# Performance Characteristics of an Orthopter-Type Vertical Axis Wind Turbine in Shear Flows
## 4 Results and Discussion
### Performance in Uniform Flow  
Figure 8a shows the power and torque coefficients (\(C_{P}\) and \(C_{T}\)) of the O-VAWT in the uniform flow. The CFD results are in good agreement with the experimental ones. The optimal tip speed ratio at which \(C_{P}\) becomes the maximum is less than unity; 0.4 in the experiments and 0.5 in the  
CFD simulations. As mentioned in the introduction, this low optimal tip speed ratio is a favorable feature for the built environment from the viewpoint of aerodynamic noise. With increasing \(\lambda\), \(C_{T}\) decreases monotonically from a small tip speed ratio (\(\lambda\) = 0.2). These tendencies are commonly found among drag-type wind turbines. The wind-tunnel experimental results by [PERSON] et al. [24] for \(C_{P}\) and \(C_{T}\) of an O-VAWT with two elliptical cross-sectional blades show the same tendencies as our results. The value of the maximum \(C_{P}\) is 0.32 in our experiments and CFD simulations, while the value is 0.176 in [PERSON] et al.'s experiments. The main factors for the better performance of our O-VAWT as compared to [PERSON] et al.'s O-VAWT can be the number of blades and the cross-sectional shape of the blades. In our previous studies, the maximum Cp improved from 0.189 to 0.244 by changing the number of blades from two to three [33] and improved from 0.246 to 0.288 by changing the cross-sectional shape of blades from ellipse to rectangle [23].  
Figure 8b shows \(C_{T\_rev}\) and \(C_{T\_rot}\) computed by the CFD results. The sum of \(C_{T\_rev}\) and \(C_{T\_rot}\) is \(C_{T}\). As well as \(C_{T}\), the value of \(C_{T\_rev}\) decreases monotonically with an increase in \(\lambda\). Conversely, the value of \(C_{T\_rot}\) increases with an increase in \(\lambda\). The values of \(C_{T\_rev}\) are positive and larger than those of \(C_{T\_rot}\) except for \(\lambda\) = 0.8. At \(\lambda\) = 0.8, \(C_{T\_rev}\) is negative; however, \(C_{T\_rot}\) is positive and its absolute value is larger than that of \(C_{T\_rev}\). As a result, \(C_{T}\) is positive at \(\lambda\) = 0.8.  
Figure 9 shows the variations of blade torque coefficient (\(C_{TB}\)), its rotor-revolution component (\(C_{TB\_rev}\)) and blade-rotation component (\(C_{TB\_rot}\)) with respect to azimuthal angle (\(\varphi\)) at \(\lambda\) = 0.4 and 0.6. The value of \(C_{TB}\) is significantly large in the upwind region of the advancing side (\(\varphi\approx 180^{\circ}\) to \(270^{\circ}\)) of the rotor, being the maximum at \(\varphi\approx 210^{\circ}\). In the range of \(\varphi\) where \(C_{TB}\) is significantly large, the contribution of \(C_{TB\_rev}\) is dominant. Except for this range, \(C_{TB\_rev}\) does not always positively contribute to \(C_{TB}\). At \(\varphi\) where the value of \(C_{TB\_rev}\) is negative, the value of \(C_{TB\_rot}\) is generally positive and the rotation of the blade positively contributes to \(C_{TB}\). Due to this positive contribution of \(C_{TB\_rot}\) to \(C_{TB}\), the value of \(C_{TB}\) is positive at almost all \(\varphi\) and the variation of \(C_{TB}\) of the O-VAWT with respect to \(\varphi\) is smaller as compared to that of a Savonius-type VAWT. The variation of \(C_{TB}\) of a Savonius-type VAWT with respect to \(\varphi\) in Figure 9 is a result of CFD simulation by [PERSON] et al. [37]. The maximum \(C_{P}\) and the optimal \(\lambda\) of the Savonius-type VAWT were 0.258 and 1.0, respectively.  
Figure 8: Performance of the O-VAWT in the uniform flow; (**a**) the variation of power coefficient (\(C_{P}\)) and torque coefficient (\(C_{T}\)) with tip speed ratios (\(\lambda\)) by the experiments and CFD simulations, (**b**) rotor-revolution (\(C_{T\_rev}\)) and blade-rotation (\(C_{T\_rot}\)) components of torque coefficients (\(C_{T}\)) computed by the CFD simulations. The wind-tunnel experimental results by [PERSON] et al. [24] for \(C_{P}\) and \(C_{T}\) of an O-VAWT with two elliptical cross-sectional blades are added for reference.