## 4 Results and Discussion
### Influence of Fluid-Structure Interaction on Aerodynamic Performance of Wind Turbines
#### 4.1.3 Response Analysis of Blade Structure  
According to the modeling verification in Section 3, it can be seen that the overall mass of the blade tip is the lightest part in linear density, so the displacement of the whole blade is the largest in the blade at the same wind speed, and the comparison of the displacement of the jitter direction of the blade tip with the official report is shown in Table 7. It can be seen from the table that the results of the STAR CCM+ and ABAQUS co-simulation at each wind speed simulated in this paper are consistent with the FAST simulation results from NREL laboratory, and the maximum error of the flapping direction as the main deformation direction is less than 6%, which verified the accuracy and reliability of the bidirectional fluid-structure coupling method adopted in this paper. Since the main deformation direction of the wind turbine blade is the flapping direction, Figure 16 is the comparison graph of the blade tip flapping displacement under different wind speeds. It can be seen from the figure that with the increase in wind speed, the deformation of blade tip flapping direction increases continuously, and reaches the maximum value at the rated wind speed of 11.4 m/s, when the blade deformation is the largest and the stress is also the largest. As mentioned above, the load of the wind turbine reaches the maximum value at the rated wind speed. Under normal operating conditions, the main form of wind turbine blade deformation is deformation in the waving direction, while deformation in the edging direction and torsional deformation are very small, which will put the blade root forces to the test. Therefore, \"Von Mises Stress\" in ABAQUS post-processing was used to describe the stress analysis [24]. This method is based on the concept of equivalent stresses based on the von Mises criterion, which can be used to assess the fatigue and failure of the structure and to clearly describe the most dangerous areas of the structure. It can be seen from Figure 17 that the overall equivalent force of the wind turbine blade is about 15% of its length, distributed in the middle of the blade and near the root of the blade. The reason for this is that the deformation of the blade is mainly concentrated in the middle and upper part of the blade, and the closer to the blade root, the higher the stresses. However, since the material layout at the blade root is the best, the mass line density and the blade bedding are thick here. This leads to greater equivalent stresses at a relatively thin distance from the blade root length of 15%. However, the color distribution in the figure shows that the maximum stress distribution in the blade is not on the surface of the blade skin. Figure 18 shows the equivalent stress distribution of the blade shell surface and web under two high wind speeds of 10 m/s and 11.4 m/s. It can be found that the maximum stress of the blade shell surface is on the main beam, about 15% of the length from the blade root, and the maximum stress of the web is also here, and the maximum stress of the web is obviously larger than the maximum stress of the blade shell surface. The existence of the web plate effectively improves the bending resistance and is also one of the main stressed parts. Figure 17: Stress distribution of blades under different wind speeds. (**a**) Stress distribution on windward surface of wind turbine at 11.4 m/s. (**b**) Stress distribution on leeward side of wind turbine at 11.4 m/s. (**c**) Stress distribution on windward surface of wind turbine at 5.0 m/s. (**d**) Stress distribution on leeward side of wind turbine at 5.0 m/s. Table 8 shows the comparison of the maximum equivalent forces of the blade surface and web for different uniform wind speeds. The difference between the maximum equivalent force of blade surface and web is about 2 times. The presence of the web reduces the force of the main beam of the blade to a large extent.