Extended Abstract (568K)7.2
Impacts of Fire/Atmosphere Coupling and Boundary–Layer Turbulence on Firebrand Propagation
Mary Ann Jenkins, York University, Toronto, ON, Canada; and S. Bhutia and R. Sun
Firebrand spotting is one of the most vexing problems associated with wildland fires,
challenging the lives and efforts of fire–fighting planners. This work is an effort to model
numerically the event of brand spotting for the purposes of reviewing past modelling approaches, demonstrating a new modelling approach, and recommending changes to the
current operational prediction of firebrand spotting. First, a simple, two–dimensional treatment of the process of brand lofting is examined under the restrictive conditions typical of
the traditional operational forecast model. Using this model, the differences in trajectories
of combusting and non–combusting particles are investigated. Next, the process of brand
lofting is examined using a coupled fire/atmosphere LES (Large Eddy Simulator). The
coupled LES resolves important features associated with flow in the Atmospheric Boundary
Layer (ABL) and produces a more realistic scenario of three-dimensional time–varying cou-
pled ABL–wildfire–induced circulations, which is not possible using the traditional simple
two-dimensional stationary fire plume model. With the coupled LES, the propagation of
combusting and non-combusting brands released from a moving grassfire in a convectively–
driven atmospheric boundary layer (CBL) domain are investigated. It is found that brand
propagation in the coupled LES simulated CBL flow is significantly different from that
obtained by the traditional two–dimensional empirically–derived plume model approach.
Contrary to common opinion, the results of this study indicate that a model of brand
propagation in a steady mean wind field does not predict maximum brand propagation dis-
tance. Fire spotting by brands is not deterministic in the ABL, a probabilistic method for
prediction of spot fires is warranted, and combustion has a significant impact on firebrand
propagation. The results are explorative and need to be subjected to direct testing.
Session 7, Coupled Model Development
Wednesday, 14 October 2009, 1:45 PM-3:00 PM, Ballroom B
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