Group cohesion in the presence of three-dimensional hydrodynamic interactions

Swimming in groups can confer several benefits, including defence against predators, enhanced foraging success, and increased hydrodynamic efficiency. Our understanding of collective animal motion has traditionally been driven by models based on phenomenological behavioural rules, but more recent work has highlighted the critical importance of hydrodynamic interactions among a group of swimmers. To study how hydrodynamic interactions affect group cohesion, we develop a three-dimensional, inviscid, far-field model of a swimmer, a superposition of which forms a group of swimmers. In a group of two model swimmers, we observe several dynamical phases, including following, divergence, collision, and oscillation. In larger groups, more complex dynamics arise. Our results illustrate when groups can passively and stably form through hydrodynamic interactions alone, and when other action is needed to maintain cohesion.