The hydrodynamic advantages of synchronized swimming in a rectangular pattern.

Area of Science:

• Fluid dynamics
• Biophysics
• Animal behavior

Background:

• Fish schooling behavior is hypothesized to provide hydrodynamic advantages, potentially reducing swimming costs.
• Two primary mechanisms, the vortex hypothesis and the channeling effect, have been proposed to explain these benefits.
• Experimental evidence supports hydrodynamic advantages, but the relative importance of these mechanisms remains debated.

Purpose of the Study:

• To investigate the physical mechanisms behind hydrodynamic advantages in fish schooling using three-dimensional simulations.
• To differentiate the contributions of the vortex hypothesis and the channeling effect in rectangular schooling patterns.
• To provide computational evidence at realistic Reynolds numbers.

Main Methods:

• Employed large-eddy simulations (LES) to model self-propelled, synchronized swimmers in rectangular formations.
• Simulated various lateral distances between swimmers to assess the impact on flow dynamics.
• Analyzed flow patterns and wake structures to identify contributing hydrodynamic mechanisms.

Main Results:

• Found strong evidence supporting the channeling effect, where reduced lateral distance enhances forward flow velocity between fish.
• Observed that coherent wake structures break down into disorganized vortices in rectangular patterns, diminishing the vortex hypothesis's relevance.
• Demonstrated that fish in tighter rectangular schools swim up to 20% faster with similar energy expenditure compared to solitary fish.

Conclusions:

• The channeling effect is a significant mechanism for hydrodynamic advantage in rectangular fish schools.
• The vortex hypothesis appears less relevant for rectangular patterns, though it may apply to other formations (e.g., diamond).
• Optimizing lateral spacing in schools can enhance swimming performance and efficiency.