Pairwise and Collective Interactions of a Model Swimmer at Intermediate Reynolds Numbers

In between the two extremes of Stokes flow, home to microorganisms, and inviscid flow, home to human swimmers and large fish, resides the less studied intermediate Reynolds regime where millimeter to centimeter sized organisms thrive. Here, both viscosity and inertia play a role in an organism’s movement, and few models have been developed which relate movement to general underlying physical mechanisms. In this presentation, we investigate pairwise interactions between reciprocal, asymmetric dumb-bell swimmers at intermediate Reynolds numbers (Re). Even for a single swimmer we find interesting behavior: a transition in the swimming direction from a small-sphere-leading to a large-sphere-leading regime. Their pairwise interactions are just as surprising. We computationally vary a broad range of parameters and find a wealth of states: steady pairs that cooperatively swim differently from individual swimmers under identical conditions, bi-stable pairs, orbits, and diverging paths. Averaged flow fields are analyzed to further understand these configurations. We continue by studying the collective behavior of large numbers of swimmers (up to a hundred) in order to identify pairwise versus many-body emergent behaviors.