Tunas are known for their extraordinary swimming performance, which is accomplished through various specializations. The caudal keels, a pair of lateral keel-like structures along the caudal peduncle, are a remarkable specialization in tunas and have convergently arisen in other fast-swimming marine animals. In the present study, the hydrodynamic function of caudal keels in tuna was numerically investigated. A three-dimensional model of yellowfin tuna with caudal keels was constructed based on previous morphological and anatomical studies. Vortical structures and pressure distributions are analyzed to determine the mechanisms of thunniform propulsion. A leading-edge vortex and a trailing-edge vortex are attached to the caudal fin and enhance the thrust. By comparing models of tuna with and without caudal keels, it is demonstrated that caudal keels generate streamwise vortices that result in negative pressure and reduce the transverse force amplitude. Moreover, the orientations of the streamwise vortices induced by caudal keels are opposite to those on the pressure side of the caudal fin. Therefore, caudal keels reduce the negative effects of the streamwise vortices adjacent to the caudal fin and thereby enhance the thrust on the caudal fin. A systematic study of the effects of variations in the Strouhal number (St), the Reynolds number (Re), and the cross-sectional shape of the body on the swimming of tuna is also presented. The effects of caudal keels are magnified as Re and St increase, whereas the cross-sectional shape has no major influence on the caudal keel mechanism.

中文翻译：

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金枪鱼专业化：尾龙骨功能的数值研究

Tunas因其出色的游泳性能而闻名，这可以通过各种专业来实现。尾龙骨是沿尾柄的一对侧向龙骨状结构，是金枪鱼的显着专长，并已在其他快速游动的海洋动物中汇聚。在本研究中，对金枪鱼尾龙骨的水动力功能进行了数值研究。基于先前的形态学和解剖学研究，构建了带有尾龙骨的黄鳍金枪鱼的三维模型。分析了涡流的结构和压力分布，确定了楔形推进的机制。前缘涡流和后缘涡流连接到尾鳍并增强了推力。通过比较有和没有尾龙骨的金枪鱼模型，结果表明，尾龙骨会产生沿流的涡流，从而产生负压并减小横向力幅度。此外，由尾龙骨引起的流向涡流的方向与尾鳍压力侧的方向相反。因此，尾龙骨减小了与尾鳍相邻的沿流涡旋的负面影响，从而增强了对尾鳍的推力。对Strouhal数变化的影响的系统研究（尾龙骨减少了与尾鳍相邻的水流涡流的负面影响，从而增强了尾鳍的推力。对Strouhal数变化的影响的系统研究（尾龙骨减少了与尾鳍相邻的水流涡流的负面影响，从而增强了尾鳍的推力。对Strouhal数变化的影响的系统研究（St），雷诺数（Re）以及金枪鱼游泳时身体的横截面形状。随着Re和St的增加，尾龙骨的影响被放大，而横截面形状对尾龙骨的机理没有重大影响。