Impacts of Fire/Atmosphere Coupling and Boundary–Layer Turbulence on Firebrand Propagation

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.