Hydrodynamic Interactions in Schooling Fish: Disentangling Mechanisms via Highly Resolved Direct Numerical Simulations and Force Partitioning

A fish swimming in the proximity of others may obtain hydrodynamic benefits such as increased thrust generation and reduced power consumption due to the interaction with the flow induced by neighbors. Several different mechanisms (drafting, "channeling," constructive wake-interference, vortex phase matching, LEV enhancement, etc.) have been proposed but results have often been contradictory and difficult to reconcile. Ultimately, the swimming performance is determined by the time-varying pressure forces on the fish, but pressure is simultaneously affected by vortices, viscous diffusion, and added-mass effects, thereby making it difficult to determine causality. In the present study, the hydrodynamic interactions between a pair of swimming fish are examined by performing a series of high-resolution, three dimensional simulations. The simulation results show that the thrust and power of the trailing fish interacting with the wake of the leading fish changes about +/- 10% and +/- 5%, respectively, depending on the streamwise distance and the tail beat phase difference. The mechanisms underlying these interactional effects are analyzed by applying a force partitioning method, which enables quantification of the effect of vortices around the fish on the force generation.