  When swimming animals form cohesive groups, they can reap several benefits.
Our understanding of collective animal motion has traditionally been driven by
models based on phenomenological behavioral rules, but more recent work has
highlighted the critical importance of hydrodynamic interactions among a group
of inertial swimmers. To study how hydrodynamic interactions affect group
cohesion, we develop a three-dimensional, inviscid, far-field model of a
swimmer. In a group of two model swimmers, we observe several dynamical phases,
including following, divergence, collision, and cohesion. Our results
illustrate when cohesive groups can passively form through hydrodynamic
interactions alone, and when other action is needed to maintain cohesion. We
find that misalignment between swimmers makes passive cohesion less likely;
nevertheless, it is possible for a cohesive group to form through passive
hydrodynamic interactions alone. We also find that the geometry of swimmers
critically affects the group dynamics due to its role in how swimmers sample
the velocity gradient of the flow.
