Dominique Morvan

Numerical study of a crown fire spreading toward a fuel break using a multiphase physical model

The propagation of a wildfire through a Mediterranean pine stand was simulated using a multiphase physical model of fire behaviour. The heterogeneous character of the vegetation was taken into account using families of solid particles, i.e. the solid phases (foliage, twigs, grass). The thermal decomposition of the solid fuel by drying and pyrolysis, and the combustion of chars were considered, as well as the radiative and convective heat transfer between the gas and the vegetation. In the gaseous phase, turbulence was modelled using a two transport equations model (RNG k-e) and the rate of combustion, which was assumed to be controlled by the turbulent mixing of fuel and oxygen, was calculated using an eddy dissipation concept. The radiation transfer equation, which includes absorption and emission of both the gas-soot mixture and the vegetation, was solved to calculate the contribution of radiation to the energy balance equations. Numerical solutions were calculated in a two-dimensional domain (vertical plane). Results showed the ability of this approach to simulate the propagation of a crown fire and to test the efficiency of a fuel break with success. The effects of the terrain slope were also tested. Some effects on fire behaviour of vortices resulting from the interaction of the wind flow with the canopy layer are shown.

Effect of vegetation heterogeneity on radiative transfer in forest fires

Wildland fires are driven by the heat transferred from the fire source to the unburned fuel bed and this transfer is likely to be affected by the spatial heterogeneity of fuel element distributions at different scales from shoot to stand. In a context of theoretical fire modelling, we investigated the impact of a departure from randomness of fuel distributions on the radiative transfer of energy. Our methodology was derived from the approach developed for solar radiation in heterogeneous canopies or clouds and was modified to suit an analysis of fire behaviour. Some fine and coarse fuel distributions for several Mediterranean fuel types were derived from field measurements and plant architecture modelling. A comparison of the average irradiances in different fuels showed whether heterogeneity effects were significant or not. Results showed that both marked spatial variability in fuel distribution (low cover fraction and large clumps) and a high vegetation density were required to provide significant effects. The radiative transfer in heterogeneous maritime pines and in dense shrub stands was significantly affected by heterogeneity, mainly at crown and shoot scales. Less pronounced effects were observed in Aleppo pine stand and light shrubs. In terms of fuel modelling, the 2-m resolution used in a fire model such as FIRETEC seems to be sufficient for the fuel types investigated here, with the exception of dense small clumps in shrublands. An effective coefficient was proposed for these latter cases.

Optimized parallel approach for 3D modelling of forest fire behaviour

In this paper we present methods for parallelization of 3D CFD forest fire modelling code on Non-uniform memory computers in frame of the OpenMP environment. Mathematical model is presented first. Then, some peculiarities of this class of computers are considered, along with properties and limitations of the OpenMP model. Techniques for efficient parallelization are discussed, considering different types of data processing algorithms. Finally, performance results for the parallelized algorithm are presented and analyzed (for up to 16 processors).

Efficient Parallelization of the Preconditioned Conjugate Gradient Method

In this paper we present methods for efficient parallelization of the solution of pressure Poisson equation arising in 3D CFD forest fire modeling. The solution procedure employs the Conjugate Gradient method with implicit Modified ILU (MILU) preconditioner. The basic idea for parallelizing recursive incomplete-decomposition algorithms is to use a direct nested twisted approach in combination with a staircase method. Parallelization of MILU-CG solver is implemented in OpenMP environment for Non-uniform memory (NuMA) computer systems. Performance results of the parallelized algorithm are presented and analyzed for different number of processors (up to 16).

Wind Effects, Unsteady Behaviors, and Regimes of Propagation of Surface Fires in Open Field

The subject of this article concerns the unsteady effects (fire intensity, wind) upon the propagation and, more generally, the behavior of surface fires in open fields. The study focused on two sources of unsteadiness: the first one resulting from the regime of propagation (wind driven or plume dominated), which can affect greatly the behavior of the flame front and consequently the fire intensity, the second one resulting from the wind gusts associated with the conditions of flow of wind in real conditions. The study was based on numerical simulations, using a multiphase formulation, and on spectral analysis of the time evolution of the fire line intensity. The calculations were performed in 2D for a homogeneous vegetation layer (grassland) and for a large interval of wind conditions (10 m open wind velocity U10 ranged between 1 m/s and 25 m/s). The results have highlighted the link between the unsteady character of flame front behavior and the regime of propagation (plume dominated, wind driven). A particular interest was focused on the role played by two potential sources of instabilities, namely the Kelvin-Helmholtz instability (wind effects) and the thermo-convective instability (plume effects), upon the behavior of fires. A second set of simulations has been carried out using unsteady wind conditions, reproduced using sinusoidal boundary conditions for the streamwise velocity, with a frequency ranging between 0.5 Hz and 3 Hz.

