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Wildfires are among the most devastating events in nature and have profound environmental and socioeconomical impacts around the world. A particularly important mechanism of wildfire spread refers to the transport and eventual deposition of burning debris known as firebrands. Under favorable conditions, firebrands can travel several kilometers downwind, igniting new fires upon landing. The erratic nature of firebrand transport makes spotting challenging to predict and control. Firebrand ignition is an important pathway of fire spread into communities, straining fire suppression resources and posing considerable risk to lives and property. Despite the extensive literature, many aspects of spotting behavior remain understudied. Particularly, no previous study has provided an in-depth, fundamental insight on the role of turbulence structures on firebrand transport and spotting behavior in complex environments. This lack of understanding compromises our capability in accurately predicting firebrand transport, therefore hindering risk mitigation and management efforts. This research aims to advance our understanding of the spotting phenomenon through a computational study of particle-turbulence interactions that govern firebrand behavior during flight and after landing. The present research involves a Lagrangian particle tracking model, capable of simulating thousands of firebrands with varying mass, size, and temperature. This model is employed in combination with large-eddy simulations to examine how topography-driven turbulence influences the flight and post-landing dynamics of firebrands across different particle sizes. Additionally, this research investigates how different turbulence scales, including small-scale motions comparable to firebrand sizes, influence the large-scale transport of inertial and settling particles in boundary layer flows, with applications to improving models of particle transport in the atmospheric boundary layer. Collectively, the insights of this research contribute towards advancing our understanding of firebrand spotting, therefore improving strategies for fire-risk mitigation around the world.
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Characterizing firebrands and their kinematics during lofting
Spot fires pose a major risk and add to the already complex physics, which makes fire spread so hard to predict, especially in the wildland urban interface. Firebrands can not only cross fuel breaks and thwart other suppression efforts but also directly damage infrastructure and block evacuation routes. Transport models and computational fluid dynamics tools often make simplifications when predicting spot fire risk, but there is a relative lack of experimental data to validate such parameterizations. To this end, we present a field experiment performed at the University of California Berkeley Blodgett Research Forest in California where we recorded the flame and firebrands emanating from a nighttime hand-drawn pile fire using high-frequency imaging. We used image-processing to characterize the fire intensity and turbulence as well as particle tracking velocimetry to measure ejected firebrand kinematics as they are lofted by the plume. We further collected embers that settled around the fire at varying distances and measured their size, shape, density, and settling distributions. We also examine existing physics-based time-averaged models of firebrand lofting and note discrepancies between such models, often used due to their speed and simplicity, and our experimental observations. Finally, we discuss some implications our observations could have on future modeling efforts by considering the time-dependent fire dynamics, intermittency in the plume turbulence, and in the firebrand generation rate. To the best of our knowledge, these are the first in situ observations of firebrand generation and lofting from representative fuels, addressing a major source of data gap and uncertainty in the wildland fire literature.
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- PAR ID:
- 10552014
- Publisher / Repository:
- AIP
- Date Published:
- Journal Name:
- Physics of Fluids
- Volume:
- 36
- Issue:
- 10
- ISSN:
- 1070-6631
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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- Free Publicly Accessible Full Text
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Accepted Manuscript
- Journal Article:
- https://doi.org/10.1063/5.0227024
