## 3 WRF-CFD Wind Field Downscaling Technology
### Simulation of Nested Wind Field by CFD Model
#### 3.2.3 Validity and Aerodynamic Characteristics  
To verify the validity of CFD, the pulsating wind field of the design code (GBT 50102-2014 2014) type A landform was numerically simulated based on the small-scale CFD model. The wind pressure coefficient is defined as follows:  
\[C_{Pl}=\frac{P_{i}-P_{\infty}}{P_{0}-P_{\infty}} \tag{5}\]  
\[\mu_{i,\theta}=\frac{\overline{C}_{Pl,\theta}}{\left(Z_{i}/h\right)^{2a}} \tag{6}\]  
Figure 8: Schematic diagram of CFD mesh division. (**a**) Overall meshing. (**b**) Meshing of the encryption zone. (**c**) Elevation layout of measuring points. (**d**) Boundary conditions.  
where \(\mu_{i,\theta}\) is the wind pressure coefficient; \(C_{Pi,\theta}\) is the dimensionless wind pressure; \(Z_{i}\) is the height of measuring point \(i\); \(h\) is the height of reference point; \(P_{i}\) is the pressure of measuring point \(i\); \(P_{0}\) and \(P_{\infty}\) are the total pressure and static pressure at the reference height, respectively.  
Figure 9 shows the numerical analysis results of the wind pressure coefficient of the typical weak throat layer (layers 8, 9, and 10) of the type A wind field and compares it with the results of existing related research [6; 28; 29]. The analysis shows that the numerically predicted wind pressure coefficients in the upwind and leeward areas are in good agreement, while in the crosswind area they are slightly larger than those of the norm; however, they are more consistent with the results of the wind tunnel test.  
To compare and analyze the aerodynamic characteristics of the typhoon wind field, Figure 10 shows the average wind pressure coefficient distribution of each layer and Figure 11 shows a schematic comparison of the outer surface pressure coefficient of a cooling tower between typhoon and type A normal wind. Based on this, the accurate 3D typhoon wind load distribution parameters that can be used as input for subsequent analysis of the typhoon-induced collapse of the entire cooling tower were extracted. As can be seen from the figure, the maximum wind pressure coefficient of each layer of the cooling tower appears on the \(0^{\circ}\) meridian; furthermore, this maximum is greater at the top of the tower (Layer 1) and becomes smaller towards the bottom of the tower (Layer 12). The minimum wind pressure coefficient of each layer of the cooling tower appears at \(72^{\circ}\) and \(288^{\circ}\) meridian. Compared with the normal wind, the gradient of the average pressure coefficient of the windward area of the tower is more significant under the action of the typhoon. The average pressure coefficients of the windward and crosswind surfaces in the typhoon case are increased significantly compared with the normal wind case. In particular, the maximum increase of the windward side was 46.8% and the maximum increase of the crosswind surface was 34.6%, which all occurred at the bottom of the tower. Results are as expected due to wind profile change with higher wind load and a lower height.  
Figure 9: Simulated wind pressure distribution coefficient curves.  
Figure 11: Comparison diagram of the pressure coefficients of cooling towers between typhoon and normal wind. (**a**) Front view of normal wind. (**b**) Overview of normal wind. (**c**) Front view of typhoon. (**d**) Overview of typhoon.  
Figure 10: Distribution of average pressure coefficients of the cooling tower under different wind fields. (**a**) Normal wind. (**b**) Typhoon.