. Although flow interactions are thought to be beneficial for schooling, their exact effects on the speed, energetics, and stability of the group remain elusive. Recent experiments suggest that flow interactions stabilize in-tandem formations of heaving foils. Here, we propose a minimal approach based on the vortex sheet model that captures salient features of the flow interactions among flapping swimmers, and we study the free swimming of a pair of in-line swimmers driven with identical heaving or pitching motions. We find that, independent of the flapping mode, the follower passively stabilizes at discrete locations in the wake of the leader, consistent with the heaving foil experiments, but pitching swimmers exhibit tighter and more cohesive formations. Further, in comparison to swimming alone, pitching motions increase the energetic efficiency of the group while heaving motions result in a slight increase in the swimming speed. These results recapitulate that flow interactions provide a passive mechanism that promotes school cohesion, and provide novel insight into the role of the flapping mode in controlling the emergent properties of the school.