On the torque mechanism of Savonius rotors

The aerodynamic performance and the flow fields of Savonius rotors at various overlap ratios have been investigated by measuring the pressure distributions on the blades and by visualizing the flow fields in and around the rotors with and without rotation. Experiments have been performed on four rotors having two semicircular blades but with different overlap ratios ranging 0 to 0.5. The static torque performance is improved by increasing the overlap ratio especially on the returning blade, which is due to the pressure recovery effect by the flow through the overlap. On the other hand, the torque and the power performance of the rotating rotor reaches a maximum at an overlap ratio of 0.15. This effect is largely created by the Coanda-like flow on the convex side of the advancing blade, which is strengthened by the flow through the overlap at this small overlap ratio. However, this phenomenon is weakened as the overlap ratio is further increased suggesting a deteriorated performance of the rotor. Observations of the flow inside the rotor indicate an increased recirculating region at such large overlap ratios, which also suggests a reduced aerodynamic efficiency for rotors with large overlap.