Front’s dynamics of quasi-infinite grassland fires

Abstract

Numerical simulations of grassland fires were carried out to study the effects of the wind speed on the dynamics of the fire front. This study was performed using a detailed physical model based on a multiphase formulation, consisting in solving the balance equations (mass, momentum, energy ...) governing the behavior of the coupled system formed by the vegetation layer and the surrounding atmosphere. The objective of this study is to focus on the 3D interactions between a quasi-infinite fire front propagating through a homogeneous vegetation layer (a grassland) and an atmospheric boundary layer flow representing an idealized wind flow. The behavior of a surface fire results from the competition between the buoyant force of the thermal plume and the inertial force of the wind. In 3D, the interaction between these two forces is at the origin of the structuration of the fire front as a succession of peaks and troughs that modify the flames dynamics in a manner different to what can be observed in a simplified 2D assumption. To reproduce a quasi-infinite fire front, the numerical simulations were carried out using periodic boundary conditions along the two lateral sides of the computational domain. The numerical results highlights the 3D structuration of the fire both vertically and horizontally, and the significant effect of an increase of the wind velocity on the rate of spread (ROS) and on the fire intensity. Results also show that the correlation between Froude’s number (based on the fire-front structures characteristic-wavelength) and Byram’s number is the same at small scale (litter fires) and at large scale (grassland fires).