  We computationally studied the pair interactions and collective behavior of
asymmetric, dumbbell swimmers over a range of intermediate Reynolds numbers and
initial configurations. Depending on the initial positions and the Re, we found
that two swimmers either repelled and swum away from one another or assembled
one of four stable pairs: in-line and in-tandem, both parallel and
anti-parallel. When in these stable pairs, swimmers were coordinated, swum
together, and generated fluid flows as one. We compared the stable pairs'
speeds, swim direction and fluid flows to those of the single swimmer. The
in-line stable pairs behaved much like the single swimmer transitioning from
puller-like to pusher-like stroke-averaged flow fields. In contrast, for the
in-tandem pairs we discovered differences in the swim direction transition, as
well as the stroke-averaged fluid flow directions. Notably, the in-tandem V
pair switched its swim direction at a higher $\text{Re}$ than the single
swimmer while the in-tandem orbiting pair switched at a lower $\text{Re}$. We
also studied a system of 122 swimmers and found the collective behavior
transitioned from in-line network-like connections to small, transient
in-tandem clusters as the Reynolds number increased, consistent with the
in-line to in-tandem pairwise behavior. Details in the collective behavior
involved the formation of triples and other many-body hydrodynamic interactions
that were not captured by either pair or single swimmer behavior. Our findings
demonstrate the richness and complexity of the collective behavior of
intermediate-$\text{Re}$ swimmers.
