Identifying Characteristics of Wildfire Towers and Troughs

High frequency (30 Hz) two-dimensional particle image velocimetry data recorded during a field experiment exploring fire spread from point ignition in hand-spread pine needles under calm ambient wind conditions are analysed in this study. In the initial stages, as the flame spreads approximately radially away from the ignition point in the absence of a preferred wind-forcing direction, it entrains cooler ambient air into the warmer fire core, thereby experiencing a dynamic pressure resistance. The fire-front, comprising a flame that is tilted inward, is surrounded by a region of downdraft. Coherent structures describe the initial shape of the fire-front and its response to local wind shifts while also revealing possible fire-spread mechanisms. Vortex tubes originating outside the fire spiral inward and get stretched thinner at the fire-front leading to higher vorticity there. These tubes comprise circulation structures that induce a radially outward velocity close to the fuel bed, which pushes hot gases outward, thereby causing the fire to spread. Moreover, these circulation structures confirm the presence of counter-rotating vortex pairs that are known to be a key mechanism for fire spread. The axis of the vortex tubes changes its orientation alternately towards and away from the surface of the fuel bed, causing the vortex tubes to be kinked. The strong updraft observed at the location of the fire-front could potentially advect and tilt the kinked vortex tube vertically upward leading to fire-whirl formation. As the fire evolves, its perimeter disintegrates in response to flow instabilities to form smaller fire “pockets”. These pockets are confined to certain points in the flow field that remain relatively fixed for a while and resemble the behavior of a chaotic system in the vicinity of an attractor. Increased magnitudes of the turbulent fluxes of horizontal momentum, computed at certain such fixed points along the fire-front, are symptomatic of irregular fire bursts and help contextualize the fire spread. Most importantly, the time-varying transport terms of the turbulent kinetic energy budget equation computed at adjacent fixed points indicate that local fires along the fire-front primarily interact via the horizontal turbulent transport term.

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Section: Resultssupporting

confidence: 88%

“…Several studies 20 – 23 have utilized FIRETEC, which employs a finite-volume solver for its system of governing equations. Alternating regions of upwash and downwash motions 24 were discovered upwind of the headfire. Streamlines were found to deviate into the flanking direction upwind of the fire-front 22 .…”

Section: Introductionmentioning

confidence: 97%

Investigating the turbulent dynamics of small-scale surface fires

Desai

Goodrick

Banerjee

2022

Sci Rep

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“…The pockets of fire at each fixed point are regions of updraft, which are separated by regions of downdraft. The envelope of the fire-perimeter of each of these pockets constitutes the fire-front, which consequentially comprises alternating regions of upwash and downwash verifying similar observations made in the literature 24,34 . Finally, a three-dimensional flow picture is constructed from an isoparametric view of the net flow ( u = u î + v ĵ + w k ) at t = 100 s as shown in Fig.…”

Section: Resultssupporting

confidence: 88%

“…Several studies [20][21][22][23] have utilized FIRETEC, which employs a finite-volume solver for its system of governing equations. Alternating regions of upwash and downwash motions 24 were discovered upwind of the headfire. Streamlines were found to deviate into the flanking direction upwind of the fire-front 22 .…”

