4.1.3. Near Flow Field of Each Section of Blade

Figure 11 shows the interaction between the different blade sections and the tower when the blade rotates to the shadow area of the tower under surge motion. Position 1 was selected for further analysis. For the convenience of analysis, the variation of pressure fields near the blade surface and the distribution of absolute velocity streamline are discussed, respectively.

**Figure 10.** Distribution of pressure in each section of blade at position 1 and position 2.

By comparing the pressure field of the wind turbine with and without the tower at the same cross-section, it is seen that there is an obvious negative pressure field behind the suction surface of the blades. On the root and middle section of the blade, it can be observed that the suction leading edge negative pressure field of the wind turbine without the tower is larger and the negative pressure value is lower, while the pressure surface is basically the same, and the interference of the tower on the pressure field in the tip section is relatively small. These phenomena are in good agreement with the blade surface pressure distribution results described in the previous section. In addition, the whole negative pressure field at the root and middle section of the blade is obviously compressed due to the interference of the tower, which leads to a decrease in the pressure difference between the pressure surface and suction surface of the blade, thus reducing the overall work capacity of the blade. This further explains that the average power of the wind turbine decreases due to the influence of the tower shadow effect in the above results.

**Figure 11.** Streamline and pressure contours on different blade sections under surge motion.

Meanwhile, the streamline change of the wind turbine with the tower is not obvious compared with the wind turbine without the tower under surge motion, and the main change is concentrated in the wake of the tower. In the 0.32 R section, the streamline behind the tower shifts greatly, and the tower shadow effect is the most obvious in the interference of the flow field. In the 0.63 R section, the streamline on both sides of the tower shifts, and the flow field in this section is affected by both the tower shadow effect and the enhanced blade rotation effect. In the 0.94 R section, there is an obvious stall separation in the flow field behind the tower, and the separation point shifts to the direction of blade rotation. The area near the root of the blade is the main area affected by the tower shadow effect under surge motion, and the effect of the tower shadow effect weakens on the near wake flow field with the increase of blade height.

#### 4.1.4. Wake Field behind the Wind Turbine

The main load source of the wind turbine is axial flow; the wake flow characteristics are particularly important for the analysis of wind turbine aerodynamics. By comparing the wake field with or without the tower, this section further illustrates the influence of platform motion on the aerodynamics of the wind turbine.

Figure 12 shows the velocity distribution of the wake field under platform surge motion. The symmetry of the wake field is disturbed and the flow field is slightly compressed. When the wind turbine moves forward, the average velocity of the wake field is greater than that of the backward motion. Considering the effect of the tower shadow, the influence range of the high-speed wake behind the hub expands, but it has little influence on the tip vortex, and the rear of the tower is accompanied by a large vortex shedding range and obvious vortex motion.

**Figure 12.** Velocity contours of wake field under surge motion.
