# Performance Characteristics of an Orthopter-Type Vertical Axis Wind Turbine in Shear Flows
## 5 Conclusions  
We investigated the effects of horizontal shear flow on the performance characteristics of an orthopter-type vertical axis wind turbine (O-VAWT) by conducting wind tunnel experiments and computational fluid dynamics (CFD) simulations. In addition to a uniform flow, two types of shear flow were used as the approaching flow to the O-VAWT. One type was an advancing side faster shear flow (ASF-SF), which had a higher velocity on the advancing side of the rotor. The other type was a retreating side faster shear flow (RSF-SF), which had a higher velocity on the retreating side of the rotor. For each type of shear flow, we set three different velocity ratios (\(\Gamma\) = 0.28, 0.40 and 0.51), which were the ratios of the difference between the highest velocity and the lowest velocity in a shear flow to the sum of the highest and lowest velocities. The main findings are summarized as follows:  
1. In the ASF-SF cases, the power coefficients (\(C_{P}\)) were significantly higher than the uniform flow case at all tip speed ratios (\(\lambda\)) and increased with \(\Gamma\). The experimental results for the maximum \(C_{P}\) of the ASF-SF case with \(\Gamma\)= 0.51 and the uniform flow case were 0.43 when \(\lambda\)= 0.6 and 0.32 when \(\lambda\) = 0.4, respectively. Around the optimal \(\lambda\), the blade torque coefficient (\(C_{TB}\)) on the advancing side of the rotor was, in general, significantly higher than the uniform flow case and increased with \(\Gamma\), predominantly contributing to the increase in \(C_{P}\). The CFD results for the maximum discrepancies of \(C_{TB}\) on the advancing side of the rotor between the ASF-SF case with \(\Gamma\)= 0.51 and the uniform flow case were 0.38 when \(\lambda\)= 0.4 and 0.44 when \(\lambda\)= 0.6. The high values of \(C_{TB}\) of the ASF-SF cases on the advancing side of the rotor were mainly caused by the higher pressure on the upwind side of the blade due to the higher speed of the approaching flow and by the lower pressure near the outer edge of the downwind side of the blade due to the formation of a larger vortex. 2. In the RSF-SF cases, \(C_{P}\) increased with \(\Gamma\). However, when \(\Gamma\) = 0.28, \(C_{P}\) was lower than the uniform flow case at all \(\lambda\). When \(\Gamma\) = 0.51, \(C_{P}\) was higher than the uniform flow case except at low \(\lambda\); however, it was lower than the ASF-SF case with \(\Gamma\) = 0.28. The experimental results for the maximum \(C_{P}\) of the RSF-SF case with \(\Gamma\) = 0.28, the RSF-SF case with \(\Gamma\) = 0.51 and the ASF-SF case with \(\Gamma\) = 0.28 were 0.29 when \(\lambda\) = 0.4, 0.33 when \(\lambda\) = 0.5 and 0.35 when \(\lambda\) = 0.5, respectively. Around the optimal \(\lambda\), the blade torque coefficient (\(C_{TB}\)) on the retreating side of the rotor was, in general, higher than the uniform flow case and increased with \(\Gamma\), predominantly contributing to the increase in \(C_{P}\). The CFD results for the maximum discrepancies of \(C_{TB}\) on the retreating side of the rotor between the RSF-SF case with \(\Gamma\) = 0.51 and the uniform flow case were 0.60 when \(\lambda\) = 0.4 and 0.36 when \(\lambda\) = 0.6. The high values of \(C_{TB}\) of the RSF-SF cases on the retreating side of the rotor were mainly caused by the higher pressure on the outer side of the blade on the upwind side of the rotor, due to the higher speed of the approaching flow. By contrast, \(C_{TB}\) on the advancing side of the rotor was, in general, lower than the uniform flow case, due to the lower pressure on the upwind side of the blade. 3. \(C_{TB}\) consists of the rotor-revolution component (\(C_{TB\_rev}\)) and the blade-rotation component (\(C_{TB\_rot}\)). In all the shear flow cases, as well as the uniform flow case, the contributions of \(C_{TB\_rev}\) to \(C_{TB}\) were dominant. The dependencies of \(C_{TB\_rev}\) and \(C_{TB\_rot}\) on \(\Gamma\) had the opposite tendencies. These findings are useful for micro-siting of an O-VAWT in the area where shear flows occur.