Numerical prediction of the thermal stress induced by the burning of an ornamental vegetation at WUI
Authors: Graziani Anthony , Meerpoel-Petri Karina , Tihay-Felicelli Virginie , Santoni Paul-Antoine , Morandini Frédéric , Perez-Ramirez Yolanda , Pieri Antoine , Mell William 
Published: October 2022

Over the last decades, urban expansion and global warming have increased the occurrence of wildland fires propagating at the vicinity of buildings at WUI. In this scenario, ornamental vegetation has been identified as a vector of fire propagation close to habitations, which can significantly increase the risk of damage [1]. In such context, it is necessary to quantify the thermal stress generated by an ornamental plant over a building to predict the vulnerability of construction materials. To this end, numerical simulation is a good candidate to easily multiply burning cases at field scale and explore the effects. The present study focuses on the numerical prediction of the thermal stress induced by the burning of an ornamental vegetation over targets facing the fire. The study involves a numerical modelling of the burning of rockrose hedges at field scale using the physics based code WFDS. The solver is based on a large eddy simulation approach for fluid dynamics and energy transfer through the fluid phase. A three steps thermal degradation model (dehydration, pyrolysis, char oxidation) with Arrhenius laws [2] is used for the fuel. The raised vegetation is represented with a Fuel Element approach which models the solid fuel as a set of static Lagrangian particles of different sizes and distributed within the volume to reproduce the arrangement of the shrub. The accuracy of WFDS to reproduce the combustion of plants has already been demonstrated at laboratory scales [2-6] but studies at field scale involving raised vegetation are few. Numerical results are compared to experimental measurements recorded during a set of experiments conducted at field scale, which involves the burning of reconstructed rockrose hedges of 6m length, 1m width and two heights (1m and 2m). The geometry mimics the typical shape of ornamental hedges that can be found to separate buildings in south of France. Visible cameras are distributed around the setup to capture the geometry of the flame front. Four couples of heat flux meters are positioned at 3m in front of the centreline and side of the hedge, which represents the theoretical position of the wall of a building according to the current fire safety regulation in France. Comparison between numerical model and experimental results shows good agreement for the local measurement of the heat stress at the location of the targets. Total and radiant heat fluxes fit with experimental data during the fire growth and the fully developed phases, which represent the period where the thermal stress is the highest. Peaks of total and radiant heat flux are the same order value but can be overestimated depending of the location of the sensors due to the wind dynamics that is not fully implementable in WFDS. Results show that the accuracy of the numerical model is enough to predict the thermal stress received by targets during the fully developed fire at field scale and could be used to numerically determine the vulnerability of material buildings in different scenarios.