Abstract
Oscillatory modes of swimming are used by a majority of aquatic swimmers to generate thrust. This work seeks to understand the phenomenological relationship between the body and caudal fin for fast and efficient thunniform swimming. Phase-averaged velocity data was collected and analyzed in order to understand the effects of body-fin kinematics on the wake behind a two degree-of-freedom fish model. The model is based on the yellowfin tuna (Thunnus albacares) which is known to be both fast and efficient. Velocity data was obtained along the side of the tail and caudal fin region as well as in the wake downstream of the caudal fin. Body-generated vortices were found to be small and have an insignificant effect on the caudal fin wake. The evolution of leading edge vortices formed on the caudal fin varied depending on the body-fin kinematics. The circulation produced at the trailing edge during each half-cycle was found to be relatively insensitive to the freestream velocity, but also varied with body-fin kinematics. Overall, the generation of vorticity in the wake was found to dependent on the trailing edge motion profile and velocity. Even relatively minor deviations from the commonly used model of sinusoidal motion is shown to change the strength and organization of coherent structures in the wake, which have been shown in the literature to be related to performance metrics such as thrust and efficiency.
Highlights
A majority of aquatic animals use oscillations of their body and fins to produce thrust for locomotion
We are clearly capturing a number of coherent phenomena in these wakes: boundary layers and discrete vortices along the body and tail portion of the model, coherent vortices being generated by the motion of the anterior, or leading, swept edge of the caudal fin, and strong spanwise vortices and shear layers being generated by and shed from the caudal fin trailing edge
All these structures show some variation in organization or timing among the different kinematic groups, and will be discussed
Summary
A majority of aquatic animals use oscillations of their body and fins to produce thrust for locomotion. There are numerous forms of oscillatory motions that are used in aquatic swimming; thunniform swimmers are most commonly studied for their ability to swim efficiently and quickly [1,2,3] This form of swimming is characterized by a relatively stiff body with the majority of lateral motion restricted to the posterior 10% of the body, which consists of the peduncle region and caudal fin [2]. The fact that the lateral motion is concentrated in the posterior portion of the body has led many researchers to eliminate the complexity that arises from fin-fin and body-fin interaction by studying the caudal fin in isolation In these cases, the caudal fin is typically modeled as a relatively simple pitching and/or heaving airfoil or thin plate, both rigid and flexible, in experiments and simulations. Several recent numerical works have modeled the entire body of fish to understand thrust generation
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