The hydrodynamics of eel swimming: I. Wake structure

Area of Science:

• Biomechanics
• Fluid Dynamics
• Ichthyology

Background:

• Eels use whole-body undulation for swimming, a mode with poorly understood hydrodynamic consequences.
• Previous hypotheses suggested eel wakes consist of unlinked vortex rings.
• Comparison with other fish swimming modes, like carangiform, is needed.

Purpose of the Study:

• To detail the hydrodynamics of American eels (Anguilla rostrata) during steady swimming.
• To quantify wake structure, swimming efficiency, force, and power output.
• To compare eel swimming hydrodynamics with other fish and existing models.

Main Methods:

• High-resolution particle image velocimetry (PIV) was employed to analyze the flow field.
• Measurements focused on quantifying wake structure and swimming performance metrics.
• Data were compared against large-amplitude elongated body theory and computational fluid dynamics models.

Main Results:

• The eel wake is characterized by lateral fluid jets and vortices generated by shear layer instability, not unlinked vortex rings.
• High vorticity pulses are shed primarily at the tail tip.
• Unlike carangiform swimmers, eels exhibit minimal downstream wake flow, indicating a balance of thrust and drag.

Conclusions:

• Eel swimming hydrodynamics differ significantly from carangiform swimmers due to their body shape and lack of a narrow caudal peduncle.
• The uniform body shape of eels leads to a spatial and temporal integration of thrust and drag, resulting in a low-momentum wake.
• The study refines our understanding of fish locomotion and the diverse strategies employed in aquatic environments.