Meeting Abstract
Fish schooling is typically observed in many fish species, which is thought to provide hydrodynamic advantages. Previous theoretical models have predicted improvements in swimming performance of fish schools due to two physical mechanisms: vortex hypothesis, which states that the relative velocity between fish and the flow is reduced through the induced velocity of the organized vortex structure of the incoming wake; and the channeling effect, which states that the relative velocity is reduced by the enhancement of the flow between swimmers in the direction of swimming. Although experimental observations confirm hydrodynamic advantages, there is still debate regarding the two mechanisms. We have carried out the first three-dimensional simulations at realistic Reynolds numbers to investigate these physical mechanisms. Through large-eddy simulations of self-propelled synchronized swimmers in various rectangular patterns, we found evidence in support of the channeling effect, which enhances the flow velocity between swimmers in the direction of swimming as the lateral distance between swimmers decreases. Our simulations show that the coherent structures, in contrast to the wake of a single swimmer, break down into small, disorganized vortical structures, which have a low chance for constructive vortex interaction for rectangular pattern. Therefore, the vortex hypothesis, which is hypothesized to be the main mechanism for diamond patterns, was not found in rectangular patterns, and needs to be further studied for diamond patterns in the future. By exploiting the channeling mechanism, a fish in a rectangular school swam faster as the lateral distance decreased, while consuming similar amount of energy. The fish in the rectangular school with the smallest lateral distance (0.3 fish lengths) swam 20% faster than a solitary swimmer while consuming similar amount of energy.
