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A 3D multi-physical model referred to as "FireStar3D" has been developed in order to predict the behavior of wildfires at a local scale (< 500m). In the continuity of a previous work limited to 2D configurations, this model consists of solving the conservation equations of the coupled system composed of the vegetation and the surrounding gaseous medium. In particular, the model is able to account explicitly for all the mechanisms of degradation of the vegetation (by drying, pyrolysis, and heterogeneous combustion) and the various interactions between the gas mixture (ambient air + pyrolysis and combustion products) and the vegetation cover such as drag force, heat transfer by convection and radiation, and mass transfer. Compared to previous work, some new features were introduced in the modelling of the surface combustion of charcoal, the calculation of the heat transfer coefficient between the solid fuel particles and the surrounding atmosphere, and many improvements were brought to the numerical method to enable affordable 3D simulations. The partial validation of the model was based on some comparisons with experimental data collected at small scale fires carried out in the Missoula Fire Sciences Lab's wind tunnel, through various solid-fuel layers and in well controlled conditions. A relative good agreement was obtained for most of the simulations that were conducted. A parametric study of the dependence of the rate of spread on the wind speed and on the fuelbed characteristics is presented.

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This study reports 3D numerical simulations of the ignition and the propagation of grassland fires. The mathematical model is based on a multiphase formulation and on a homogenization approach that consists in averaging the conservation equations (mass, momentum, energy …) governing the evolution of variables representing the state of the vegetation/atmosphere system, inside a control volume containing both the solidvegetation phase and the surrounding gaseous phase. This preliminary operation results in the introduction of source/sink additional terms representing the interaction between the gaseous phase and the solid-fuel particles. This study was conducted at large scale in grassland because it represents the scale at which the behavior of the fire front presents most similarities with full scale wildfires and also because of the existence of a large number of relatively well controlled experiments performed in Australia and in the United States. The simulations were performed for a tall grass, on a flat terrain, and for six values of the 10-m open wind speed ranged between 1 and 12 m/s. The results are in fairly good agreement with experimental data, with the predictions of operational empirical and semi-empirical models, such as the McArthur model (MK5) in Australia and the Rothermel model (BEHAVE) in USA, as well as with the predictions of other fully 3D physical fire models (FIRETEC and WFDS). The comparison with the literature was mainly based on the estimation of the rate of fire spread (ROS) and of the fire intensity, as well as on the analysis of the fire-front shape.