Enhanced energy harvesting in spacing optimized three-cylinder systems through coupled vortex-induced vibration and wake-induced galloping

Flow-induced vibration (FIV) of cylinders provides a promising pathway for ambient wind energy harvesting, yet its practical application is constrained by a narrow lock-in region and limited operational bandwidth. This study proposes a three-cylinder piezoelectric energy harvester (TC-PEH) that leverages the coupled dynamics of vortex-induced vibration (VIV) and wake-induced galloping (WIG) to overcome these limitations. Wind tunnel experiments were conducted over wind speeds from 0.5 to 10 m/s to systematically examine the influence of both uniform and non-uniform spacing ratio configurations. The findings demonstrate that spacing ratio L/D plays a decisive role in governing the FIV regime, at L/D = 1.3, the system exhibits strongly coupled, upstream-dominated WIG, while larger spacings lead to decoupled VIV and WIG behavior. According to the proposed regime, the optimal uniform arrangement (L/D = 1.3) delivers a 15-fold increase in average output power and a 4.1-fold rise in output voltage relative to a single-cylinder baseline, with consistently stable voltage across most of the operating range. Furthermore, a strategically engineered non-uniform spacing (L1/D = 1.5, L2/D = 1.3) enhances maximum output power by 25% and induces downstream synergy, achieving peak output voltages up to 40 V. These results confirm that spacing optimization enables robust VIV-WIG coupling in multi-cylinder arrays. This work establishes a high-efficiency, wide-bandwidth design framework for FIV-based energy harvesters serving micro-electro-mechanical systems (MEMS) applications.