Abstract
A model is developed to explore fire–atmosphere interactions due to the convective sink and vorticity sources in a highly simplified and idealized form, in order to examine their effect on spread and the stability of various fire front geometries. The model is constructed in a cellular automata framework, is linear, and represents a background flow, convective sink, and vortices induced by the fire plume at every burning cell. We use standard techniques to solve the resulting Poisson equations with careful attention to the boundary conditions. A modified Bresenham algorithm is developed to represent convection. The three basic flow types—large-scale background flow, sink flow, and vortex circulation—interact in a complex fashion as the geometry of the fire evolves. Fire-generated vortex–sink interactions produce a range of fire behavior, including unsteady spread rate, lateral spreading, and dynamic fingering. In this simplified framework, pulsation is found associated with evolving fire-line width, a fire-front acceleration in junction fires, and the breakup of longer initial fire lines into multiple head fires. Fuel is very simply represented by a single burn time parameter. The model fuel is uniform yet patchiness occurs due to a dynamic interaction of diffusive and convective effects. The interplay of fire-induced wind and the geometry of the fire front depends also on the fuel burn time.
Highlights
Sophisticated computational fluid dynamics and combustion models represent a myriad of effects and feedbacks inherent in the complex problem of wildland fire spread.Chemical and thermodynamic combustion processes, fuel structure and composition, terrain, and atmospheric boundary layer dynamics and thermodynamics are all ingredients to such models, depending on the situation being investigated
We have developed a simplified model to explore fire–atmosphere interactions, and their effect on spread
The model couples the fire and atmosphere by parameterizing a fire-induced velocity field that includes both convective sinks and vortices. This velocity field is coupled to the fire spread by using a modification of Bresenham’s line algorithm
Summary
Sophisticated computational fluid dynamics and combustion models represent a myriad of effects and feedbacks inherent in the complex problem of wildland fire spread. Chemical and thermodynamic combustion processes, fuel structure and composition, terrain, and atmospheric boundary layer dynamics and thermodynamics are all ingredients to such models, depending on the situation being investigated. Canopy flow is recognized as a turbulent mixing layer with large eddies controlling vertical fluxes [1]; small fires within a canopy are subject to this control to a large degree, while more intense fires develop their own boundary layer flow. For instance, a grass fuel layer with no canopy, a different set of boundary layer turbulent structures have been used to describe interactions between boundary layer flow and fire spread [2]. A common thread is the convergence and divergence effects of buoyant convection and large eddies on nearsurface flow. Near-field turbulence due to buoyant plume effects develops from Kelvin–
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