Gilbert Accary

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).

Wildfire behavior study in a Mediterranean pine stand using a physically based model

The aim of this article is to study the propagation of a wildfire, propagating through two eco-systems characteristics of the Mediterranean region located in south of France: the garrigue and the aleppo pines forest. The behavior of a line fire is analysed using a physical model based on the resolution of conservation equations (mass, momentum, energy) governing the coupled system formed by the vegetation and the surrounding atmosphere. The effects of wind (ranged from 3 to 30 m/s) and slope of the terrain (ranged from 0 to 40°) have been studied for surface fires propagating through shrubs (quercus coccifera) composing the surface vegetation of a garrigue and the understorey vegetation of a young pines stand (pinus halepensis). Then the configuration has been generalized, in adding a solid fuel layer above the first one, representing the canopy of a young pines stand. The numerical results have highlighted the strong connection between the surface fire and the canopy. In all cases (on flat terrain) the active crown fire regime has never been reached, and the behaviour of fires, predicted with the model, was more representative of passive crown fires (torching).

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.

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.

Numerical Simulation of Grassland-Fires Behavior Using FireStar3D Model

This study reports 3D numerical simulations of the ignition and the propagation of grassland fires. The mathematical model consists in solving the conservation equations governing the behavior of the coupled system formed by the vegetation and the ambient atmosphere. Based on a finite volume discretization, the computation code is parallelized using OpenMP directives and had been the subject of numerous validations. the model was previously tested on small scale in case of homogeneous litter fires; in this study it is proposed to test it on larger scale in case of grassland fires. the results are in fairly agreement with experimental data, with predictions of operational empirical-models developed in Australia (MK5) and in the USA (BEHAVE), as well as with predictions of physical models (FIRETEC, WFDS, FireStar2D). The comparison with the literature is based on the estimation of the rate of fire spread (ROS) and the analysis of the fire-front shape.

FireStar3D: 3D finite volume model for the prediction of wildfires

A fully-physical wildfire model was used to simulate a confined fire in a wind tunnel. A preliminary study was carried out in the case of homogeneous fuel bed to evaluate the potential of the model to predict fire behavior. The obtained results were analyzed in terms of fire front rate of spread (ROS) and shape, for different wind velocities and fuel moisture content values and compared with experimental data from the literature (measurements and empirical correlations).

Numerical modeling of coherent structures attendant on impurity propagation in the atmospheric boundary layer over a forest canopy

Three-dimensional large eddy simulation is used to solve the problem for a homogeneous forest canopy. The development of the Kelvin-Helmholtz instability above the canopy leads to the formation of coherent structures in the atmosphere flow, which are reproduced in the calculations. The statistical characteristics of the flow obtained from the numerical modeling are compared with experimental data. The passive admixture transfer from the canopy to the clean atmosphere is studied for two cases, namely, for constant and variable coupled concentration of the impurity in the canopy.