Tuna-Like Swimmers Experience a Fluid-Mediated Stable Side-by-Side Formation

TL;DR

This study demonstrates that bio-robotic tuna-like swimmers naturally form a hydrodynamically stable side-by-side formation due to a flow channeling effect, with minimal speed reduction and potential biological relevance.

Contribution

It reveals a novel fluid-mediated mechanism for stable schooling formation in tuna-like swimmers, independent of phase synchronization, based on experiments and simulations.

Findings

Stable side-by-side formation due to flow channeling effect.

Minimal reduction in swimming speed within the formation.

Formation stability is insensitive to phase differences.

Abstract

New free-swimming experiments and simulations are conducted on a pair of three-dimensional, bio-robotic swimmers composed of a body and tail section based on Yellowfin tuna, Thunnus albacares. It is discovered that the pair converges spontaneously to a side-by-side schooling formation that is stable to perturbations in the swimming direction at a fixed lateral spacing. We reveal that for close lateral spacings of 43% of the body length and thick, tuna-like bodies with a 22% thickness-to-length ratio, the flow between the 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. This quasi-steady mechanism makes the stability of the formation insensitive to the phase synchronization between the bio-robots in contrast to previous results for schooling foils. Moreover, in the side-by-side formation tunalike 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.