Abstract
Wildfire behavior is dictated by the complex interaction of numerous physical phenomena including dynamic ambient and fire-induced winds, heat transfer, aerodynamic drag on the wind by the fuel and combustion. These phenomena create complex feedback effects between the fire and its surroundings. In this study, we aim to study the mechanisms by which buoyant flame dynamics along with vortical motions and instabilities control wildfire propagation. Specifically, this study employs a suite of simulations conducted with the physics-based coupled fire-atmosphere behavior model (FIRETEC). The simulations are initialized with a fire line and the fires are allowed to propagate on a grass bed, where the fuel heights and wind conditions are varied systematically. Flow variables are extracted to identify the characteristics of the alternating counter-rotational vortices, called towers and troughs, that drive convective heat transfer and fire spread. These vortices have previously been observed in wildfires and laboratory fires, and have also been observed to arise spontaneously in FIRETEC due to the fundamental physics incorporated in the model. However, these past observations have been qualitative in nature and no quantitative studies can be found in the literature which connected these coherent structures fundamental to fire behavior with the constitutive flow variables. To that end, a variety of state variables are examined in the context of these coherent structures under various wind profile and grass height conditions. Identification of various correlated signatures and fire-atmosphere feedbacks in simulations provides a hypothesis that can be tested in future observational or experimental efforts, potentially assisting experimental design, and can aid in the interpretation of data from in situ detectors.
Highlights
Wildfires occur across the globe, with varied degrees of impact on nature and society.Low intensity wildfires can be beneficial from a landscape ecology perspective, while high-intensity fires, especially at the wildland-urban interface can be catastrophic in terms of threat to infrastructure, economic losses as well as loss of lives
Noting that the coherent structures are a direct result of heat transfer coupling the fuel combustion to the gas phase, the tower and trough structures are naturally only observed during active heating and combustion
The current work investigated the characteristic features of tower and trough type structures using numerical simulations, which has been recently identified as the dominant coherent structures associated with fire front propagation
Summary
Wildfires occur across the globe, with varied degrees of impact on nature and society. Low intensity wildfires can be beneficial from a landscape ecology perspective, while high-intensity fires, especially at the wildland-urban interface can be catastrophic in terms of threat to infrastructure, economic losses as well as loss of lives. The fundamental physics governing wildland fires are immensely complex. Because landscape-scale fires often impact human and ecological well-being, an improved understanding of fire physics and accurate wildfire models are necessary to anticipate and mitigate catastrophic scenarios, as well as designing prescribed burns. Prescribed burns are usually low intensity fires that are aimed at reducing heavy fuel loads in order to reduce future fire intensity. A detailed understanding of the energetics, regulating factors and scales responsible for fire propagation are important from a management perspective [1]. This work applies a coupled fire/atmosphere model (FIRETEC) to elucidate our understanding of phenomena that have
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