Flow interactions lead to self-organized flight formations disrupted by self-amplifying waves

Area of Science:

• Collective animal behavior
• Fluid dynamics
• Robotics

Background:

• Collective locomotion in animals is often compared to states of matter, with group phenomena emerging from individual interactions.
• Visco-inertial flows are key mediators of physical interactions in fish schools and bird flocks.
• Understanding these flow-mediated interactions is crucial for explaining collective movement patterns.

Purpose of the Study:

• To investigate the role of visco-inertial flows in mediating interactions within collectively locomoting animal groups.
• To explore how pairwise flow interactions influence arrangement and stability in 'mock flocks'.
• To identify mechanisms for self-ordering and disturbance amplification in flow-mediated collectives.

Main Methods:

• Utilizing robotic experiments with flapping wings in forward flight to simulate 'mock flocks'.
• Conducting force measurements and applying perturbations to analyze wake interactions.
• Developing a wake interaction model incorporating spring-like forces and resonance cascades.

Main Results:

• Pairwise flow interactions promote crystalline or lattice-like arrangements, but order is disrupted by growing positional waves.
• Larger groups exhibit flow-induced oscillatory modes ('flonons') that amplify and cause collisions.
• Introducing individual variability stabilizes larger groups by suppressing flonon amplification.

Conclusions:

• Self-ordering in collectives is mediated by spring-like forces, while disturbance amplification occurs via resonance cascades.
• Flow-induced oscillatory modes ('flonons') are a key factor in collective instability.
• Variability among individuals can stabilize large collectives by disrupting resonance and promoting positional disorder.