MASTERS, J.C.*; MINETTI, A.; Manchester Metropolitan University, United Kingdom; Manchester Metropolitan University, United Kingdom: Wake structure of a swimming roach (rutilus rutilus) as a function of speed/tailbeat frequency

The social and behavioural benefits of schooling in fish have been investigated in depth in the past; however, very little research has focused on the hydrodynamic benefits derived by schooling. The current study investigates the effect of the speed/tailbeat frequency combination on the structure of the downstream wake of a fish swimming at steady state. Detailed knowledge of the angle of formation and downstream persistence of vortices is crucial in determining the optimal trailing distance and lateral position that a fish should assume in order to derive the most hydrodynamic benefit. We employ a three-dimensional computational fluid dynamic model of a swimming fish which takes into account the variation in speed during a tailbeat cycle. The frame-of-reference is attached to the fish, and velocity conditions equal in magnitude but opposite in direction to the velocity of the fish are applied to the boundaries. At each time step the instantaneous velocity is calculated and applied to the model. For each speed/tailbeat frequency combination, we calculate an initial estimate of the average speed of the fish by using the Strouhal number and then allow the fish to accelerate to steady state. We performed simulations for three different tailbeat frequencies: 3.7Hz, 4.6Hz, and 5.5Hz. We found that vortex size and persistence increase as the fish approaches a steady state, while the width of the backward jet decreases. For the different steady state speed/tailbeat frequency combinations, the structure of the wake remained qualitatively unchanged, suggesting that the trailing fish should locate roughly the same optimal distance and position regardless of speed/tailbeat frequency combination.