Numerical investigation of an actively and passively controlled skeleton-reinforced caudal fin

We numerically investigate the propulsion performance of a skeleton-reinforced caudal fin with both active and passive control mechanisms. In our fluid–structure interaction model, the embedded rays are depicted as nonlinear beams while the flow is simulated using a Navier–Stokes solver. Kinematically, the fin is activated by a sway motion at the basal ends of the rays and distributed time-varying forces along each ray individually. The dynamics of the fin is closely associated with the exact distribution of phase lags (between the sway motion and external forces) among the rays. We find that the fin’s performance can be significantly enhanced by active control when the mean phase lag is less than 90 deg. Among different deformation patterns, the cupping deformation (C mode) produces the best propulsion performance and the W-shape deformations (W mode) have a similar (yet less pronounced) effect. Asymmetric deformations such as the heterocercal mode (H mode) and undulation mode (S mode) are able to generate vertical forces. Compared with the H mode, the S mode creates less thrust force, but it significantly reduces the transverse force, making it more suitable in cases when there is no other mechanism to balance this transverse force (for example, during the braking maneuver).