Numerical simulation of self-propelled aquatic swimming in uniform and vortical flows

Fish swimming has fascinated scientists for a long time but important questions regarding the effect of scale (Reynolds number), body shape and kinematics, and approach flow on swimming performance still remain unanswered. In this paper we review our previous computational work with tethered and self-propelled virtual swimmers in a free stream and present new results of tethered swimmers in the wake of a cylinder in order to provide answers to some of these questions. The work with tethered swimmers showed that carangiform swimmers (e.g. mackerel) are more efficient in the inertial regime while anguilliform swimmers (e.g. lamprey) are more efficient in the transitional regime. To isolate the effects of body shape and kinematics, we created two hybrid virtual swimmers—a mackerel swimming like lamprey and a lamprey swimming like a mackerel—and made them race each other in the same hydrodynamic environment by performing self-propelled simulations. We found that the mackerel body always reached higher velocities in all flow regimes but is more efficient only in the inertial regime. The lamprey body was found to be more efficient in the transitional regime. The lamprey kinematics reached higher velocities and was more efficient in the transitional regime while the mackerel kinematics in the inertial regime. The simulations of a tethered mackerel in the wake of a circular cylinder show that the cylinder wake gives rise to larger thrust-type force relative to that of the same mackerel swimming in uniform ambient flow.