Hydrodynamic classification of fish-like undulating swimmers in high-density schools

Two-dimensional numerical simulations of carangiform fish swimming are used to investigate the mechanisms for hydrodynamic benefit via fluid interactions in large planar fish schools. It is observed that the average efficiency of the 10-fish school swimming is increased by 30% over a single swimmer, along with a thrust production improvement of 114%. The performance results and flow analyses uncover the associated hydrodynamic interaction mechanisms in large schools. First, anterior body suction arises from the proximity of the suction side of the tail to the head of the next fish. Next, the block effect is shown to occur as fish are added to the back of the school, and the partial block effect is demonstrated along the edges of the school. Finally, the wall effect is proven to enhance the flow of momentum downstream and thus increase the net forward force of the school. Because these primary body-body interactions are based on the arrangement of surrounding fish, a classification of individual fish within the school arises based on the interactions for each group and is reflected in the performance of the individuals. It is shown that the school can be separated as front fish, middle fish, edge fish, and back fish based not only on the geometric position but also performance and wake characteristics. Finally, the groupings and mechanisms observed are proven to be consistent over a range of Reynolds numbers and school arrangements.