Sensitivity of atypical lateral fire spread to wind and slope

Simpson, Colin C.; Sharples, Jason J.; Evans, Jason P.

doi:10.1002/2015gl067343

Cited by 30 publications

(28 citation statements)

References 15 publications

(28 reference statements)

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“…Figure 2a,b illustrates a schematic of this tilting and stretching process in a nonuniform buoyant velocity field where an eddy-generating mechanism exists. Readers are referred to Sharples et al (2015) and Simpson et al (2016) for theoretical details on the tilting and stretching of vortices during wildland fire scenarios. The converging flow in the vortex column of a fire whirl also concentrates existing vorticity, as shown in Figure 2c.…”

Section: Essential Conditions For Fire Whirl Formationmentioning

confidence: 99%

Fire Whirls

Tohidi

Gollner

Xiao

2018

Annu. Rev. Fluid Mech.

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Fire whirls present a powerful intensification of combustion, long studied in the fire research community because of the dangers they present during large urban and wildland fires. However, their destructive power has hidden many features of their formation, growth, and propagation. Therefore, most of what is known about fire whirls comes from scale modeling experiments in the laboratory. Both the methods of formation, which are dominated by wind and geometry, and the inner structure of the whirl, including velocity and temperature fields, have been studied at this scale. Quasi-steady fire whirls directly over a fuel source form the bulk of current experimental knowledge, although many other cases exist in nature. The structure of fire whirls has yet to be reliably measured at large scales; however, scaling laws have been relatively successful in modeling the conditions for formation from small to large scales. This review surveys the state of knowledge concerning the fluid dynamics of fire whirls, including the conditions for their formation, their structure, and the mechanisms that control their unique state. We highlight recent discoveries and survey potential avenues for future research, including using the properties of fire whirls for efficient remediation and energy generation.

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Section: Essential Conditions For Fire Whirl Formationmentioning

confidence: 99%

Fire Whirls

Tohidi

Gollner

Xiao

2018

Annu. Rev. Fluid Mech.

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“…Sharples et al (2012) initially expressed these thresholds in terms of a first-order wind-terrain filter, or binary variable χ, which assumes a value of 1 in regions prone to VLS occurrence and 0 elsewhere. While, the first-order filter identified the entire leeward slope as prone to VLS occurrence, subsequent laboratory experiments, wildfire observations and numerical simulations have revealed that the rapid lateral spread associated with VLS really only occurs in a relatively narrow portion of the leeward slope near the top of the hill (Quill and Sharples, 2015;Raposo et al, 2015;Simpson et al, 2016). This region could be better identified using a second-order VLS filter, based on the second-derivative of elevation, but for the idealised cases considered in this paper, a crude approximation will suffice.…”

Section: Vorticity-driven Lateral Spreadmentioning

confidence: 99%

“…This second dependence becomes critical when the spread of a wildfire is dominated by the dynamic modes of fire propagation, which arise in response to multi-scale interactions between the fire and the local atmosphere. Examples of these dynamic modes of fire propagation include eruptive fire spread (Viegas, 2006;Viegas and Pita, 2004) and vorticity-driven lateral spread (Sharples et al, 2012;Simpson et al, 2013Simpson et al, , 2014Simpson et al, , 2016.…”

Section: Introductionmentioning

confidence: 99%

Incorporating firebrands and spot fires into vorticity-driven wildfire behaviour models

2019

El Sawah, S. (Ed.) MODSIM2019, 23rd International Congress on Modelling and Simulation.

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Complex modes of fire behaviour resulting from local coupling between the fire and the atmosphere are a significant challenge for rapid operational wildfire spread simulations. While threedimensional fully coupled fire-atmosphere models are able to account for many types of fire behaviour, their computational demands are prohibitive in an operational context. Two-dimensional fire spread models have much lower computational overhead, but are generally not able to account for complex local coupling effects and cannot provide a three-dimensional flow structure suitable for modelling the transport of firebrands. In this paper we investigate extending two-dimensional fire spread simulations to model local coupling effects resulting from wind flow over a ridge that can result in a number of non-intuitive modes of fire behaviour. These include fire propagation opposite to the direction of the prevailing wind on the lee slope of ridges caused by re-circulation on the lee slope, called vorticity-driven lateral spread (VLS). Furthermore we develop extensions of these two-dimensional models to incorporate three-dimensional firebrand transport and show that enhanced downwind spot fire formation can result under certain VLS conditions. The spread of fires under VLS conditions is driven by vortices in the ground plane. A model for the production and effects of these vortices was incorporated into computational simulations using a vector potential formulation in similar manner to a scalar 'pyrogenic potential' model, detailed in earlier studies. Firebrands were incorporated using a Lagrangian scheme to model transport through the atmosphere and a sub-scale model for spot fire creation and growth. The firebrand transport took factors such as drag, gravity and buoyancy into account. As effect of plume buoyancy on firebrands under real-world conditions for this scenario is currently unknown, the plume buoyancy was parameterised using a exponential decay model. The sensitivity of the decay parameter in this model was then examined in relation to the resulting spot fire distribution and area burnt. All simulations were carried out using Spark, a wildfire prediction framework. The coupled VLS and firebrand transport simulations indicated that a higher value of decay parameter, representing a higher cooling rate of the plume, acted to enhance the lateral spread as firebrands were lofted for shorter times and were caught in the vortices at the edge of the lateral spread region. In contrast, a lower value of decay parameter, representing a lower cooling of the plume, resulted in widespread downwind spot fires and larger burnt areas. This appeared to be due to longer lofting times resulting in firebrands being transported further downwind and away from the vortices within the lateral spread region. The model appears, at least qualitatively, to match observed lateral spread and 'deep flaming' fire behaviour although many of the parameters in the model require further research and experimental calibration. Further development of the model ...

