Date Published: March 31, 2017
Publisher: Public Library of Science
Author(s): Ningyu Li, Huanxing Liu, Yumin Su, Iman Borazjani.
Numerical simulations are employed to study the hydrodynamics of self-propelled thunniform swimming. The swimmer is modeled as a tuna-like flexible body undulating with kinematics of thunniform type. The wake evolution follows the vortex structures arranged nearly vertical to the forward direction, vortex dipole formation resulting in the propulsion motion, and finally a reverse Kármán vortex street. We also carry out a systematic parametric study of various aspects of the fluid dynamics behind the freely swimming behavior, including the swimming speed, hydrodynamic forces, power requirement and wake vortices. The present results show that the fin thrust as well as swimming velocity is an increasing function of both tail undulating amplitude Ap and oscillating amplitude of the caudal fin θm. Whereas change on the propulsive performance with Ap is associated with the strength of wake vortices and the area of suction region on the fin, the swimming performance improves with θm due to the favorable tilting of the fin that make the pressure difference force more oriented toward the thrust direction. Moreover, the energy loss in the transverse direction and the power requirement increase with Ap but decrease with θm, and this indicates that for achieving a desired swimming speed increasing θm seems more efficiently than increasing Ap. Furthermore, we have compared the current simulations with the published experimental studies on undulatory swimming. Comparisons show that our work tackles the flow regime of natural thunniform swimmers and follows the principal scaling law of undulatory locomotion reported. Finally, this study enables a detailed quantitative analysis, which is difficult to obtain by experiments, of the force production of the thunniform mode as well as its connection to the self-propelled swimming kinematics and vortex wake structure. The current findings help provide insights into the swimming performance and mechanisms of self-propelled thunniform locomotion.
Despite impressive innovations in underwater vehicles, both the military and scientific communities are expecting to benefit from vehicles with better performance, and the biomimetic propulsion system which applies principles abstracted from fish swimming has been increasingly used [1–4]. Fish which primarily employ body and/or caudal fin (BCF) swimming mode for propulsion and maneuvering are divided into five families in accordance with the manner they swim: anguilliform, sub-carangiform, carangiform, thunniform and ostraciiform [5–7]. For thunniform fish, the undulation is limited to the rear 1/3 of the body and reaches the maximal amplitude at the end of the tail peduncle [8, 9]. As the main propulsive device, driven by the tail peduncle, the caudal fin sways and oscillates [10, 11]. As a primary locomotion mode for many fast moving swimmers, thunniform swimming has attracted increasing attention.
Numerical simulations are carried out to study the hydronamics of a thunniform swimmer which is undulated laterally in the viscous fluid and moved freely under a self-propelled 3-DoF condition. The complex interaction of the fish with surrounding viscous flow is achieved by the FSI method with an in-house developed UDF.