Flow Interactions and Performance Benefits in Fish Schools*

In nature fish are known to aggregate into large schools or collectives. It has long been argued that these collectives are formed for social reasons, as a protection strategy against predators, and even to save energy.  However, our knowledge of the fluid dynamic interactions that occur among individuals and their collective energetics is based mostly on two-dimensional analyses.  Here, we examine the role of three-dimensionality in altering the flow interactions, the force generation, and the energetics of two interacting swimmers.  We model our two swimmers as simple finite-span pitching propulsors.  Additionally, the positions of the two propulsors are configured in mixtures of canonical side-by-side and in-line arrangements.  The forces and moments are then measured as the two wings undergo sinusoidal pitching motions over a large range of synchrony and spacing.  Our aim is to identify the flow mechanisms that improve or degrade propulsive performance of individuals within a collective as well as to identify flow mechanisms that give rise to fluid-mediated forces among individuals.