# Experimental and Numerical Evaluation of a Wildland-Urban Interface Fire Scenario
## 2 Materials and Methods
### Development of the Full-Physics Model
To model the field fire test, similar strategies and assumptions were used. The full test field was reproduced, as depicted in Figure 3. Figure 4 provides detailed information on the meshes adopted for the numerical model. For reproducing the terrain (modelled as an obstruction), the topography provided for the numerical model was extracted from the Digital Elevation Model (DEM; [[https://spacedata.copernicus.eu/collections/copernicus-digital-elevation-model](https://spacedata.copernicus.eu/collections/copernicus-digital-elevation-model)]([https://spacedata.copernicus.eu/collections/copernicus-digital-elevation-model](https://spacedata.copernicus.eu/collections/copernicus-digital-elevation-model)) (accessed on 4 December 2023)) of Copernicus. The extension of the domain is 55 m \(\times\) 90 m \(\times\) 33 m (in the \(x\), \(y\), and \(z\) directions). In total, 74 meshes were used, and different grid resolutions (with grid size cells ranging from 3.125 cm to 50 cm) were adopted with smaller resolutions when approaching the building component at the top of the field. In Figure 4, the 74 meshes adopted are represented, using different colours. The vegetation was modelled following the strategies described before, considering a fuel moisture content (FMC) of 30% for the trees and 8% for the shrubs. Also, a mass per volume of 20 kg/m\({}^{3}\) and a particle surface density of 0.3 kg/m\({}^{2}\) were used for the shrubs and grass, respectively. In this case, the key grid size is 25 cm since it is well-established in the literature for WUI scenarios [49]. Lateral boundaries (X = 0 m, X = 65 m, Y = 0 m, Y = 95 m) and the top boundary (Z = 33 m) were modelled as open. The wind field was specified via the Monin-Obukhov [50] similarity parameters as described by [PERSON] et al. [36], considering a wind speed of 1.7 m/s blowing from south to north. \begin{table}
\begin{tabular}{c c c} \hline
**Quantity** & **Value** & **Quantity** & **Value** \\ \hline Heat of Combustion & 14,516 kJ/kg \({}^{1}\) & Thermal Conductivity \({}^{3}\) & 2.0 W/m K \\ \multicolumn{2}{c}{Soot Yield} & 0.02 kg/kg & Reference Temperature & 300 \({}^{\circ}\)C \({}^{4}\), 100 \({}^{\circ}\)C \({}^{5}\) \\ Initial Temperature & 28 \({}^{\circ}\)C & Vegetation Density & 514 kg/m\({}^{3}\) \\ Heat of Reaction & 416 kJ/kg & Drag Coefficient & 2.8 \\ Specific Heat Capacity \({}^{2}\) & 2.0 l 1.5 l 4.184 kJ/kg K & Packing Ratio (f, s, m, l) \({}^{6}\) & 24 l 68 l 48 l 44 10 \\ Chemical Formula & CH\({}_{1.7}\)O\({}_{0.74}\)N\({}_{0.002}\) & SVR \({}^{7}\) (f, s, m, l) \({}^{6}\) & 3940 l 2667 1888 1500 m\({}^{-1}\) \\ Density \({}^{2}\) & 1000 l 100 l 1300 kg/m\({}^{3}\) & Length (f, s, m, l) \({}^{6}\) & 0.05 l 0.10 l 0.10 l 0.10 l 0.10 l \\ \hline \end{tabular} \({}^{1}\) Vacca and Agueda [24], \({}^{2}\) Moisture, Char Veg and Dirt, \({}^{3}\) Moisture, Vegetation and Char, \({}^{4}\) Vegetation Grass; \({}^{5}\) Moisture; \({}^{6}\) (f, s, m, l)—foliage, small, medium, large; \({}^{7}\) Surface Volume Ratio.  
\end{table} 
Table 1: Physical and thermal properties used to reproduce the Douglas fir trees. Three categories of natural fuel were considered in this model: grass, shrubs, and trees. These three natural fuels were modelled considering the Lagrangian Particle Model (LPM). In Table 2, all properties used as input are presented. Additionally, non-natural fuels, such as polyurethane (the core of the steel sandwich panel) and steel, were also modelled. Details can be found in Table 3. However, it is worth mentioning that the pyrolysis of the sandwich model was not modelled in this investigation, as the main objective was to characterize and quantify the thermal attack by the wildfire.