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“…Over the past 10 years, many studies have investigated the formation and impacts of pyroconvective processes and pyrocumulonimbus events (Fromm et al 2006(Fromm et al , 2008a(Fromm et al ,b, 2010Peterson et al, 2015Peterson et al, , 2017Dowdy et al 2017). Plume dynamics and wind modification are well known as a major source of uncertainty for modelling bushfire spread, and in particular knowledge gaps relating to coupled fire-atmospheric dynamics including the occurrence and growth of pyrocumulus (pyroCu) and pyrocumulonimbus (pyroCb) events (Peace et al 2011;Potter 2012;Simpson et al 2016). Despite the recent attention pyroCb cloud has received, little research has been able to relate the formation of pyroCu or pyroCb to wind field characteristics at the surface and aloft in observations (McRae et al 2015).…”

Section: Introduction and Summary Of Previous Studiesmentioning

confidence: 99%

The use of spectrum width radar data for bushfire model verification

McCarthy

Guyot

McGowan

et al. 2017

Syme, G., Hatton MacDonald, D., Fulton, B. And Piantadosi, J. (Eds) MODSIM2017, 22nd International Congress on Modelling and Si

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In wildland fires, the turbulent motions of air in the convective plume can significantly affect fire spread through processes such as spotting, local wind field modifications and the initiation of pyrocumulus and pyrocumulonimbus (i.e., cumuliform cloud types associated with fire activity). Radar has been used in a variety of studies to gauge the escalation of such processes. However, these studies have relied heavily on the radar moment of reflectivity despite a poor understanding of the nature of the particles producing the back-scatter. In the last decade, many models have emerged capable of simulation of fireatmosphere interactions. Validation of these models requires observations of the turbulent motions in plumes, but such observations from very large fires, including those that develop pyrocumulonimbus, are very rare. Doppler velocity can be used to estimate radial windspeeds, but specialised manual interpretation is required to characterise the coherent vortices, which offers little utility to modellers. Reflectivity and Doppler velocity therefore are limited in the modelling of to bushfire. The use of spectrum width, defined as the standard deviation of the Doppler velocity, has received no attention in the literature to date in relation to its potential to quantifiably validate modelling of fire-atmosphere interactions. This variable has long been known to act as a proxy for the sub-grid turbulence in radar sampling volumes. The Bushfire Convective Plume Experiment (BCPE) involved a 2.5 year field campaign to capture observations of pyroconvection primarily with a mobile X-band Doppler radar, to collect a range of different types of observed fields including spectrum width. This field campaign involved collaborating with agencies responding to wildfires through the bushfire season, as well as attending prescribed burning for bushfire hazard reduction. The mobile radar observations were supported by other field observations, including weather balloons to observe vertical wind and stability structure of the atmosphere, as well as rapiddeployable Automatic Weather Stations (AWS), time-lapse cameras and fire severity reconstructions. One particular finding from the BCPE relevant to the modelling community is associated with the Doppler velocity and spectrum width results. In effect, it was found that spatial approximations of the turbulence distributions within bushfire plumes can be produced, along with resolving the structures of large vortices within the plume through the two variables. Here we present a summary of some previous radar-based studies of plume dynamics, as well as observations from one of the BCPE fire cases, showing the spectrum width and Doppler analyses of the Mt Bolton bushfire (in Victoria, southeast Australia). The Mt Bolton fire event was captured in high resolution using a portable X-band radar in the course of the BCPE field campaign. The resolution allowed for the calculation of Plume Relative Winds (or PRW). These were calculated by subtracting an estimate of the advection in th...

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Sensitivity of atypical lateral fire spread to wind and slope

Cited by 30 publications

References 15 publications

Fire Whirls

Fire Whirls

Incorporating firebrands and spot fires into vorticity-driven wildfire behaviour models

The use of spectrum width radar data for bushfire model verification

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