Passively adaptive wind energy harvester featuring a double-airfoil bluff body with adjustable attack angles

In nature, the wind always fluctuates in magnitude. Hence, it is essential to consider the adaptability and durability of wind energy harvesters. This paper proposes a passively adaptive piezoelectric wind energy harvester with a double-airfoil bluff body to enhance performance subjected to time-varying wind velocity. The proposed double-airfoil bluff body is inspired by the fact that the attack angle can significantly affect the aerodynamic characteristics of the airfoil. An associated aero-electromechanical model is numerically developed to investigate the influence of the attack angle on the energy harvesting performance. Based on the aero-electromechanical model, the numerical results demonstrate three typical working modes that appear under different attack angles: vortex-induced vibration, galloping, and vibration suppression. These working modes can promise the proposed harvester to effectively and safely scavenge ambient wind energy. The physical mechanisms behind the three working modes are thoroughly explored given the work-energy principle and evolution of the vortex structure. The performance enhancement of the proposed harvester can be attributed to the positive aerodynamic work and an airfoil dynamic stall. Experiments are thereafter conducted to validate the theoretical results. Compared to the conventional galloping-based harvester with a square prism, the maximum voltage improvement percentage of the proposed harvester is over 62.3% within the investigated wind speed range when the attack angle is set to 10°.