A semi-submerged vortex-induced bending-torsional vibration energy harvester: design, modeling, and experimental validation

Hydrokinetic energy is abundantly available in natural environments such as rivers and canals. Although various piezoelectric, triboelectric, and electromagnetic energy harvesters have been proposed to harness flow-induced vibrations, most cantilever-based piezoelectric designs utilize symmetric bluff bodies that primarily induce transverse bending. While torsional motion may arise incidentally due to gravitational or structural asymmetries, existing systems generally do not exploit torsional or coupled-mode dynamics for energy harvesting. This study presents a semi-submerged vortex-induced vibration energy harvester that selectively operates within surface-layer flows, without requiring intrusive support structures. The device consists of a cantilever beam with integrated piezoelectric patches and an eccentric cylindrical bluff body designed to actively induce coupled bending–torsional vibration through asymmetric vortex shedding. Piezoelectric layers are strategically configured to capture strain energy from both bending and torsion, enabling multi-mode energy harvesting. The governing equations are derived and validated through air-based frequency sweep experiments. Subsequent water tunnel experiments investigated the effects of tip mass, beam length, and immersion depth on system performance. Results show that increasing the tip mass within an optimal range enhances both vibration amplitude and energy conversion efficiency. In certain parameter regimes, internal resonance conditions are triggered, resulting in dual-peak voltage outputs of $4.366V$ (at $0.168m/s$) and $14.981V$ (at $0.456m/s$), demonstrating improved adaptability across a range of flow conditions.