Vortex induced vibration of a deformable cylinder for wide lock-in range energy harvesting

Energy harvesting from ambient sources has gained attention due to increasing energy demands. Despite VIV-based harvesters showing significant potential, their lock-in region, where significant power is generated, is narrow. Given the continuously varying ambient conditions of fluid currents, harvesters can easily fall into de-synchronization, yielding low energy output. Existing solutions like tunable masses or multiple degrees of freedom systems increase complexity and weight, limiting practical applications. This work introduces a novel variable-diameter cylinder mechanism–a practical technique that actively tunes the cylinder's geometry in real time to enhance energy harvesting efficiency from VIV. The mechanism employs an expanding pulley system that deforms the elastic circular cylinder radially, dynamically altering its diameter to adjust key non-dimensional parameters governing VIV. This real-time adaptability counteracts ambient fluctuations, significantly widening the lock-in range. The cylinder incorporates a piezoelectric patch, and the fluid-structure-piezoelectric interaction problem was analyzed numerically to determine the cylinder motion, voltage, and power output. A partitioned Lagrange-Eulerian approach was employed, coupling a fluid solver with a custom-coded structural piezoelectric solver via the preCICE library, which enables data mapping and exchange between solvers at run time. The analysis was conducted for different diameter profiles, and the results showed an enhancement in the harvester synchronization width and maximum amplitude by 70% and 117%, respectively, compared to the constant diameter case. Additionally, a 113% increase in peak voltage was achieved, and more than 8 times greater power was generated. The analysis also showed the effect of load resistance on harvesting performance, and the varying diameter cases demonstrated resilience against the shunt-damping effect. The proposed control technique is versatile, as it can be used for VIV suppression as well as energy harvesting applications.