The drag coefficient of windmill rotors

Abstract

The aims of the project were to design and construct an apparatus to find the drag of a horizontal axis windmill rotor, to observe the change (if any) of the Cd with changes in the rotational speed, and to find and compare the drag coefficients of different types of rotors. Model rotors were constructed and tested using the apparatus mounted in a wind tunnel. Windmills today are mainly used for generating electricity from small units used on boats and in isolated locations through auxiliary generating units on aircraft to large wind turbines - and for raising water from wells. They offer an economically attractive option since the only costs involved are the cast of the equipment and set-up and maintenance costs. A windmill is economic if it operates continually and for a long time : it should be built to last. The structure should withstand any wind farces which might be encountered, not only in normal operating conditions but also in gusts and storms, i.e. survival conditions. When designing a windmill the drag farces of the tower, the nacelle and the rotor have to be calculated and considered. The rotor drag constitutes a considerable part of the force on the windmill structure, and results in a bending moment about the base. The drag of the rotor acting alone is found in the project, for a variety of windmill rotors, giving an indication of the drag coefficients which might be encountered at various speeds. More importantly, the variations in Cd with changes in the ratio of the tip speed and the wind speed are studied [...].