# Experimental and Numerical Evaluation of a Wildland-Urban Interface Fire Scenario
## 2 Materials and Methods
### Development of the Full-Physics Model  
The full-physics model (physics and chemistry of fire spread are simulated) was developed using the Fire Dynamics Simulator (FDS 6.7.8) software [38]. FDS is a CFD software developed by the National Institute of Standards and Technology (NIST). It was initially developed to model compartment fires, but it is also a capable tool for modelling wildfires, and specifically wildfires in the WUI, focusing mainly on microscale scenarios. Full-physics CFD models demand very high computational resources, making them unsuitable for reproducing large wildfire scenarios. However, for microscale scenarios, this tool is of utmost importance to further understand heat transfer phenomena and fire spread from forest fires to fires in the built environment. In this investigation, FDS version 6.7.8 was used. FDS numerically solves a form of the Navier-Stokes equations, appropriate for low-speed Mach number, thermally driven fluid, focusing on smoke and heat transport from fires [30]. Fluid dynamics is solved using three governing equations referring to the conservation of mass, momentum, and energy [30]. FDS provides the Lagrangian Particle Model (LPM) to model different types of vegetation and consequent fire spread in the vegetation. Grass, trees, and firebrands can be modelled as a collection of Lagrangian particles that are eventually heated by radiation and convection [36]. The vegetation, including the canopy of the trees and shrubs, is able to absorb and emit heat and undergo thermal degradation. Using the LPM, the fire spread rate is computed by the model provided that all thermo-physical properties are provided. The pyrolysis model consists of three reactions, namely the endothermic moisture evaporation, the endothermic pyrolysis of dry vegetation, and exothermic char oxidation. The pyrolysis model obeyed an Arrhenius rate law, which defines a finite-rate reaction where the temperature-dependent reactions are characterized and based on the activation energy and Boltzmann distribution [38]. Figure 2: Positioning of the disk type-K thermocouples (**a**) on the real structures and (**b**) schematic view. Exploring the capabilities of the software and showing the ability to reproduce vegetation in fire, a set of preliminary numerical models were developed, aiming to reproduce the behaviour of single trees and shrubs in fire. Available results in the literature were selected. The experimental study on single Douglas fir trees in fire reported by [PERSON] et al. [40] and the study on the ignition of a cylindrical wire mesh basket filled with _Pinus pinuster_, reported by [PERSON] et al. [45] and [PERSON] et al. [46], were reproduced with FDS. To model the shrubs, it is assumed that they are composed primarily of cellulose (C\({}_{2.28}\)H\({}_{6.34}\)O\({}_{2.44}\)). The heat of combustion is 15,600 kJ/kg. The fuel density is 640 kg/m\({}^{3}\), and the bulk density is 20 kg/m\({}^{3}\). The dimensionless drag coefficient used is 2.8. Additional thermophysical data and material properties can be found in [PERSON] et al. [36]. During these preliminary numerical tests on a single tree and shrub, the computational domain was a volume of 1 \(\times\) 1 \(\times\) 2.4 m\({}^{3}\) (in \(x\), \(y\), and \(z\) directions) with cubic grid cells of 2 cm. To model the Douglas fir trees in the fire, the physical and thermal properties presented in Table 1 were used. The Douglas fir trees presented different heights and moisture content, ranging from 2 to 5 m and from 14% to 49%, respectively. To improve the calibration, different fuel chemical compositions were tested, namely C\({}_{3.4}\)H\({}_{6.2}\)O\({}_{5.2}\)[47], CH\({}_{1.7}\)O\({}_{0.74}\)N\({}_{0.002}\)[30], and C\({}_{6}\)H\({}_{10}\)O\({}_{5}\) (i.e., cellulose), as found in [PERSON] [48]. The heat of combustion was 14,516 kJ/kg [24]. Referring to grid size, two mesh sizes were tested: 25 cm and 10 cm. The 10 cm size was the same computational grid used by [PERSON] et al. [39].