Tuna-like swimmers experience a fluid-mediated stable side-by-side formation

New free-swimming experiments and simulations are conducted on pairs of three-dimensional, bio-robotic swimmers composed of a body and tail section based on Yellowfin tuna,Thunnus albacares. Two robot body shapes are studied: thetuna-likeswimmers have a 22% thickness-to-length ratio, while the second type of bio-robots, designated asthin-tunas, are 63% thinner than the tuna-like swimmers. It is discovered that the pair of thicker, tuna-like bio-robots converges spontaneously to a side-by-side schooling formation that is stable to perturbations in the swimming direction at a fixed lateral spacing. The same stable schooling formation is not observed for the thin-tuna bio-robots. We reveal that for close lateral spacings of 43% of the body length, the flow between the thicker tuna-like swimmers is accelerated in a 'channeling effect' due to flow constriction. Consequently, this creates a low-pressure zone that is the primary mechanism generating a fluid-mediated restorative force, thereby making the side-by-side formation hydrodynamically stable. The thinner body profile of the thin-tunas, on the other hand, does not produce a flow constriction sufficiently strong to maintain the side-by-side formation stable in our free-swimming experiments and simulations. This quasi-steady flow mechanism makes the stability of the formation of tuna-like swimmers insensitive to the phase synchronization between the bio-robots in contrast to previous results for schooling foils. Moreover, in the side-by-side formation tuna-like swimmers are seen to have only a small reduction in their swimming speed and a concurrent small rise in their cost of transport. By leveraging this channeling effect, bio-robotic schools may be able to maintain a schooling formation with little or no control. This flow mechanism may also be present in biological schools of tuna-like fish where it may sculpt the formations observed in nature.