## 3 WRF-CFD Wind Field Downscaling Technology
### Simulation of Nested Wind Field by CFD Model
#### 3.2.2 Mesh Division and Parameter Settings  
To ensure that the wake can fully develop, the calculation domain size is set as \(\mathrm{x=16\times D}\), \(\mathrm{y=20\times D}\), and \(\mathrm{z=4\times H}\), where \(\mathrm{x}\) is the crosswind direction, \(\mathrm{y}\) is the downwind direction, \(\mathrm{z}\) is the vertical direction, \(\mathrm{D}\) is the diameter of the bottom of the tower, and \(\mathrm{H}\) is the height of the tower. The maximum blockage rate of the tower shape does not exceed 5%, which meets the requirements of the design code. To facilitate calculation efficiency and accuracy while considering the complex shape of the cooling tower and the more complex internal wind field near the structure, the entire calculation domain is divided by a hybrid network. The discrete form of the mesh divides it into external regions and internal encrypted regions. The external regions are divided by high-quality hexahedral structured mesh, while the internal encrypted regions are divided by tetrahedral unstructured mesh. Before determining the final mesh scheme, mesh independence was verified. Table 2 shows the mesh quality and pressure coefficient of the windward surface of the tower throat area under different mesh schemes. As can be seen from the table, by increasing the total number of cells, the mesh quality, mesh skew, and windward pressure coefficient gradually converge. There is no significant difference in the mesh quality and calculation results when using 12.8 million or 18.8 million cells. Considering the efficiency and accuracy comprehensively, this paper selects the total number of 12.8 million cells as the final mesh scheme. In the internal encryption area, the minimum mesh size of the structural model is 1m and that of other calculation domains is 15m. The wall surface \(\mathrm{y+value}\) is from 29.8 to 35.2, which can ensure that the underlying mesh is located in the logarithmic law layer. In conclusion, the quality and quantity of the mesh meet the calculation requirements. Due to the full-scale model and standard air materials, the Reynolds number of the numerical simulation is on the order of \(10^{8}\), which is similar to the Reynolds number of the actual cooling tower structure at the design wind speed. The boundary conditions of the calculation domain include velocity inlets, pressure outlets, free-slip walls (sides and tops), and non-slip walls (ground and tower walls). Figure 8 shows the CFD mesh division, measurement point layout, and boundary conditions. The outlet boundary conditions are defined as pressure outlets, where the relative pressure is set to 0. Non-slip walls are used on the ground and the tower surface of the calculation domain and free-slip walls are used on both the sides and top surface of the calculation domain. The wind field velocity inlet was defined by a user-defined function (UDF) of the above-mentioned mesoscale WRF model typhoon field simulation results. The turbulence intensity profile [27] adopts the distribution form of the Chinese design code A-type landform. The above velocity entry is shown as follows:  
\[U_{z}=U_{0}(Z/Z_{0})^{a} \tag{1}\]  
\[I_{z}=I_{10}(Z/10)^{-a} \tag{2}\] \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Mesh Scheme** & \(\backslash\) & \(\backslash\) & \(\backslash\) & \(\backslash\) & \(\backslash\) & \(\backslash\) \\ \hline Total number of cells & 1.3 million & 5.1 million & 9.8 million & 12.8 million & 18.8 million \\ Mesh minimum & & 0.14 & 0.37 & 0.52 & 0.62 & 0.64 \\ orthogonal mass & & & & & \\ Mesh skew & 0.96 & 0.88 & 0.81 & 0.73 & 0.72 \\ Windward pressure & & & & & \\ coefficient & & & & & \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Mesh quality and pressure coefficient of the windward surface of the tower throat area under different mesh schemes.