## 2 Materials and Methods
### Different Scale Grid Division Strategy
#### 2.2.3 FDS Grid Parameter Configuration  
In this section, the wind field obtained from nested coupling simulations of WRF and Fluent serves as the boundary conditions for fire numerical simulations of high-rise buildings using FDS. The specific settings are as follows:  
(1) Physical model and grid division  
As shown in Figure 9, the building dimensions are 40 m in length, 10 m in width, and 100 m in height, with all windows closed. The primary objective of this study is to investigate the influence of external winds on the development of facade fires. Therefore, temperature detectors are placed on the exterior wall surfaces, and temperature slices are  
Figure 8: CFD grid diagram.  
Figure 7: CFD three-dimensional schematic diagram.  
distributed around the building to capture fire development information. Additionally, the mass loss rate curve of XPS material, based on [PERSON]'s experimental results, is depicted in Figure 7 and implemented in FDS. Numerous thermocouples are arranged on the fire-exposed wall surface in the physical model to record temperature variations, positioned 20 cm away from the fire-exposed wall.  
The grid division schematic is illustrated in Figure 10. Considering computational resources and grid accuracy, this study employs varying grid sizes: 0.5 m \(\times\) 0.5 m \(\times\) 0.5 m near the building within 20 m, and 1.5 m \(\times\) 1.5 m \(\times\) 1.5 m elsewhere. The total number of grids is 1,680,000. To ensure computational efficiency, the domain is divided into 5 blocks for concurrent multi-core processing, thereby optimizing computational efficiency.  
(2) Boundary conditions  
The wind field boundary conditions for FDS are set as shown in Figures 11-13. The direction of the building is north-south on the long side and east-west on the short side. The location where the fire occurred is on the east side of the building, and this study names it the front of the building, meaning that the east side of the building is termed the front of  
Figure 10: Grid division diagram.  
Figure 9: FDS building model.  
the building for the sake of this work. This study selects wind field data from around high-rise buildings at a distance of 200 m (2 times the building height) from Fluent simulation results as the boundary conditions for FDS and model experiments. According to Fluent simulation results, the wind field varies significantly with vertical height. Therefore, to simulate a more realistic strong wind environment in FDS fire numerical simulations, the vertical direction is divided into four layers. Data from heights of 10 m, 50 m, 100 m, and 200 m are selected as boundary conditions for FDS, with data at 10 m applied to heights from 0 to 10 m, data at 50 m applied to heights from 10 to 50 m, data at 100 m applied to heights from 50 to 100 m, and data at 200 m applied to heights from 100 to 200 m.  
On the same horizontal plane, two points are selected each in the north-south and east-west directions. For wind directions perpendicular to the building, the average of each  
Figure 11: Schematic diagram of wind speed extraction plane.  
Figure 12: Schematic diagram of wind direction.  
Figure 13: Three-dimensional diagram of boundary wind speed extraction.  
point's wind speed is taken as the boundary wind speed. For oblique winds, the wind vector is decomposed into cosine and sine components based on the wind direction deviation angle, and both components are used for comprehensive wind input. For experimental wind field data, oblique winds do not require decomposition and are directly inputted with a single wind direction. To reflect the variation in the wind field at building heights constrained by experimental conditions, the experimental wind field is divided into two layers vertically. The lower layer uses the average wind speed at heights of 10 m and 50 m, and the upper layer uses the average wind speed at heights of 100 m and 200 m.