Computational hydrodynamics of animal swimming: boundary element method and three-dimensional vortex wake structure

Area of Science:

• Fluid dynamics
• Biomechanics
• Computational science

Background:

• Fish swimming hydrodynamics are complex, involving efficient propulsion and maneuvering.
• Previous studies utilized methods like slender body theory, lifting surface theories, panel methods, and Navier-Stokes solvers.
• Understanding the three-dimensional vortex wake structures is crucial but remains challenging.

Purpose of the Study:

• To investigate fish swimming hydrodynamics using a novel boundary element method (BEM) code.
• To analyze the vortex wake structures generated by undulatory swimming.
• To elucidate the fluid dynamic mechanisms behind high-efficiency propulsion and agility in fish.

Main Methods:

• Development and application of an unsteady three-dimensional BEM code (3DynaFS) based on a potential flow model.
• Modeling realistic fish body geometries and arbitrary movements.
• Computation of pressure distribution, vorticity, and velocity fields.

Main Results:

• The 3DynaFS code successfully modeled realistic swimming motions and wake evolution.
• Simulations of a swimming saithe (Pollachius virens) revealed reduced three-dimensional effects.
• Undulatory swimming resulted in a weakened tail tip vortex and a dominant reverse Karman Vortex Street in the wake.

Conclusions:

• The boundary element method is effective for studying fish swimming hydrodynamics.
• Undulatory swimming patterns can simplify wake structures, leading to a reverse Karman Vortex Street.
• This finding provides insight into the fluid dynamics enabling efficient and agile fish locomotion.