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.