Modelling wildland fire propagation by tracking random fronts
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Abstract. Wildland fire propagation is studied in the liter- ature by two alternative approaches, namely the reaction– diffusion equation and the level-set method. These two ap- proaches are considered alternatives to each other because the solution of the reaction–diffusion equation is generally a continuous smooth function that has an exponential de- cay, and it is not zero in an infinite domain, while the level- set method, which is a front tracking technique, generates a sharp function that is not zero inside a compact domain. However, these two approaches can indeed be considered complementary and reconciled. Turbulent hot-air transport and fire spotting are phenomena with a random nature and they are extremely important in wildland fire propagation. Consequently, the fire front gets a random character, too; hence, a tracking method for random fronts is needed. In par- ticular, the level-set contour is randomised here according to the probability density function of the interface particle dis- placement. Actually, when the level-set method is developed for tracking a front interface with a random motion, the re- sulting averaged process emerges to be governed by an evo- lution equation of the reaction–diffusion type. In this recon- ciled approach, the rate of spread of the fire keeps the same key and characterising role that is typical of the level-set ap- proach. The resulting model emerges to be suitable for sim- ulating effects due to turbulent convection, such as fire flank and backing fire, the faster fire spread being because of the actions by hot-air pre-heating and by ember landing, and also due to the fire overcoming a fire-break zone, which is a case not resolved by models based on the level-set method. More- over, from the proposed formulation, a correction follows for the formula of the rate of spread which is due to the mean jump length of firebrands in the downwind direction for the leeward sector of the fireline contour. The presented study constitutes a proof of concept, and it needs to be subjected to a future validation.