# Experimental and Numerical Evaluation of a Wildland-Urban Interface Fire Scenario
## 1 Introduction
In the case of Portugal, the WUI density was the highest in thinly urbanized, rural areas which are located in the most fire-prone regions [19]. This indicates the predominance of the most critical areas as the ones in the WUI intermix, where scattered houses intermingle with wildland vegetation. More recently, [PERSON] et al. [20] presented the first high-resolution WUI map of Europe, showing that the WUI covers approximately 7.4% of the European area. Different mapping approaches have been adopted worldwide, mainly using two types of land cover, namely built-up areas and flammable vegetation, but considering different approaches to quantify the two types of land cover [2; 7; 19; 20]. One of the greatest challenges associated with the fire risk in the WUI is the vulnerability of structures and infrastructures to ignition mechanisms [21] resulting from exposure to direct flame contact and heat fluxes from fires or firebrands. It is worth noting that [PERSON] and [PERSON] stated that 90% of building ignitions occur due to firebrands [22]. The WUI fire problem is extremely complex due to the interaction between multiple influencing factors [23]. The most relevant influencing factors are the fuel type arrangement and accumulation, weather conditions and impacts of climate change, human activities, community design and structure vulnerability, fire suppression capabilities and management strategies, and finally community preparedness and awareness. In the vicinity of the built environment, a comprehensive understanding of the ignition and combustion of both natural and non-natural fuels is crucial [24; 25]. Spot fires, primarily initiated by firebrands, are microscale events occurring in the \"Home Ignition Zone\" (HIZ) as defined by [PERSON] [26]. Protecting this HIZ relies on the definition of a protected space where preventive actions like cleaning, avoiding the deposit of fuels, and maintaining a safe distance from vegetation are undertaken regularly. However, additional measures are needed at the structure level, ensuring that, in the vicinity of forests, structures fulfil additional requirements in terms of fire safety (structures exposed to external fires), going beyond the traditional requirements for compartment fires. Some standards already present specific guidelines for the design of buildings in the vicinity of forest areas, such as the AS 3959 [27]. Assessing the behaviour of structures exposed to forest fires is nowadays a critical global concern, requiring extensive experimental [28; 29; 30; 31; 32] and numerical investigations [33; 34; 35; 36; 37] to ensure a deep understanding of the involved phenomena. This deeper understanding will enable the development and implementation of innovative and intelligent technical solutions and the extensive use of the performance-based design approach to enhance the resistance and resilience of the built environment. Computational Fluid Dynamics tools, such as the Fire Dynamics Simulator (FDS) [38], have been widely used to model fire scenarios at the microscale level, assessing and quantifying the characteristics of the modelled fire scenario. FDS is well-suited for simulating complex fire scenarios, including modelling of fire dynamics, combustion, and heat transfer, since it can reproduce intricate interactions between fire and the surrounding environment. Moreover, the reported research involves complex scenarios where the fire interacts with structural geometries representative of the built environment. FDS [38] is a well-validated tool [39] but still requires further developments, for instance on the generation of firebrands and their interaction with structures. Firebrands are modelled as Lagrangian particles including their thermo-physical properties, geometric parameters, transport, and behaviour [40]; however, the basic characteristics, namely the release rate and speed, must be defined as inputs to the model. As some examples of the highly relevant investigations conducted using FDS, one can mention the work performed by [PERSON] et al. [41] and [PERSON] et al.