Energy-harvesting characteristics of flapping wings with the free-surface effect: Fid-Bau Portal

Energy-harvesting characteristics of flapping wings with the free-surface effect

Zhu, Bing / Cheng, Wenhao / Geng, Jingyan / Zhang, Junwei

Flapping-wing devices working in an energy-harvesting mode have the advantage of environmental adaptation. To analyze the energy-extraction characteristics of a flapping wing with the free-surface effect, transient-numerical studies were carried out based on the homogeneous two-phase volume-of-flow model, the shear stress transport k–ω turbulence model, and dynamic-grid technology. These studies took into consideration the influences of the dimensionless submergence depth $S_{d}$ along with the Froude number Fr. The following results were obtained. (1) In the subcritical condition of Fr < 1, there was a critical hydrofoil submergence depth. When it was greater than this critical value, the existence of the free surface was able to promote the energy-extraction efficiency. On the contrary, the closer the flapping wing was to the free surface, the lower its energy-harvesting efficiency was. (2) When the hydrofoil submergence depth was small, the energy-harvesting efficiency first increased and then decreased with the increase in Fr. Furthermore, the smaller $S_{d}$ was, the faster the energy-extraction efficiency of the flapping wing decreased. While $S_{d}$ was large, for example $S_{d}$ > 9, the energy-extraction efficiency first increased and then gradually approached the unbounded-flow condition as Fr increased, but it was always lower than the unbounded-flow case. (3) Compared with the case of unbounded flow, the existence of the free surface affected the motion of the leading-edge vortex, thereby changing the magnitude and direction of the lift force and pitch moment. The relative position of the free-surface wave crest to the wing also affected the pressure distribution around the flapping-wing surface, which in turn affected the energy-harvesting properties. Additionally, Fr affected the formation and shedding of the vortex around the flapping wing, and the movement synchronization between the leading-edge vortex and the flapping wing was extremely important to the energy-harvesting performance.