Collective behavior and hydrodynamic advantage of side-by-side self-propelled flapping foils

Area of Science:

• Fluid Dynamics
• Biomimetics
• Hydrodynamics

Background:

• Understanding the hydrodynamic advantages of fish schools is complex, especially for large aggregations.
• The underlying mechanisms enabling collective hydrodynamic benefits in schooling aquatic animals remain largely unclear.

Purpose of the Study:

• To numerically investigate the hydrodynamic performance of large schools of self-propelled foils in side-by-side configurations.
• To analyze the effects of in-phase and out-of-phase flapping motions between neighboring foils on school performance.
• To identify optimal configurations for enhanced speed and reduced energy consumption in artificial schooling systems.

Main Methods:

• Numerical simulations were conducted on large schools comprising four, six, and eight self-propelled foils.
• Foils were arranged in a side-by-side configuration and driven by synchronized harmonic flapping motions.
• The study examined various combinations of in-phase and out-of-phase flapping between adjacent foils.

Main Results:

• Stable side-by-side configurations were spontaneously formed by foils using synchronized flapping motions.
• Out-of-phase flapping increased energy consumption for a specific cruising speed compared to a single foil.
• In-phase flapping resulted in lower speeds for a specific flapping frequency.
• Hybrid states, combining both in-phase and out-of-phase flapping, were found to enhance speed and save power.

Conclusions:

• Hybrid flapping states in multi-foil schools offer superior hydrodynamic performance, improving speed and energy efficiency.
• The enhanced performance is attributed to synchronized collaborative wake vortex structures and bow configurations.
• These findings provide insights into the self-organized collective behavior and hydrodynamic advantages observed in large schools.