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Flow interactions between uncoordinated flapping swimmers give rise to group cohesion
Joel W Newbolt1,2, Jun Zhang3,2,4, Leif Ristroph3
1Applied Math Lab, Courant Institute, New York University, New York, NY 10012.
Summary
Fish and birds in groups can stay together thanks to fluid dynamics. Robotic hydrofoils show how varied flapping motions and wake interactions allow cohesive swimming, even with different speeds.
Area of Science:
- Fluid dynamics
- Biomechanics
- Collective animal behavior
Background:
- Many aquatic and avian species exhibit schooling or flocking behavior.
- The precise role of fluid-mediated interactions in maintaining group cohesion is not fully understood.
- Existing fluid-dynamic models often oversimplify by assuming uniform individual motion.
Purpose of the Study:
- To investigate the influence of individual flapping kinematics on collective locomotion in a simplified two-body system.
- To explore how fluid interactions, particularly wake dynamics, contribute to cohesive group swimming.
- To determine if dissimilar flapping patterns can lead to stable group formations.
Main Methods:
- A minimal robotic school of two tandem hydrofoils was designed.
- Independent and systematic variation of each hydrofoil's flapping kinematics.
- Observation of free forward swimming motion resulting from fluid forces.
- Formulation of a reduced-order model to explain observed phenomena.
Main Results:
- Uncoordinated hydrofoils with dissimilar kinematics achieved cohesive swimming without separation or collision.
- Follower hydrofoils found stable positions within the leader's wake, controllable by amplitude and phase.
- Lower-speed followers maintained pace, while higher-speed followers oscillated within the wake.
- A reduced-order model accurately predicted observed swimming modes based on relative flapping speed and wake flow.
Conclusions:
- Flapping kinematics can be actively controlled to navigate within fluid wakes.
- Fluid interactions, specifically wake dynamics, serve as a crucial mechanism for promoting group cohesion in schooling and flocking.
- This study provides insights into the fundamental principles governing collective animal locomotion.
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