Abstract
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.
Highlights
We present the nominal grid adopted in the current simulations. 3.42 million elements are used for the whole simulation domain
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-degree of freedom (DoF)
Hydrodynamic analysis on different parts of fish body shows that the caudal fin is a major source of thrust production, and interestingly the deforming rear body can provide a small thrust in a certain period of time, which is similar to that found in anguilliform swimmers
Summary
Despite impressive innovations in underwater vehicles, both the military and scientific communities are expecting to benefit from vehicles with better performance, and the biomimetic. Carling et al [44], Kern and Koumoutsakos [45], Hu et al [46], Katumata et al [47] and van Rees et al [48] simulated self-propelled anguilliform swimming In their investigations, the swimming speed was not prescribed beforehand but obtained as a solution, and a quantitative study of the body motion, wake flow field and hydrodynamic forces was provided. Systematic parametric studies of the swimming speed, hydrodynamic forces, propulsive efficiency and wake structure of thunniform mode have not been well addressed in the literature, since keeping one of parameters constant while changing others in a repeatable manner is hard to achieve in live-fish experiments [8, 52,53,54]. Based on the present numerical simulations, we comment on the overall hydrodynamics of the self-propelled thunniform swimming
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