# Performance Characteristics of an Orthopter-Type Vertical Axis Wind Turbine in Shear Flows
## 4 Results and Discussion
### Performance in Shear Flows  
Figure 10 compares the experimental results for \(C_{P}\) of the O-VAWT in the cases of the advancing side faster shear flow (ASF-SF), the retreating side faster shear flow (RSF-SF) and the uniform flow. In the cases of the ASF-SF, \(C_{P}\) is higher than in the case of the uniform flow at all \(\lambda\) and increases with an increase in \(\Gamma\). In the cases of the RSF-SF, similar to the cases of the ASF-SF, \(C_{P}\) increases with an increase in \(\Gamma\). However, when \(\Gamma=0.28\), the values of \(C_{P}\) are lower than those of the uniform flow case. Concerning the optimal \(\lambda\), there is a trend that it shifts to higher \(\lambda\) in both cases of shear flows as compared to the uniform flow case, except for the RSF-SF with \(\Gamma=0.28\). Both in the cases of the ASF-SF (Figure 10(a)) and the RSF-SF (Figure 10(b)), the CFD results for \(Cp\)-\(\lambda\) curves are in good agreement with the experimental ones. In the following discussion, we use the CFD results for the torque of the O-VAWT and the flow field to explain the effects of shear flows on the characteristics of the power performance. Figure 10: Power coefficient (\(C_{P}\)) of O-VAWT in shear flows by the experiments. Figure 9: The variation of blade torque coefficients (\(C_{TB}\)), its rotor-revolution component (\(C_{TB,rev}\)) and its blade-rotation component (\(C_{TB,rot}\)) with azimuthal angle (\(\varphi\)) at; (**a**) \(\lambda=0.4\) and (**b**) \(\lambda=0.6\). Note that \(C_{TB}\), \(C_{TB,rev}\) and \(C_{TB,rot}\) are coefficients of one blade. The CFD simulation result by [PERSON] et al. [37] for the variation of \(C_{TB}\) of a Savonius-type VAWT with \(\varphi\) is added for comparison. Figure 12 shows the variations of blade torque coefficient (\(C_{TB}\)), its rotor-revolution component (\(C_{TB\_rev}\)) and its blade-rotation component (\(C_{TB\_rot}\)) with azimuthal angle (\(\varphi\)) in the cases of the ASF-SF. It is confirmed that these profiles are qualitatively the same between the cases of \(\lambda=0.4\) and \(\lambda=0.6\). Therefore, it is considered that the effects of the shear flow on the characteristics of the blade torque variations with \(\varphi\) do not significantly change around the optimal \(\lambda\). As compared to the case of the uniform flow, \(C_{TB}\) is higher on most of the advancing side of the rotor (\(\varphi\approx 210^{\circ}\) to \(330^{\circ}\)) and lower in the upwind region of the retreating side of the rotor, specifically at \(\varphi\approx 120^{\circ}\) to \(150^{\circ}\). In particular, with an increase in \(\Gamma\), \(C_{TB}\) increases on most of the advancing side of the rotor (\(\varphi\approx 210^{\circ}\) to \(330^{\circ}\)). The optimal \(\varphi\) at which \(C_{TB}\) is the maximum shifts to the downwind direction on the advancing side of the rotor (\(\varphi\approx 240^{\circ}\)) as compared to the case of the uniform flow. The effects of the shear flow on the variations of \(C_{TB\_rev}\) with \(\varphi\) is almost the same as \(C_{TB}\). As compared to the case of the uniform flow, \(C_{TB\_rev}\) is higher on most of the advancing side of the rotor (\(\varphi\approx 210^{\circ}\) to \(330^{\circ}\)). In contrast, \(C_{TB\_rot}\) is lower in most of the upwind region of the advancing side (\(\varphi\approx 180^{\circ}\) to \(240^{\circ}\)) and slightly higher in the upwind region of the retreating side of the rotor, specifically at \(120^{\circ}\) to \(150^{\circ}\) as compared to the case of the uniform flow. With an increase in \(\Gamma\), \(C_{TB\_rot}\) decreases in the upwind region of the advancing side of the rotor (\(\varphi\approx 180^{\circ}\) to \(240^{\circ}\)). Due to its negative values of \(C_{TB\_rot}\), the optimal \(\varphi\) at which \(C_{TB}\) is the maximum slightly shifts to the downwind direction as compared to \(C_{TB\_rev}\).