# Experimental and Numerical Evaluation of a Wildland-Urban Interface Fire Scenario
## 2 Materials and Methods
### Development of the Full-Physics Model
\begin{table}
\begin{tabular}{c c c c} \hline \hline
**SHRUB Quantity** & **Value** & **GRASS Quantity** & **Value** \\ \hline Density & 640 kg/m\({}^{3}\) & Density & 500 kg/m\({}^{3}\) \\ Thermal Conductivity & 0.11 W/m K & Thermal conductivity & 0.1 W/m K \\ Heat of reaction & 416 kJ/kg & Specific heat & 1.5 kJ/kg K \\ Max Reaction Rate & 12 s\({}^{-1}\) & Nu\_spec/Nu\_matl & 0.7/0.3 \\ Drag coefficient & 2.8 & Heat of combustion & 15,600 kJ/kg \\ Moisture Fraction & 0.52 & Heat of reaction & 418 kJ/kg \\ Surface Volume Ratio & 4100 m\({}^{-1}\) & Drag coefficient & 1.0 \\ Length & 0.05 m & Reference temperature & 300 \({}^{\circ}\)C \\ Geometry & cylindric & Pyrolysis range & 80 \({}^{\circ}\)C \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Physical and thermal properties used to reproduce shrubs and grass. Figure 4: Adopted meshes in the FDS model. Figure 3: Geometric configuration of the field test modelled using FDS. The cumulative rate of spread for the first stage of the fire was approximately 0.06 m/s. The maximum flame height recorded was approximately 10 m. During the fire spread, a curved (or V-shape, also following the shape of the fireline) shape was observed (see Figure 5), similar to what was reported by other authors [51; 52; 53], even if, in this specific scenario, the imposed fireline contributed more to the observed shape of the fire. The largest amount of available fuel was located near to the top, with a shrub height between 1 and 1.5 m. At this location, dead fuel (moisture content of approximately 8%) cut from the perimeter of the field was positioned to increase the fuel load near the building component. Under direct exposure, the building component started to pyrolyze and ignited, completely burning the inner combustible core of the sandwich panel. The ignition started in the top part of the building component, which is consistent with the information collected from the disk type-K thermocouples. The ignition of the panel started at about 1200 s, and the wildfire extinction occurred at about 1650 s, which means that the recorded temperatures from 1650 s up to 2200 s correspond strictly to the combustion (including the decay phase) of the building component on fire. From 1200 s up to 1650 s, the recorded data was mainly a result of the building component being on fire, with a reduced contribution from the wildfire since it was already in its decay phase. The degradation of the mechanical properties of steel led to significant deformations in the sandwich panel and the separation of the steel parts from the core, allowing the complete combustion of the core. The temperature evolution recorded by the thermocouples is depicted in Figure 6. The position of the thermocouples is depicted in Figure 2. The temperature increase in the exposed surface of the building component was extremely fast, and the peak temperature was attained in the thermocouples located at the top of the building component. The maximum temperature of 1037.6 \({}^{\circ}\)C was recorded by thermocouples 11 (located on Panel C identified in Figure 2). Considering all recorded data, the maximum average temperature recorded was approximately 638 \({}^{\circ}\)C. During the test, it was observed that the ignition of the construction component occurred approximately at the beginning of the decay phase of the wildfire. As detailed in Figure 6, the building component kept burning even after the extinction of the wildfire. This was due to the presence of non-natural fuel (PUR foam) inside the panel. Figure 6: Temperature evolution is monitored by thermocouples: (**a**) all, (**b**) top, (**c**) middle and (**d**) bottom. Snapshots retrieved from the IR images are presented in Figure 7, where the maximum temperature in that specific frame is indicated.