Physics-Based Fire Models Solve Combustion in Cells and Cost Hours per Minute
At the top of the model ladder sit simulators such as FIRETEC and the Wildland-Urban Interface Fire Dynamics Simulator (WFDS), which do not use a spread equation at all. They divide the air and the vegetation into three-dimensional cells and solve conservation of mass, momentum, energy and chemical species in each one, with fuel elements that heat, dry, pyrolyse and burn as computed rather than assumed. Spread is not prescribed; it emerges, along with the buoyant plume, the radiative preheating of fuel ahead of the front and the wind flowing through and around the canopy. Because they invent no fire-specific fitted coefficients, they can be applied to fuel arrangements no empirical model was ever calibrated for, such as a shrub cleared to a particular spacing or a house among trees. The price is stark: cells are typically a metre or two, so a simulation covers a few hundred metres for a few minutes of fire and runs for hours to days on a cluster. That budget is why these models are research and design tools, not forecast tools. After this Concept you can say what a physics-based fire model computes and what its resolution costs.
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At the top of the model ladder sit simulators such as FIRETEC and the Wildland-Urban Interface Fire Dynamics Simulator (WFDS), which do not use a spread equation at all. They divide the air and the vegetation into three-dimensional cells and solve conservation of mass, momentum, energy and chemical species in each one, with fuel elements that heat, dry, pyrolyse and burn as computed rather than assumed. Spread is not prescribed; it emerges, along with the buoyant plume, the radiative preheating of fuel ahead of the front and the wind flowing through and around the canopy. Because they invent no fire-specific fitted coefficients, they can be applied to fuel arrangements no empirical model was ever calibrated for, such as a shrub cleared to a particular spacing or a house among trees. The price is stark: cells are typically a metre or two, so a simulation covers a few hundred metres for a few minutes of fire and runs for hours to days on a cluster. That budget is why these models are research and design tools, not forecast tools. After this Concept you can say what a physics-based fire model computes and what its resolution costs.
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