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Vertical axis wind turbines (VAWTs) are easier to install and maintain since the rotor shaft they attached to is typically set transverse to the wind direction. Therefore, the generator and rotation transmission mechanisms can be placed at the base level, where it is close to the ground. An additional benefit of this setup is that there is no need to point it into the wind and thus reduce the associated systems and machine complexity. Despite these advantages, from the early lift-based Darrieus rotors and later cycloturbines to drag-based Savonius rotor, the VAWTs have been facing many challenges including low starting torque, low peak efficiency, narrow operating range, pulsatory torque, and dynamic stability problems. Some derived systems including Giromills design and Helical airfoils can help improve one disadvantage (such as low starting torque) but usually at the sacrifice of another advantage (such as high peak efficiency). Researchers suggest pitch control systems such as the Self-Acting Stabilized Pitch Control, the Sinusoidal Forced Pitch Variation, the Aero Pitch, etc. for performance improvement. Experimental results have shown that they all can improve the starting torque, provide a broader operating range, and result in higher efficiency than the more conventional fixed pitch VAWTs.

In this work, the CFD Module of the COMSOL Multiphysics^® simulation software is used to study the drag force on a VAWT consisting of three NACA 0012 airfoils. Each of the airfoils is capable of pivoting individually and thus changing the angle of attack between the wind and airfoils with respect to their supporting arms that are fixed to the vertically rotating main axis. The goal is to understand how the pitch control of the airfoils affects the drag force on the VAWT. COMSOL Multiphysics^® simulation software

models are developed to simulate the flow of air through the three airfoils at selected rotational positions and various airfoil pivoting angles at these positions. Drag forces on the VAWT are obtained by integrating both the pressure and viscous stresses over the surface of the three airfoils. Projection along the drag direction based on the normal vector on the surface is computed considering pressure is a scalar to obtain the pressure force, and the predefined variable representing viscous stress is directly used for computation of viscous force. In case that the angle of attack is not zero (wind direction is not in line with the chord of airfoils), similar projection along the direction of drag is performed to calculate the forces.

The figures below show the velocity fields for the rotational position in which the supporting arm of the leftmost airfoil is horizontal and the pivoting angles for all three airfoils are zero-degree, as well as zero-degree for the top and 30-degree pivoting angles for the other two airfoils.