# The Influence of Tilt Angle on the Aerodynamic Performance of a Wind Turbine
## 3 Results and Discussion
### The Effect of the Tilt Angle on the Aerodynamic Performance of the Wind Turbine  
In this study, nacelle tilt angles of 0, 4, 8 and 12\({}^{\circ}\) were investigated. Figure 7 shows the structure of the wind turbine at different tilt angles. In the picture, \(\beta\) is the pre-coning angle, and \(\gamma\) is the shaft tilt angle. The azimuth of the rotor is defined as \(\psi\), as presented in Figure 8. In Figure 8, the blue rotor is the initial position with the 0-azimuth angle. Subsequent analysis is based on the results after the wind turbine has stabilized. Under different tilt angles, the change in wind turbine thrust and power with the azimuth is shown in Figure 9. Comparing the no-tower curve with the other four curves, it can be seen that the thrust and power generate periodic fluctuations due to the influence of the tower. When the blades pass through the tower, the thrust and power will periodically decrease. This phenomenon is called the blade-tower interaction (BTI) [19]. The BTI effects begin at approximately 30\({}^{\circ}\) rotor azimuth and dissipate at approximately 100\({}^{\circ}\) rotor azimuth, as presented in Figure 10. This agrees with previous studies, which all show effects in approximately this same 70\({}^{\circ}\) range [19]. Figure 9 shows the difference between the thrust and power at approximately 60, 180 and 300\({}^{\circ}\) azimuth with the same nacelle tilt. This phenomenon is due to the interaction between the blade and the tower creating a random vortex. As the nacelle tilt increases, the blade and tower interactions gradually weaken. Therefore, this phenomenon becomes less important as the nacelle tilt increases. In Figure 10, when \(\psi\) is approximately 65\({}^{\circ}\), the thrust and power of the wind turbine at 4 and 8\({}^{\circ}\) nacelle tilt are higher than 0 and 12\({}^{\circ}\) nacelle tilt. Figure 6: Comparisons of thrust and power. Figure 8: Definition of the azimuth. Figure 10: Comparison of thrust and power at different nacelle tilt angles (partial enlargement). Figure 7: Structure of the wind turbine at different tilt angles. Figure 9: Comparison of thrust and power at different nacelle tilt angles. The position of the blade relative to the tower with the 60\({}^{\circ}\) azimuth is shown in Figure 11. Instantaneous pressure magnitude and streamlines at blade sections r/R = 0.5, r/R = 0.7 and r/R = 0.9 (Blade 1) of the wind turbine are presented in Figures 12-14. In the low span (r/R = 0.5) suction side, the flow separation phenomenon can be observed. However, the flow remains attached for higher radial sections (r/R = 0.7 and r/R = 0.9). In addition, with the increase of the nacelle tilt, the flow separation of the low span suction side is gradually weakened. The variation of the pressure distribution around different sections airfoil with the nacelle tilt can also be observed. [PERSON] et al. [20] studied different methods of calculating the angle of attack of the wind turbine section airfoil. However, in CFD, when considering the interaction between the blade and the tower, it is difficult to calculate the angle of attack of the blade section airfoil. Therefore, only the effect of tilt angle on the blade section airfoil load is considered in this paper. Figure 15 shows the  
Figure 14: Instantaneous pressure magnitude and streamlines diagram (r/R = 0.9). Figure 9.0 Instantaneous pressure magnitude and streamlines diagram (r/R = 0.7). distribution of azimuth average thrust and tangential force along the blade span. From the figure, we can see that in terms of thrust, when the tilt angle is 4\({}^{\circ}\), the distribution of the thrust along the blade span does not change much compared to the 0\({}^{\circ}\) tilt angle. However, when the tilt angle is increased to 8 and 12\({}^{\circ}\), the thrust of the section airfoil at the blade tip is lower than the values at 0 and 4\({}^{\circ}\) tilt angle. In terms of tangential force, the tangential force gradually decreases as the tilt angle increases, for up to 0.5 of the span.