Experimental and numerical studies on a passively deformed coupled-pitching hydrofoil under the semi-activated mode

Flexibility is expected to positively affect the hydrodynamic and energy-harvesting performance of deformable hydrofoils, which have a simple structure and can be easily implemented in practical engineering applications. However, the characteristics of the fluid-structure interaction and power capturing are complicated. In this study, to reveal the fluid-structure interaction (FSI) mechanism and the effects of the passive deformation on the energy-harvesting performance, a flexible hydrofoil to conduct a coupled-pitching motion under the semi-activated mode was proposed and investigated experimentally and numerically. In the experimental study, a deformation-measuring technology based on a digital imaging algorithm was developed and employed for water channel tests. The effects of the hydrofoil profile and activated pitching amplitude on the deformation status, torque output, and energy-harvesting performance were studied experimentally. In the numerical study, a three-dimensional unsteady two-way FSI model was established and validated using experimental results. The influence mechanisms of the damping torque coefficient, activated pitching amplitude, and elasticity modulus on the passive deformation, its phase difference with the activated pitching angle, and the hydrodynamic and energy-harvesting performances were studied. Compared with the rigid hydrofoil, the energy-harvesting efficiency and power coefficient of the passively deformable hydrofoil could be increased by 4.8 % and 8.0 %, respectively, as the phase difference zone was close to 3π/4. In addition, positive and negative phase difference zones for performance enhancement were identified, which provides a future direction for optimizing flexible hydrofoils and control strategies.