Development of a novel wake-induced rotational galloping wind energy harvester and the identification of its working mechanism

This study proposes a novel, high-performance electromagnetic wind energy harvesting system using wake-induced rotational galloping. The design idea is based on the premise that there exist multiple time delays between the angular displacement of a downstream bluff body and the torque generated on it, which induces system instability. A two-fold electromagnetic system was employed to efficiently impart torsional elasticity and convert mechanical energy into electrical energy. Furthermore, this study reports, for the first time, a mathematical model associated with wake-induced galloping in an autonomous form to explain the aerodynamic memory effect that causes system instability.

To identify the mechanism of the rotational galloping phenomenon occurring in the proposed energy harvesting system, comprehensive computational fluid dynamic analyses were first performed. Subsequently, the resulting aerodynamic moment diagram, in terms of the angular displacement of the downstream bluff body, was transformed into a nonlinear autonomous form through a Fourier analysis and trigonometric relations. The extended Hamilton’s principle was applied to derive a set of coupled governing equations, which were then combined with the autonomous form of the aerodynamic moment.

To determine the final system configuration, a series of preliminary and main experiments were performed with various geometric parameters. Subsequently, an eigen-analysis was performed, and a set of delay differential equations was solved and analyzed to determine the system stability and predict its dynamic and energetic behaviors under various conditions. A comparison between the results of the model with those of the experiment showed a good agreement, validating the accuracy of the proposed autonomous nonlinear time-delay model. The proposed energy harvesting system achieved an average output of 9.3 mW and a power density of 131 W/m³ at a wind velocity of 10 m/s, illustrating its efficiency and optimal energy harvesting performance.

From the viewpoint of how this study offers practical value, various column-shaped obstacles commonly used in urban areas negatively impact the performance of traditional galloping-based energy harvesters; however, with the proposed system, they generate vortices to cause instability, thereby helping improve energy harvesting efficiency.