Analysis of the Caudal Fin Propulsion Mechanisms by BCF Swimming Fish from the Perspective of Force Element Theory

• Main Authors: YAU-TING TZENG, 曾耀霆
• Other Authors: 朱錦洲
• Format: Others
• Language: zh-TW
• Published: 2013
• Online Access: http://ndltd.ncl.edu.tw/handle/96439730765154142593

碩士 === 國立臺灣大學 === 應用力學研究所 === 101 === In this study, we investigate three-dimensional thrust mechanisms of finite rigid and deformable flapping plates simulated as a caudal fin of the BCF (body and/or caudal fin) swimming fish at low Reynold numbers Re=500 from the perspective of force element theory. Three values in Strouhal Number (St=0.2, 0.4 and 0.6) ranged in the nature regime and four stiffness (a_0=0, 0.1, 0.15, and 0.2) of the plate are considered. It is shown that the thrust generation of the flapping plate is mainly dominated by the acceleration of the body C_Da as well as vorticity in the flow field and on the body surface C_Dv and C_Df; moreover, C_Dv and C_Df will dramatically increase accompanied by the increasing in St and a_0 due to the generation of stronger vortices, including two sides of the tip vortices and the trailing-edge vortices. Further, we could precisely quantify the force contribution of each vortex strucuture in the flow field. Carefully examining, it is shown that the leading-edge vortex generated in a stroke of flapping motion provodes resistance contribution. However, the vortices around two sides and the trailing edge of plate enhanced by the deformation have contribution to the thrust except the cases of rigid plate. The main reason that results in nearly zero thrust contribution for the heaving rigid plates is due to geometric effects of the auxiliary potential. Therefore, given by a slight deflection, the flapping plate could sufficiently gain thrust forces to move forward. In a final, the propulsive efficiency η of flapping plates is computed for different St, and show that η attains to the optimum at St=0.4, whilst the thrust force is the greatest at St=0.6. Therefore, no matter how fish pursue high swimming velocities or high cruising endurance, it must be trade-off under a swimming strategy.