Physique et modélisation du comportement des feux de forêt (thèse 2015 - 2019)
Publications scientifiques au M2P2
2020
Nicolas Frangieh, Gilbert Accary, Dominique Morvan, Sofiane Meradji, Oleg Bessonov. Wildfires front dynamics: 3D structures and intensity at small and large scales. Combustion and Flame, 2020, 211, pp.54-67. ⟨10.1016/j.combustflame.2019.09.017⟩. ⟨hal-02892557⟩ Plus de détails...
The 3D structure of a fire front propagating through a homogeneous porous solid-fuel layer was studied numerically at laboratory and field scales. At laboratory scale, wind-tunnel fires propagating through laser-cut cardboard fuel were numerically reproduced, while at field scale, simulations of grassland fires with quasi-infinite fire front were carried out for different wind speeds. These simulations were performed using FIRESTAR3D, based on a multiphase formulation that includes the main physical phenomena governing fire behavior. An unsteady RANS approach and a Large Eddy Simulation (LES) approach were used to simulate the reactive turbulent flow, whereas turbulent combustion was modeled using Eddy Dissipation Concept (EDC). Unlike other 3D wildfire tools available in the community, such as FIRETEC and WFDS, the model is based on an implicit, low-Mach number resolution of the governing equations, and makes no empirical assumptions in the resolution of the radiative transfer equation. The comparison with the experimental data concerned mainly the Rate of Spread (ROS) of fire, the fireline intensity, the flame-zone depth, and the wavelength characterizing the crest-and-trough structure of the fire front along the transverse direction. Particular attention was drawn to the similitude in the fire front dynamics between small and large scales. In order to highlight the physical mechanisms responsible for this dynamics, a dimensional analysis was carried out by introducing Byram's convection number N-C based on the fireline intensity and Froude's numbers Fr based on the characteristic wavelength of the fire-front structure. The analysis shows that all the results (wind-tunnel fires and grassland fires, experimental and numerical) collapsed on a single scaling law in the form Fr = N-C(-)2/3.
Nicolas Frangieh, Gilbert Accary, Dominique Morvan, Sofiane Meradji, Oleg Bessonov. Wildfires front dynamics: 3D structures and intensity at small and large scales. Combustion and Flame, 2020, 211, pp.54-67. ⟨10.1016/j.combustflame.2019.09.017⟩. ⟨hal-02892557⟩
Gilbert Accary, Duncan Sutherland, Nicolas Frangieh, Khalid Moinuddin, Ibrahim Shamseddine, et al.. Physics-Based Simulations of Flow and Fire Development Downstream of a Canopy. Atmosphere, 2020, 11 (7), pp.683. ⟨10.3390/atmos11070683⟩. ⟨hal-02957445⟩ Plus de détails...
The behavior of a grassland fire propagating downstream of a forest canopy has been simulated numerically using the fully physics-based wildfire model FIRESTAR3D. This configuration reproduces quite accurately the situation encountered when a wildfire spreads from a forest to an open grassland, as can be the case in a fuel break or a clearing, or during a prescribed burning operation. One of the objectives of this study was to evaluate the impact of the presence of a canopy upstream of a grassfire, especially the modifications of the local wind conditions before and inside a clearing or a fuel break. The knowledge of this kind of information constitutes a major element in improving the safety conditions of forest managers and firefighters in charge of firefighting or prescribed burning operations in such configurations. Another objective was to study the behavior of the fire under realistic turbulent flow conditions, i.e., flow resulting from the interaction between an atmospheric boundary layer (ABL) with a surrounding canopy. Therefore, the study was divided into two phases. The first phase consisted of generating an ABL/canopy turbulent flow above a pine forest (10 m high, 200 m long) using periodic boundary conditions along the streamwise direction. Large Eddy Simulations (LES) were carried out for a sufficiently long time to achieve a quasi-fully developed turbulence. The second phase consisted of simulating the propagation of a surface fire through a grassland, bordered upstream by a forest section (having the same characteristics used for the first step), while imposing the turbulent flow obtained from the first step as a dynamic inlet condition to the domain. The simulations were carried out for a wind speed that ranged between 1 and 12 m/s; these values have allowed the simulations to cover the two regimes of propagation of surfaces fires, namely plume-dominated and wind-driven fires.
