Hydrodynamic properties of large-amplitude pitching hydrofoil

Motivated by the extraordinary maneuvering capabilities by natural creatures, the unsteady fluid dynamics of the flapping air- and hydrofoils has been of keen sustained interest in recent decades, which are critical to the performance of the biomimetic thrusters. For the hydrofoil pitching with large amplitudes, in which situation the flow is massively separated from foil, the vortex evolutions and their implications on the hydrodynamic properties have not been described clearly. This paper numerically investigates the hydrodynamic properties of a large-amplitude 32° pitching NACA (National Advisory Committee for Aeronautics) 0012 hydrofoil, with Strouhal number 0.006–1.30. Focused on the vortex evolutions and their implications on the hydrodynamic responses, four wake types, i.e., leading-edge vortex (LEV)–trailing-edge vortex pair wake, von Kármán-like wake, inverted von Kármán wake, and deflected wake, are classified and discussed in detail, via vorticity patterns and the surface pressure distribution. The increasing Strouhal number delays the formation and the advection of the LEV and drastically alters the vortex dynamics. The asymmetric wake reinforces the thrust generation, which facilitates the transition from the drag to thrust. In addition, the effect of the pitching amplitude on the thrust generation and the wake types is also investigated, with pitching amplitude 15°–40°. With the pitching amplitude increases, the thrust and the efficiency monotonously reduce, and the thresholds for different wake types are also postponed.