## 4 Results and Discussion
### Influence of Fluid-Structure Interaction on Aerodynamic Performance of Wind Turbines
#### 4.1.1 Effect of Coupling on Aerodynamic Loads  
Due to the common simulation of STAR CCM+ and ABAQUS, the settings of both software need to be consistent, so the same time step needs to be set in ABAQUS. Due to the high accuracy requirement of bidirectional fluid-structure coupling, the time of 0.5 degrees of blade rotation is chosen as the coupling time step in this paper. In addition, Table 6 and Figure 12 show the comparison of thrust and torque results using bidirectional fluid-structure coupling technique, CFD technique and official NREL reports at different wind speeds.  
Figure 11: Grid of FEM blade. (**a**) Grid of hub. (**b**) Meshing of blade surface and web.  
Figure 12: Comparison of thrust and torque at different wind speeds. (**a**) Contrast of thrust. (**b**) Contrast of torque.  
It can be seen from Figure 12 that the thrust and torque simulated with the bidirectional fluid-structure coupling technique are smaller than those of the pure CFD method. The thrust of the wind turbine is determined by the swept area of the wind turbine blade surface. The coupled calculation takes into account the deformation of the blade, which leads to a slight reduction in the swept area of the wind turbine blade and a slight decrease in the thrust force. The torque of the wind turbine becomes smaller as the wind speed increases, because as the wind speed increases, the rotation speed of the turbine becomes faster, the degree of twisting of the blades around the blade span axis increases, and the torque decreases instead.  
The results in Figure 13 show that the aerodynamic performance of the wind turbine blade decreases due to deformation. As the wind speed increases, the thrust and torque losses caused by blade deformation also increase. From the perspective of power generation, a wind turbine with a design power of 5000 kw will lose 238.276 kw under the rated operating conditions of 11.4 m/s. The effect of aeroelasticity is not negligible. Although there are higher losses, this paper introduces the concept of power/thrust coefficient variation:  
\[\delta C_{P}=\frac{P_{1}-P_{0}}{0.5\rho U_{\infty}^{3}\pi R^{2}} \tag{10}\]  
\[\delta C_{T}=\frac{T_{1}-T_{0}}{0.5\rho U_{\infty}^{2}\pi R^{2}} \tag{11}\]  
where, \(P_{1}\) and \(T_{1}\) represent the power and thrust obtained by CFD calculation, \(P_{0}\) and \(T_{0}\) represent the power and thrust obtained by two-way fluid-structure coupling calculation. \(\rho\) is the air density, \(U_{\infty}\) is the uniform wind speed, and \(R\) is the radius of the wind turbine.  
According to the calculation, when the wind speed increased from 5 m/s to 11.4 m/s, \(\delta C_{P}\) decreased from 0.0502 to 0.0210, and \(\delta C_{T}\) decreased from 0.0360 to 0.0123. This indicates that although the generation power lost due to blade deformation increased with  
\begin{table}
\begin{tabular}{c c c c c c c} \hline
**Wind Speed** & **Thrust (CFD)** & **Thrust (T-W)** & **Error (\%)** & **Torque (CFD)** & **Torque (T-W)** & **Relative** \\
**(m/s)** & **(kN)** & **(kN)** & **(kN)** & **(kN-m)** & **(kN-m)** & **Difference (\%)** \\ \hline
5 & 183.424 & 176.531 & 3.758 & 573.37 & 511.256 & 10.833 \\
7 & 302.551 & 295.362 & 2.376 & 1362.095 & 1216.516 & 10.687 \\
8 & 376.698 & 369.253 & 1.976 & 1843.85 & 1703.519 & 7.611 \\
10 & 585.679 & 573.679 & 2.049 & 2851.56 & 2675.707 & 6.167 \\
11 & 705.122 & 693.95 & 1.726 & 3523.565 & 3368.225 & 5.260 \\
11.4 & 714.682 & 702.445 & 1.712 & 3898.55 & 3710.487 & 4.824 \\ \hline \end{tabular}
\end{table}
Table 6: Comparison of thrust and torque results calculated by CFD and two-way fluid-structure interaction.  
Figure 13: Thrust and torque change under different wind speeds.  
the increase in wind speed, the change of power coefficient decreased significantly, and the wind speed doubled. \(\delta C_{P}\) is doubled, so in order to improve the power generation efficiency, the rated working condition is still the first choice.