# Experimental and Numerical Evaluation of a Wildland-Urban Interface Fire Scenario
## 2 Materials and Methods
### Full-Physics Model Results  
Explicitly modelling wildfire scenarios using full-physics models is a very complex and demanding task that still requires enhancements to ensure that the involved phenomena are accurately and reliably reproduced. To gain a deeper understanding of the most suitable modelling assumptions and strategies, the fire behaviour of individual trees, reported by [PERSON] et al. [39], and of a cylindrical wire mesh basket filled with _Pinus pinaster_, reported by [PERSON] et al. [45] and [PERSON] et al. [46], was reproduced. Observing the results of the preliminary tests on single trees and shrubs, plotted in Figure 8, it can be stated that the numerical models can reproduce the combustion of different types of vegetation with a good level of accuracy. In Figure 8a, for the case of a tree with a height of 2 m and moisture content (MC) of 14%, the peak MLR values are 2.03 and 1.97 kg/m\({}^{2}\)/s, respectively, for C\({}_{3.4}\)H\({}_{6.2}\)O\({}_{2.5}\) and the experimental curve. This means a peak-to-peak MLR difference of 3%. In addition, for a tree of 5 m height and MC of 26% (see Figure 8b), the peak-to-peak MLR difference is 2%, with peak MLR values of 0.409 and 0.418 expressed in kg/s (for C\({}_{3.4}\)H\({}_{6.2}\)O\({}_{2.5}\) and the experimental curve). Concerning the observed behaviour of the shrub, it is worth noting that the numerical model slightly overpredicts the mass loss rate. Using the previously described strategies and modelling assumptions, the full field fire test was reproduced. The simulation was run on the high-performance computer (HPC) available at the University of Coimbra ([[https://www.uc.pt/lca](https://www.uc.pt/lca)]([https://www.uc.pt/lca](https://www.uc.pt/lca)), accessed on 4 December 2023). Using 34 CPU cores, the simulation took 312 h to complete. Generally, the full-physics model was able to reproduce the experimental fire scenario both in terms of fire spread and thermal attack on the construction component. The obtained rate of spread was very similar to the experimental one. In Figure 9, the fire spread of the wildfire for different time steps is depicted. Comparing with the information presented in Figure 5, a good agreement can be observed since the fire spread rate and the time from the ignition point to the building component are very similar (approximately 1200 s), indicating that the developed numerical model accurately reproduced the observed fire behaviour and spread. Figure 7: Snapshots taken from the IR video recording. The temperatures monitored at the exposed surface increased extremely fast at about 1000 s after the beginning of the simulation (reproducing accurately what was observed in the experimental test). Also, the rate of temperature increase in the exposed face of the construction element shows a very good agreement with the experimental results, as depicted in Figure 10 (slope of the experimental and numerical curves plotted). Finally, the extinction of the wildfire in the simulation occurred after approximately 1520 s, which once again is in relatively good agreement (less than 9% difference) with what was observed in the experimental test (1670 s). Figure 8: Comparison between experimental results and FDS results: (**a**,**b**) single Douglas fir trees on fire with 2 and 5 m and moisture content of 14 and 26% and (**c**) shrubs on fire. Figure 9: Snapshots at different time steps of the numerical simulation reproducing the experimental fire scenario. In Figure 10, the experimental results (dashed lines) are compared with the numerical ones (solid lines). A very good agreement was observed in the initial 1200 s of the reported tests since only the vegetation in fire was being reproduced. The results start to diverge from the moment the sandwich panels ignite.