Effect of inertia on the collective dynamics of an active suspension of mesoscale model swimmers*

We computationally examine how the collective behaviors emerge in a collection of simple model swimmers immersed in a viscous incompressible fluid as the impact of inertia, characterized by the Reynolds number (Re), gradually increases. While most studies on the collective dynamics of swimming organisms have focused either on the Stokes regime at low Re or the Eulerian regime at high Re wherein viscous and inertial forces respectively dominate, less is known about such behaviors in the intermediate Re range where the two forces play a role. We show that our model can exhibit a wide range of nontrivial swimming patterns dictated by the degree of introduced inertia. At low Re, we observe a stable network-like arrangement in which swimmers tend to follow the swimming axis of their neighbors. As Re increases, a side-by-side swimming arrangement sets in, as swimmers favor alignment with their partner, leading to the formation of small stable moving clusters. The clusters become unstable at higher Re as the frequency of collision between the clusters increases, resulting in a rapid change of swimming partners. We relate these changes in swimming patterns to hydrodynamic signatures found in the swimmer pairwise interactions, as well as the resulting averaged flow field.