Aluminum extrusions, patterned after NACA symmetric airfoil sections, frequently are used to form Darrieus wind turbine "blades." Below are section lift and drag coefficient data [57] for a NACA 0012 section tested at \(R e=6 \times 10^{6}\) with standard roughness. The section stalled for \(\alpha>12^{\circ}\) :

Analyze the air flow relative to a blade element of a Darrieus wind turbine rotating about its troposkien axis. Develop a numerical model for the blade element. Calculate the power coefficient developed by the blade element as a function of tip-speed ratio. Compare your result with the general trend of power output for Darrieus rotors shown in Fig. 10.49.

Data From Fig. 10.49

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Angle of 02 46 8 10 12 attack, a (deg) Lift 0 0.23 0.45 0.68 0.82 0.94 1.02 coefficient, C(-) Drag 0.0098 0.0100 0.0119 0.0147 0.0194 - coefficient. CD (-)