Coupled effects of center-of-gravity position and added mass on vortex-induced vibration and galloping-based hydrokinetic piezoelectric energy harvesting

This study reveals a dual-physics control mechanism in which the center-of-gravity (CG) position and added mass act as coupled parameters governing piezoelectric energy harvesting in low-velocity water flow (0.1–0.6 m/s) across vortex-induced vibration (VIV) and galloping regimes. Experiments were conducted in an open-channel flow using a cylinder bluff body for VIV and semi-cylinder and square bluff bodies for galloping to investigate the effects of CG position and added mass on the onset velocity, operating range, and electrical output. From the obtained results, the cylinder bluff body with the top CG position and a 50 g added mass was found to achieve the highest VIV performance, yielding an onset velocity of 0.275 m/s, an operating range extending to 0.525 m/s, and a maximum power output of 0.467 mW. This enhancement is attributed to improved synchronization between the damped natural frequency and the vortex-shedding frequency, thereby expanding the lock-in region. In contrast, the semi-cylinder bluff body with the bottom CG position and a 150 g added mass achieved the highest galloping performance, yielding an onset velocity of 0.25 m/s and a maximum power output of 3.94 mW. This enhancement arises because effective inertia grows faster than restoring stiffness, sustaining larger amplitudes despite the delayed onset. Among the configurations tested, these findings identify the CG position and added mass as coupled design parameters, with a higher CG position and lower added mass favoring VIV, whereas galloping exhibits the opposite trend in hydrokinetic piezoelectric energy harvesters.