THREE-DIMENSIONAL NUMERICAL SIMULATION OF THE INTERACTION BETWEEN NATURAL CONVECTION AND RADIATION IN A DIFFERENTIALLY HEATED CAVITY IN THE LOW MACH NUMBER APPROXIMATION

Many studies have been devoted to the interaction between natural convection and radiation heat transfer in a differentially heated cavity. This problem has already been treated using the Boussinesq approximation. The main purpose of t his study is to extend this interaction to the low Mach number approximation (in 3D), for both tra nsparent and participating media. The NavierStokes and energy equations written for an ideal ga s are solved using a finite volume method, while the discrete ordinates method is used to solve the radiation transfer equation. The coupling between the energy equation and the radiation transfer is d one by adding an additional source term in the energy equation and via the radiation heat exchange between the surfaces bounding the computation domain. The work is first validated using the Bouss ine q approximation mainly by investigating the distribution of the heat flux on the hot isothermal wall. Then some simulations are presented highlighting the differences between the low Mach n umber and the Boussinesq approximations.

Numerical study of the interaction between a head fire and a backfire propagating in grassland

One of the objectives of this paper was to simulate numerically the interaction between two line fires ignited in a grassland, on a flat terrain, perpendicularly to the wind direction, in such a way that the two fire fronts (a head fire and a backfire) propagated in opposite directions parallel to the wind. The numerical simulations were conducted in 3D using the new fuel element module recently implemented in WFDS. We chose a grassland for the fuel layer, because it represents certainly one of the most studied ecosystem, in the frame of experimental campaigns to study the behaviour of surface fires at large scale. The aim of this numerical study is to understand what are the physical phenomena and the favorable conditions of ignition of a counter fire, during a fire suppression operation. The preliminary results highlighted that the two fire fronts interacted only at a relative short distance (10-20 m), following this scenario: • The thermal plume above the head fire (representing the main fire front) formed a sort of shelter, protecting the backfire to the direct effect of the wind flow • Before the merging between the two fire fronts, an in-draft flow was observed ahead of the head fire, promoting and accelerating the propagation of the backfire During the last step of the merging of the two fire fronts, a sudden increase of the heat release rate was observed, indicating a significant interaction between the two fires, which can potentially represent a safety problem for people in charge of this kind of operation.

Large Eddy Simulation of Coherent Structures over Forest Canopy

This paper deals with the numerical simulation (using a LES approach) of the interaction between an atmospheric boundary layer (ABL) and a canopy, representing a forest cover. This problem was studied for a homogeneous configuration representing the situation encountered above a continuous forest cover, and a heterogeneous configuration representing the situation similar to an edge or a clearing in a forest. The numerical results, reproduced correctly all the main characteristics of this flow, as reported in the literature: the formation of a first generation of coherent structures aligned transversally from the wind flow direction, the reorganisation and the deformation of these vortex tubes to horse shoe structures. The results obtained, introducing a discontinuity in the canopy (reproducing a clearing or a fuel break in a forest), were compared with experimental data collected in a wind tunnel. The results confirmed the existence of a strong turbulence activity inside the canopy at a distance equal to 8 times the height of the canopy, referenced in the literature as an Enhance Gust Zone (EGZ) characterized by a local peak of the skewness factor.

An improved non-equilibrium model for the ignition of living fuel

This paper deals with the modelling of living fuel ignition, suggesting that an accurate description using a multiphase formulation requires consideration of a thermal disequilibrium within the vegetation particle, between the solid (wood) and the liquid (sap). A simple model at particle scale is studied to evaluate the flux distribution between phases in order to split the net flux on the particles into the two sub-phases. An analytical solution for the split function is obtained from this model and is implemented in ForestFireFOAM, a computational fluid dynamics (CFD) solver dedicated to vegetation fire simulations, based on FireFOAM. Using this multiphase formulation, simulations are run and compared with existing data on living fuel flammability. The following aspects were considered: fuel surface temperature, ignition, flaming combustion time, mean and peak heat release rate (HRR). Acceptable results were obtained, suggesting that the thermal equilibrium might not be an acceptable assumption to properly model ignition of living fuel.

Forest Fire Research: The Latest Advances Tools for Understanding and Managing Wildland Fire

1University of Corsica Pascal Paoli, Campus Grimaldi, SPE UMR 6134 CNRS, BP 52, 20250 Corte, France 2CSIRO Ecosystem Sciences and CSIRO Climate Adaptation Flagship, GPO Box 1700 Canberra, ACT 2601, Australia 3Aix-Marseille Universite, UNIMECA, 60 Rue Joliot Curie, 13453 Marseille cedex 13, France 4U.S. Forest Service, Pacific Wildland Fire Sciences Lab, 400 N. 34th St., Suite 201, Seattle, WA 98103, USA