Underwater oscillation performance and 3D vortex distribution generated by miniature caudal fin-like propulsion with macro fiber composite actuation

Abstract

Underwater biomimetic propulsion systems that mimic the BCF (Body and/or Caudal Fin) fishes have attracted considerable attention. A miniature caudal fin-like propulsion device, whose design is based on the morphology and kinematics of the koi fish, is proposed in this paper. The propulsion is actuated by flexible fiber-based piezoelectric actuators, Macro Fiber Composite (MFC). A vacuum fabrication procedure is employed to bond the components of the proposed device. The underwater oscillation performances of the proposed propulsion at different steady-state sinusoidal excitations are experimentally investigated. Computational fluid dynamic (CFD) analysis is conducted to investigate the three-dimensional flow generated by the oscillation propulsion system. A simplified model, where the internal bending moment provided by the MFC actuators is replaced by a shear force, is created based on the obtained experimental data. The CFD simulations demonstrate that the maximum instantaneous and mean thrusts generated by the propulsion system during the stable period are 21.5 and 9.5 mN, respectively. These are consistent with the Lighthill’s elongated-body theory. Then, the uneven spatial distributions of the instantaneous vorticity around the propulsion are visualized, using seven two-dimensional planes intersecting at various locations. The CFD results suggest that the vorticity distributions around the trailing and leading edges of the caudal fin considerably differ because of the fin’s unique shape. A reverse Karman vortex street pattern is observed behind the trailing edge of the caudal fin. This indicates a thrust-producing pattern. On the other hand, a Karman vortex street pattern is distinctly observed before the leading edge. This indicates a drag-producing pattern. The geometry and shape effects of the caudal fin-like propulsion device are identified consequently. Those obtain results may be beneficial for the design of underwater vehicles with oscillation fin-like propulsion system.