Pairwise and collective behavior of dumbbell swimmers at intermediate Reynolds numbers

We computationally investigate the pairwise and collective behavior of simple, reciprocal dumbbell swimmers composed of two unequally sized spheres at intermediate Reynolds numbers (0.1 < Re < 100). For nonzero Re, a single dumbbell swims small-sphere-leading and interestingly, as Re increases, switches its swimming direction to large-sphere leading. A pair of identical dumbbells experience a rich phase space of interactions dependent on their initial configuration, Re, and individual swimming parameters. Several stable configurations are found including in-line, in-tandem V-shape, and orbiting pairs. Averaged flow fields are analyzed to further understand these stable configurations. Pair interactions are then used to develop a framework for the collective behavior of dumbbell swimmers.