mentioning

confidence: 97%

Investigating the turbulent dynamics of small-scale surface fires

Desai¹,

Goodrick²,

Banerjee³

2021

Preprint

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High frequency (30 Hz) two-dimensional particle image velocimetry data recorded during a field experiment exploring fire spread from point ignition in hand-spread pine needles under calm ambient wind conditions are analysed in this study. In the initial stages, as the flame spreads approximately radially away from the ignition point in the absence of a preferred wind-forcing direction, it entrains cooler ambient air into the warmer fire core, thereby experiencing a dynamic pressure resistance. The fire-front, comprising a flame that is tilted inward, is surrounded by a region of downdraft. Coherent structures describe the initial shape of the fire-front and its response to local wind shifts while also revealing possible fire-spread mechanisms. Vortex tubes originating outside the fire spiral inward and get stretched thinner at the fire-front leading to higher vorticity there. These tubes comprise circulation structures that induce a radially outward velocity close to the fuel bed, which pushes hot gases outward, thereby causing the fire to spread. Moreover, these circulation structures confirm the presence of counter-rotating vortex pairs that are known to be a key mechanism for fire spread. The axis of the vortex tubes changes its orientation alternately towards and away from the surface of the fuel bed, causing the vortex tubes to be kinked. The strong updraft observed at the location of the fire-front could potentially advect and tilt the kinked vortex tube vertically upward leading to fire-whirl formation. As the fire evolves, its perimeter disintegrates in response to flow instabilities to form smaller fire "pockets". These pockets are confined to certain points in the flow field that remain relatively fixed for a while and resemble the behavior of a chaotic system in the vicinity of an attractor. Increased magnitudes of the turbulent fluxes of horizontal momentum, computed at certain such fixed points along the fire-front, are symptomatic of irregular fire bursts and help contextualize the fire spread. Most importantly, the time-varying transport terms of the turbulent kinetic energy budget equation computed at adjacent fixed points indicate that local fires along the fire-front primarily interact via the horizontal turbulent transport term.The frequency and severity of wildfires have increased over the last few years and the aggravating global climate (change) presents an increased risk. According to the National Interagency Fire Center 1 , there have been 39108 fires in the year 2021, as of August 7th, 2021 in the USA and the corresponding total burned acreage has increased by 53 % from 2,286,517 acres in 2020 to 3,506,321 acres in 2021. A deeper understanding of wildfire dynamics is an urgent necessity to assist containment operations and fire incident management and prevention. While wildfire modeling has progressed significantly over the past few decades, the progress in terms of observational evidence has been slow. The interaction between the fire and the atmosphere creates a turbulent environment an...

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“…At small scales, combustion processes releasing energy through radiative and convective heating locally modify the dynamics along the fire front and can impact the geometry of the fireline and the rate of spread (ROS) (Finney et al., 2015; Frankman et al., 2012). Large temperature gradients and pressure perturbations between the environment and the fireline support convergence into the fire that enhances convection and drives plume dynamics at larger scales (Banerjee et al., 2020; Kiefer et al., 2010; Mandel et al., 2011). The interaction across scales can lead to nonlinear responses in fire‐atmosphere interactions that are challenging to disentangle.…”

Section: Introductionmentioning

confidence: 99%

A Case Study Featuring the Time Evolution of a Fire‐Induced Plume Jet Over the Rum Creek Fire: Mechanisms, Processes, and Dynamical Interplay

Strobach,

Carroll,

Baidar

et al. 2024

JGR Atmospheres

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Results are presented from the Rum Creek fire during the California Fire Dynamics Experiment (CalFiDE). An instrumented payload aboard the NOAA Twin Otter (TO) aircraft, which included a scanning micro‐pulsed Doppler lidar (DL) and in situ chemistry packages, was used to address the evolution of a buoyant plume jet (BPJ) and transport dynamics over the southwest corner of the fire between 09/01/2022 and 09/02/2022. An approach previously developed to isolate updrafts was modified to account for the Gaussian core structure when addressing the evolution of the updraft, plume‐top entrainment, lateral entrainment, and the role of fire‐atmosphere interactions on the characteristics of the BPJ during four overpasses. A persistent cross‐valley flow leading up to the BPJ was observed for all four overpasses, with flow enhancement during the second overpass that coincided with changes in the BPJ structure and turbulence characteristics surrounding the BPJ. Length scales and entrainment rates were estimated at the lateral edges and the top of the plume. In the case of the latter, an analytical form of the BPJ profile above the vertical velocity maximum of the updraft was derived using a simplified form of the vertical momentum equation following a partial budget analysis that accounted for plume‐top entrainment and the boundary layer (BL) inversion. Velocity core strength and characteristics were analyzed away from the updraft with similar relationships between depth‐to‐widths of velocity cores found in a forthcoming study focused on wildfire plumes.

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Identifying Characteristics of Wildfire Towers and Troughs

Cited by 6 publications

References 28 publications

Investigating the turbulent dynamics of small-scale surface fires

Investigating the turbulent dynamics of small-scale surface fires

Investigating the turbulent dynamics of small-scale surface fires

A Case Study Featuring the Time Evolution of a Fire‐Induced Plume Jet Over the Rum Creek Fire: Mechanisms, Processes, and Dynamical Interplay

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