Numerical and experimental investigation on effects of the primary and secondary overlaps on the performance of Savonius wind turbine

Due to simplicity of design and manufacture, self-starting characteristics and operating independently of wind direction, Savonius wind rotors have been considered by many scientists. In the present study, the effects of the primary and secondary overlap ratios on the performance of Savonius wind rotors have been investigated, by means of numerical simulation and wind tunnel tests. Eight different rotor structures were analyzed numerically; three of them were also studied by experimental methods. The effects of the primary and secondary overlap ratios, Reynolds number and the number of the blades on the power and torque coefficients were examined. For all investigated rotors, the maximum value of the power coefficient occurred at a tip speed ratio between 0.8 – 1.0 (blade tip speeds close to the wind speed). Additionally, the maximum power coefficient was found at a dimensionless primary overlap ratio of 0.2; however, increasing or decreasing the secondary overlap ratio caused a reduction in the maximum value of the power coefficient. Also, it was shown that an increase in Reynolds number resulted in the increase of the maximum and average values of power coefficient. Moreover, although adding more blades could produce a more uniform torque, it causes the maximum value of the power coefficient to decrease. By increasing the positive secondary overlap ratio, torque coefficient was increased while the negative overlap ratio reduced the torque coefficient.