Numerical Simulations of Fast-start Motions of Fish

Fast-start swimming is a very special and important type of locomotion of fish. Basically, fast-star motion can be classified into two types: C-type and S-type. It is generally thought that the C-type fast-start is adopted by fish for escape while the S-type fast-start is used in prey capture. Previous experimental data have shown that the maximum acceleration and maximum velocity of fish performing C-start are significantly greater than that for S-type motion. The present research investigates numerically the fluid flows generated by empirically modeled fast-start movements of fish. For simplicity, a 2-D potential flow is assumed, and the unsteady Kutta condition is applied to handle the vortex-shedding phenomenon at fish tail. The panel method is adopted to solve the flow field numerically, and the force and moment acting on fish body are calculated by using the unsteady Bernoulli equation. Once the force and moment acting on fish are obtained at each instant, motion of fish can then be tracked by applying Newton's second law. Effects of force and moment exerting on fish, the moving speed and distance traveled by fish under different types of S-start and C-start movements are compared and discussed. Also presented are the fluid-dynamics interpretation of the fast-start motions and explanation of key mechanisms that lead to different performances of S-start and C-start modes. Basically in S-start mode, fish receives significant thrust from the momentum of accelerating fluid enhanced by the starting vortex near its tail. While in C-start mode, fish speeds up via a turning motion induced by the fluid force and moment acting on it. Since in C-start mode, fish utilizes the lateral force and moment to assist its turning and speeding rather than resists them, fish moving with C-start mode has faster moving speed and better efficiency of performance than with S-start mode. This can explain why fish normally adopt C-type fast-start mode in their attempt to escape from danger.