## 4 Results and Discussion  
### Effect of Gabion on the Exterior CHTC  
The calculated CHTC values on the exterior surfaces of a building with or without gabion under different weather conditions (Figure 6) show the effect of this structure surrounding the exterior wall on the exterior CHTC. Generally, the gabion could significantly reduce the exterior CHTC under all simulation conditions, even in cold wave weather. The averaged CHTC value is reduced by 7.660 W/(m\({}^{2}\cdot\)K) (69%), 11.373 W/(m\({}^{2}\cdot\)K) (67%), and 18.530 W/(m\({}^{2}\cdot\)K) (63%) for three simulation conditions. As shown in Figure 6, the CHTC values on the different exterior surfaces are more even and fluctuate less when the exterior wall is surrounded by gabion. It is evident that the gabion can significantly weaken the effect of severe weather on the exterior CHTC.  
For the building without gabions, the maximum is found at the No. 3 exterior surface when higher values of CHTC are observed at the No. 2, 3, and 4 external surfaces of the building under all simulation conditions. However, with gabion, CHTC values of the above three exterior surfaces are reduced and close to those of other exterior surfaces. Further, the CHTC value of exterior surface No. 3 is reduced from 47.853 W/(m\({}^{2}\cdot\)K) to 7.925 W/(m\({}^{2}\cdot\)K) with a reduction of 83.4% during the windy cold wave weather of condition III. The flow structure is considerably influenced by the buildings' ge  
Figure 5: The exterior CHTC in the simulation results and the experiment results [38].  
\begin{table}
\begin{tabular}{c c c} \hline \hline
**Number of Cells** & **The CHTC Value (W/(m\({}^{2}\cdot\)K))** & **Relative Error (\%)** \\ \hline
2,894,941 & 7.91 & 4.91 \\
3,135,433 & 7.54 & - \\
5,383,569 & 7.16 & 5.04 \\
8,760,834 & 8.12 & 7.69 \\ \hline \hline \end{tabular}
\end{table}
Table 5: Grid independence verification.  
distribution of CHTC. Unlike other exterior surfaces, the No. 2 and 4 exterior surfaces are parallel to the direction of the incoming flow and not sheltered by other buildings or the building itself. As a result, higher wind speeds are observed near the surfaces. The No. 3 external surface has the highest surface velocity for the building without gabions due to its location, as illustrated in Figure 7. The vortices flowing from No. 3 exterior surface to No. 2 exterior surface in the corner area of the building, which decreases the air residence time, results in the highest velocity of No. 3 exterior surface. Moreover, since the wind speed is higher, the CHTCs are larger. When the gabion is set, the near-wall velocities at the No. 2, 3, and 4 exterior surfaces are effectively decreased, and thus so are the corresponding CHTCs. The reduction in CHTC might be explained by the fact that gabion can effectively reduce the surface velocity of the exterior building surface parallel to the direction of the incoming flow, demonstrating that it diminishes the effect of flow structure on the exterior CHTC.  
Figure 6: CHTC on each exterior building surface and averaged CHTC under three simulation conditions: (**a**) condition I; (**b**) condition II; (**c**) condition III; (**d**) averaged CHTC.  
Figure 7: Velocity magnitude contour and vector (at 10 m ref height for the building without gabions under condition III): (**a**) contour; (**b**) vector.