  The thrust benefits of lateral configurations of two-dimensional undulating
fish-like bodies are investigated using high-fidelity numerical simulation. The
solution of the Navier--Stokes equations is carried out with a viscous vortex
particle method. Configurations of tethered pairs of fish arranged side by side
are studied by varying the lateral separation distance and relative phase
difference. It is shown that, in mirroring symmetry, the fish in the pair
augment each other's thrust even at relatively large separations (up to ten
body lengths). At small distances, this augmentation is primarily brought about
by a peristaltic pumping in the gap between the fish, whereas at larger
distances, the thrust is affected by subtle changes in the vortex shedding at
the tail due to interactions with the other fish. In cases without symmetric
undulation, one fish always draws more benefit from the interaction than the
other. Finally, lateral configurations with three fish are studied with
mirroring symmetry between neighboring fish. Whereas the center fish draws a
net thrust benefit, this comes at the expense of a net drag on the outer two
fish. Each adjacent pair in this arrangement is slightly affected by the
presence of the third fish.
