Fluid–structure interaction study on the performance of flexible articulated caudal fin

In this paper, the propulsive performance of a flexible articulated caudal fin is investigated by fluid–structure interaction. The caudal fin is composed of two links which are connected by a hinge. One link is driven by pitching motion while the other one moves passively. Five cases of link flexibility are investigated, namely, the rigid–rigid case, the medium flexible–medium flexible case, the flexible–flexible case, the rigid–flexible case and the flexible–rigid case. Their fluid field and structure deformations are analysed and hydrodynamic forces are compared. It is found that the rigid–rigid caudal fin produces larger thrust force than other cases with a low-pitching frequency, while the rigid–flexible case performs better with a higher frequency. The mean thrust force increases with the frequency in our experiments, however, for the medium flexible–medium flexible case, an optimal frequency exists. Besides, the effect of the hinge stiffness is studied. It is seen that the medium flexible–medium flexible case exhibits a striking performance. When the hinge stiffness decreases, its mean thrust force increases and possesses larger amplitude while the forces of other cases decrease. These results can guide the design of flexible propeller with links and will be useful for the development of flexible underwater robots.

本文基于流固耦合（fluid–structure interaction）方法，针对柔性铰接尾鳍的推进性能展开研究。该尾鳍由两节通过铰链连接的连杆组成，其中一节连杆受俯仰运动驱动，另一节则被动随动。本次研究设置了五种连杆柔性组合工况，分别为刚-刚性工况、中柔性-中柔性工况、全柔性-全柔性工况、刚-柔性工况及柔-刚性工况。研究对各工况下的流场分布与结构形变开展了分析，并对比了水动力载荷。结果显示，在低俯仰频率下，刚-刚性尾鳍的推力性能优于其余工况；而在高俯仰频率下，刚-柔性工况表现更优。本实验中，平均推力随俯仰频率升高呈上升趋势，但中柔性-中柔性工况存在最优俯仰频率。此外，本文还探究了铰链刚度的影响，发现中柔性-中柔性工况展现出优异的推进性能：随着铰链刚度降低，该工况的平均推力与推力振幅均有所提升，而其余工况的推力则出现下降。上述研究成果可为带连杆结构的柔性推进器设计提供参考，同时对柔性水下机器人的研发具有借鉴意义。