Collective behavior of model swimmers at intermediate Reynolds numbers*

While most studies on the collective dynamics of swimming organisms have focused either on the microscale (Stokes regime) at low Reynolds (Re) numbers or the macroscale (Eulerian regime) at high Re wherein viscous and inertial force respectively dominate, little is known about such behaviors in the intermediate regime where the two forces concurrently play a role. Using computational fluid dynamics approaches, we examine how the collective behaviors emerge in a mixture of a simple mesoscale model swimmer immersed in a viscous incompressible fluid under the increasing effect of inertia. Specifically, we observe a range of nontrivial dynamical swimming patterns as Re increases, including different kinds of network structures and swarming. Our results suggest that inertia is one of the important factors and a possible knob to control the collective behaviors of active mesoscale particles immersed in fluids at intermediate Re regime.