Effect of rotor ellipticity on the flow-induced vibration and energy harvesting performance of a coupled power generation device with Savonius blades

Flow-induced vibration and rotation provide alternative approaches for marine current energy extraction, while the influence of rotor geometry on the coupled vibration–rotation dynamics remains unclear. This study numerically investigates a cylinder-based energy harvesting device with symmetric Savonius blades under different rotor ellipticities ( ε = 0.5, 1.0, and 1.5). A two-dimensional fluid–structure interaction model is established using the unsteady Reynolds-averaged Navier–Stokes equations with the SST k−ω turbulence model, where translational and rotational motions are simultaneously considered through an overset mesh method and user-defined functions. The attached rotor changes the wake behavior around the cylinder and leads to a response pattern different from that of the bare cylinder. At low reduced velocities, rotation is hardly developed because the generated hydrodynamic torque is insufficient to overcome the resisting moment. As the flow velocity increases, rotational motion becomes increasingly significant and interacts with the vibration response. The variation in rotor ellipticity alters the blade loading distribution and the associated wake–structure interaction. The rotor with ε = 0.5 exhibits a higher rotational response, whereas increasing ellipticity weakens angular acceleration due to the increased rotational resistance. These results highlight that rotor geometry plays an important role in determining the balance between wake excitation, structural vibration, and rotational response in vibration–rotation energy harvesting systems.