Physics-Based Simulations of Flow and Fire Development Downstream of a Canopy

Gilbert Accary,Duncan Sutherland,Duncan Sutherland,Dominique Morvan,Dominique Morvan,Ibrahim Shamseddine,Nicolas Frangieh,Nicolas Frangieh,Sofiane Meradji,Khalid Moinuddin

doi:10.3390/atmos11070683

Abstract

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.

Highlights

Every summer wildfire-prone countries brace for another devastating fire season
The flow obtained from this precursor phase was applied as a dynamic inlet condition at the entrance of the computational domain upstream the grassland in the simulations carried out in phase 2
We investigate the development of the turbulent the sub-canopy and above-canopy flow to confirm that our model correctly reproduces a well-developed canopy flow

Summary

Introduction

Every summer wildfire-prone countries brace for another devastating fire season. Within a decade of the 2009 Black Saturday wildfire in Australia, the world witnessed the Black Summer of 2019–2020 in Australia. While there is debate on whether climate change or forest management is the more important contributing factor, Atmosphere 2020, 11, 683; doi:10.3390/atmos11070683 www.mdpi.com/journal/atmosphere. Atmosphere 2020, 11, 683 the economic, ecological, and health consequences of these wildfires are huge [1]. Climate change and weather patterns occur at global scale and over a long period of time, resulting in protracted periods of drought, fuel dryness, and atmospheric conditions conducive to large fires. It is important to study the effect of forest structure on surface fires as a potential trigger to fires transitioning to an intense and fast-moving crown fire

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