Aerodynamic performance improvement of Savonius wind turbine through a passive flow control method using grooved surfaces on a deflector

A Savonius rotor is a Vertical-Axis Wind Turbine (VAWT) that converts wind force into torque on a rotating shaft. The poor efficiency of this rotor is caused by the unfavorable negative torques generated as the returning blade rotates against the incoming flow. The current study aims to improve the aerodynamic performance of a Savonius wind rotor by installing a cylinder deflector with different grooved surfaces as a unique deflector system to offset the negative impact at the wake zone downstream of the deflector. The grooved surface is essentially one way to alter the surface roughness of a cylinder, which has shown to be quite successful to control vortex shedding and wake formation behind the cylinder. In contrast to previous research, a grooved surface of a cylinder deflector was used instead of traditional deflectors to increase the aerodynamic performance by disrupting the wake zone downstream of the deflector. With this motivation, a systematic parametric study via Computational Fluid Dynamics (CFD) method was implemented for different combinations of the flow and geometric characteristics including Tip Speed Ratio (TSR) value, groove shapes (rectangular, U, and V), groove number, and size. Besides, the flow over a smooth cylinder was analyzed and compared with that of grooved cases to determine the ideal groove configurations for enhancing the total output power. The results demonstrated that the optimal deflector with the U-grooved surface augmented the aerodynamic performance the most for all TSRs. The optimal deflector has 12 U-grooved surfaces with a diameter size of 0.025 of rotor diameter. The average power coefficient enhanced dramatically up to 24.2% and 15.8% through the deflector with a U-grooved surface at TSR = 0.9 in comparison to without deflector case and smooth cylinder deflector case, respectively.