Gilbert Accary, Duncan Sutherland, Nicolas Frangieh, Khalid Moinuddin, Ibrahim Shamseddine, et al.. Physics-Based Simulations of Flow and Fire Development Downstream of a Canopy. Atmosphere, 2020, 11 (7), pp.683. ⟨10.3390/atmos11070683⟩. ⟨hal-02957445⟩
Nicolas Frangieh, Dominique Morvan, Sofiane Meradji, Gilbert Accary, Oleg Bessonov. Numerical simulation of grassland fires behavior using an implicit physical multiphase model. Fire Safety Journal, 2018, 102, pp.37-47. ⟨10.1016/j.firesaf.2018.06.004⟩. ⟨hal-02114073⟩ Plus de détails...
12 This study reports 3D numerical simulations of the ignition and the propagation of 13 grassland fires. The mathematical model is based on a multiphase formulation and on a 14 homogenization approach that consists in averaging the conservation equations (mass, 15 momentum, energy …) governing the evolution of variables representing the state of the 16 vegetation/atmosphere system, inside a control volume containing both the solid-17 vegetation phase and the surrounding gaseous phase. This preliminary operation results 18 in the introduction of source/sink additional terms representing the interaction between 19 the gaseous phase and the solid-fuel particles. This study was conducted at large scale in 20 grassland because it represents the scale at which the behavior of the fire front presents 21 most similarities with full scale wildfires and also because of the existence of a large 22 number of relatively well controlled experiments performed in Australia and in the 23 United States. The simulations were performed for a tall grass, on a flat terrain, and for 24 six values of the 10-m open wind speed ranged between 1 and 12 m/s. The results are in 25 fairly good agreement with experimental data, with the predictions of operational 26 empirical and semi-empirical models, such as the McArthur model (MK5) in Australia and 27 the Rothermel model (BEHAVE) in USA, as well as with the predictions of other fully 3D 28 physical fire models (FIRETEC and WFDS). The comparison with the literature was 29 mainly based on the estimation of the rate of fire spread (ROS) and of the fire intensity, 30 as well as on the analysis of the fire-front shape. 31 32
Nicolas Frangieh, Dominique Morvan, Sofiane Meradji, Gilbert Accary, Oleg Bessonov. Numerical simulation of grassland fires behavior using an implicit physical multiphase model. Fire Safety Journal, 2018, 102, pp.37-47. ⟨10.1016/j.firesaf.2018.06.004⟩. ⟨hal-02114073⟩
Dominique Morvan, Gilbert Accary, Sofiane Meradji, Nicolas Frangieh, Oleg Bessonov. A 3D physical model to study the behavior of vegetation fires at laboratory scale. Fire Safety Journal, 2018, 101, pp.39-52. ⟨10.1016/j.firesaf.2018.08.011⟩. ⟨hal-02114685⟩ Plus de détails...
A 3D multi-physical model referred to as “FireStar3D” has been developed in order to predict the behavior of wildfires at a local scale (<500 m). 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 works, some new features were introduced in the modeling 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.
Dominique Morvan, Gilbert Accary, Sofiane Meradji, Nicolas Frangieh, Oleg Bessonov. A 3D physical model to study the behavior of vegetation fires at laboratory scale. Fire Safety Journal, 2018, 101, pp.39-52. ⟨10.1016/j.firesaf.2018.08.011⟩. ⟨hal-02114685⟩
Dominique Morvan, Nicolas Frangieh. Wildland fires behaviour: wind effect versus Byram’s convective number and consequences upon the regime of propagation. International Journal of Wildland Fire, 2018, 27 (9), pp.636. ⟨10.1071/Wf18014⟩. ⟨hal-02114689⟩ Plus de détails...
With fuel moisture content and slope, wind velocity (U W) is one of the major physical parameters that most affects the behaviour of wildland fires. The aim of this short paper was to revisit the relationship between the rate of spread (ROS) and the wind velocity, through the role played by the two forces governing the trajectory of the flame front and the plume, i.e. the buoyancy of the plume and the inertia due to wind. A large set of experimental data (at field and laboratory scale) from the literature was analysed, by introducing the ratio between these two forces, namely Byram's convective number N C and considering the relationship between the fire ROS/wind speed ratio and Byram's number. This short note was also an opportunity to make a point on particular issues, such as the existence of two regimes of propagation of surface fires (wind-driven fire vs plume-dominated fire), the relative importance of the two modes of heat transfer (by convection and radiation) on the propagation of a fire front, and others scientific debates animating the wildland fire community.
Dominique Morvan, Nicolas Frangieh. Wildland fires behaviour: wind effect versus Byram’s convective number and consequences upon the regime of propagation. International Journal of Wildland Fire, 2018, 27 (9), pp.636. ⟨10.1071/Wf18014⟩. ⟨hal-02114689⟩
