Microscale fire modelling at the Wildland-Urban Interface
Authors: Fiorini Cesare , Craveiro Hélder D. , Santiago Aldina , Laim Luís , Silva Luís Simões da 
Published: October 2022

The direct and indirect impacts of Wildland-Urban Interface fires on infrastructures and communities have become more severe in the last few decades, mainly due to the disproportionate growth of urban areas lacking planning and management, the abandonment of rural areas and activities, and climate changes. Many regions of the southern Mediterranean, the United States, Australia, and South America have been severely affected with catastrophic losses. Building codes addressing the problem of WUI fires in the vicinity of the built environment are still scarce, but already with a few good examples, namely the Australian Standard AS 3959-2009, Construction of Buildings in Bushfire Prone Areas. But with the increasing risks, nowadays mainly driven by climate change, it is necessary to develop new approaches and codes for existing and new buildings effectively contributing to enhance the resilience of the built environment and communities in the WUI. Moreover, taking advantage of new and ever-evolving computational tools, the use of a performance-based approach, replacing or complementing prescriptive codes, shows great potential to enable a deeper understanding of the complex fire spread mechanisms from forest fires to urban fires, namely radiant heat, direct flame contact and firebrands. Physics-based modelling enables a better understanding of such phenomena, bearing in mind that up to date no accurate and reliable models for firebrands can be found. In this investigation, a performance-based approach is considered, exploring the capabilities of computational fluid dynamics and the software Fire Dynamics Simulation (FDS) to investigate and quantify WUI fire exposures. This was achieved by considering available experimental data on vegetation burning and developing and calibrating the numerical models using FDS. A Particle Method, based on Lagrangian particles was selected for this investigation, since this model is particularly suitable to simulate surface and raised vegetation fire spread. With this strategy all thermo-physical properties of the fuels must be used as input, ensuring that the fire spread can be computed by the model. Based on the calibrated models for a single tree, a new case study scenario was created (structure exposed to wildfire) and investigated aiming to assess in detail WUI fire exposures under different conditions by varying several parameters, such as wind speed and direction, distance to the structure and elevation of the terrain. Since a performance-based approach was selected and considering the basic principles associated with Fire Safety Engineering (FSE), 3 basic components must be assessed, namely the fire modelling, the thermal analysis in the structure and finally the structural analysis considering temperature increase and degradation of mechanical properties of materials. From the fire modelling investigated in this paper, some attention was devoted to assessing Adiabatic Surface Temperatures in the structure and consequently defining in a simple way to couple CFD field models to Finite Element Models (FEM) that will enable the understanding and development of ignition resistant structures in the WUI.