Nobuyuki Fujisawa

On the torque mechanism of Savonius rotors

Abstract 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.

Velocity measurements and numerical calculations of flow fields in and around Savonius rotors

Abstract The flow fields in and around Savonius rotors at various overlap ratios have been investigated to clarify the effect of overlap on the flow mechanisms. Measurements of phase-averaged velocity distributions were carried out using particle imaging velocimetry with a conditional sampling technique, and the results were compared with numerical calculations by a discrete vortex method. The measured velocity distributions indicate clearly the effect of the overlap both on the flow through the overlap and on the formation region of the vortices downstream of the rotor. Although numerical calculations can predict the basic features of the variation of the flow field with rotor angle, the flow field of a stationary rotor, especially at small rotor angles, is not well reproduced in the calculations, suggesting that the assumptions of flow separation at the tips of the blades and the two-dimensionality of the flow are invalid. However, the correlation with measurement is improved for the flow over a rotating rotor.

Pressure measurements and flow visualization study of a Savonius rotor

Abstract The power mechanism of a Savonius rotor has been studied by pressure measurements on the blade surface and by a flow visualization experiment. It is found that a low pressure region is formed on the convex side of the advancing blade contributing to the power production of the rotating rotor. This phenomenon is observed as a Coanda-like flow pattern in the flow visualization experiment, which controls the flow separation on the convex side. It is also noticed that the flow field near the rotor is effectively two-dimensional as the torque and the power coefficients evaluated from the pressure measurements at the middle plane agree closely with the results by total torque measurement.

A Comparative Study of Navier-Stokes Calculations and Experiments for the Savonius Rotor

This note compares the numerical calculations from the two-dimensional Navier-Stokes equations with experimental streakline and pressure distribution data for an operating rotor at maximum power conditions

Visualization of Flow Phenomena in and Around a Savonius Rotor

Flow phenomena occurring in and around a Savonius rotor have been studied by extensive flow visualizations and by the measurement of pressure distributions on the blade surfaces. Experiments are performed both for the stationary and rotating rotor with and without overlap. Based on the results, the variations of the torque mechanisms due to the effect of rotation and overlap are discussed by examining the flow patterns in and around the rotor and the points of flow separation and stagnation on the blades.