Hydrodynamic Analysis and Classification of Large Dense Planar Synchronous Fish Schools

As our understanding of fish schooling as a mechanism for increased speed and efficiency deepens, the need arises for hydrodynamic analysis of fish schools to grow beyond the smaller models using a few fish commonly seen in literature towards the larger schools observed in nature. To that end, the fluid interactions in a large dense planar synchronous fish school (n ≥ 10) are studied in this work. Numerical simulations of two-dimensional carangiform swimmers are used to observe the hydrodynamics of large fish schools. It is shown that the average efficiency of the 10-fish school swimming is increased by 30% over a solo swimmer, along with a thrust production improvement of 114%. The performance of each fish is analyzed in depth, and several primary interaction mechanisms are uncovered. Anterior body suction arises from the proximity of the suction side of the flapping tail to the head of the next fish, which occurs throughout the school. The block effect persists in the back of the school and an enhanced block effect is shown as subsequent fish are added behind. The partial block effect is also demonstrated along the edges of the school. The wall effect is proven to enhance the flow of momentum downstream and thus increase the net forward force of the school. This occurs throughout the school and is further enhanced by a vortex pair aiding the lateral advection beyond the subsequent fish bodies at the edge 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 into the front fish, middle fish, edge fish, and back fish. These groupings and mechanisms observed are proven to be consistent over a range of Reynolds numbers and school sizes. Finally, the effects of increasing the length of the school by adding fish in the streamwise direction are tested. It is shown that efficiency and net thrust continue to increase when adding fish; however, the net thrust starts to decrease above 16 fish in the school and a limit of efficiency is approached. This is shown to occur due to vortex structure breakdown and loss of the anterior body suction on the edge of the school.