Computational Analysis of Foil-Fish Interactions in Low Reynolds Number Flow

Schooling swimmers can benefit from vortex interaction to increase hydrodynamic performance. To examine the underlying mechanisms of how schooling fish exploit vortices shed from the front, we conducted two-dimensional computational simulations in which a fish model was placed in the vortices generated by two counter-pitching foils. An in-house immersed-boundary method based computational fluid dynamics solver was used to solve the flow field. The motion of the fish model was then calculated from the hydrodynamic pressure and shear acting on the fish. It is found that the distance between the two foils greatly alters the hydrodynamic performance regarding thrust generation and propulsive efficiency. With the counter-pitching foils placed upstream, even if the foils are far apart from each other, the wall effect increases the thrust generation on the fish compared to the solitary fish model. As the foil distance decreases (0.5L ⩽ s < L), the block effect starts to come into play. Lower pressure and velocity are observed on the head which accelerate the fish as the it approaches one of the foils. When the foil distance is much smaller (s = 0.25L), the high velocity jets generated by the foils pushes the fish downstream. Our findings discovered that foil distance impacts hydrodynamic performance diversely. This work offers a unique approach to comprehending fish schools, promising valuable insights into the mechanism behind hydrodynamic enhancement.