Experimental Study of Body-Fin Interaction and Vortex Dynamics Generated by a Two Degree-Of-Freedom Fish Model

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.

Tail shapes lead to different propulsive mechanisms in the body/caudal fin undulation of fish

“…With the presence of body before the caudal fin, the body-fin interaction is significant. 13,43 As shown in Figure 13, the vortex resulting from the posterior body (PBV) moves around the peduncle due to the pressure difference between two sides of body, and the leading edge vortex (LEV) is also generated on the anterior part of the fin. The interaction of PBV strengthens the LEV produced by the caudal fin.…”

Design of a Carangiform Swimming Robot through a Multiphysics Simulation Environment

“…Here, each revolute joint along the kinematic chain is directly actuated by a dedicated servomotor. Thus, in order to comply with the tail deformation predicted by Lighthill's travelling wave, it is easy to show how each link of the mechanism must oscillate following a harmonic motion law, such as [10,11]: The trade-off between the conflicting demands of maneuverability and propulsive efficiency caused the authors to adjust and improve upon their design, moving from the ostraciiform to the carangiform swimming mode, as detailed in the present work. As stated in [1], the motion law generally adopted in modeling the tail undulation of carangiform swimmers is Lighthill's travelling wave [5], a harmonic function with an amplitude that increases towards the caudal fin and that describes the shape of the tail as a function of time.…”

“…Here, each revolute joint along the kinematic chain is directly actuated by a dedicated servomotor. Thus, in order to comply with the tail deformation predicted by Lighthill's travelling wave, it is easy to show how each link of the mechanism must oscillate following a harmonic motion law, such as [10,11]: The highest propulsive efficiency is achieved in thunniform locomotion, where the body undulation is confined to the rigid and flapping caudal fin. On the other hand, anguilliform swimmers experience the uppermost maneuverability as they bend their bodies according to different patterns called swimming gaits.…”

“…Here, each revolute joint along the kinematic chain is directly actuated by a dedicated servomotor. Thus, in order to comply with the tail deformation predicted by Lighthill's travelling wave, it is easy to show how each link of the mechanism must oscillate following a harmonic motion law, such as [10,11]:…”

“…Here, each revolute joint along the kinematic chain is directly actuated by a dedicated servomotor. Thus, in order to comply with the tail deformation predicted by Lighthill’s travelling wave, it is easy to show how each link of the mechanism must oscillate following a harmonic motion law, such as [ 10 , 11 ]: where p j is the absolute angle of link j with respect to a reference frame attached to the robot head; a j is the oscillation amplitude; f is its frequency, which is the same for all the links; and φ j is the constant phase shift between the harmonic motion laws of the links. Position control is then required to comply with the non-linear function (1) and to synchronize the servomotors in order to maintain the constant angular position difference between the links.…”