Wake vortex structures and hydrodynamics performance of a power-extraction flapping hydrofoil

Numerical simulations are used to investigate wake vortex structures with different aspect ratios (ARs) of flapping hydrofoil on power extraction. Simulations employ a finite-volume method with an overset grid technique allowing relatively large heaving and pitching motions of the flapping hydrofoil. In a two-dimensional case, the wake is dominated by a Karman-like vortex street composed of spanwise vortices. In contrast, the wake in a three-dimensional condition is dominated by two sets of vortex structures that are oblique to the streamwise, which is significantly different from the two-dimensional condition. The two sets of vortex structures interact with each other as they convect downstream. For a large AR flapping hydrofoil, the two sets of vortex loops stay intertwined with each other. The vortex loops evolve into vortex rings for small AR. In addition, the leading edge vortex on the foil surface is compressed by the tip vortices, affecting the synchronization of vertical force and foil motion. This effect of compression is weakened as the AR increases. The power-extraction efficiency reaches an optimal value of 32.4% at AR = 8 in a three-dimensional condition.