Kyng, Matthew (2025) Development of a simplified parameterized model of firebrand landing from physics-based modelling. PhD thesis, Victoria University.

Abstract

Bushfires pose a significant risk to lives and property, both within Australia and abroad. Firebrands (particles of hot or burning debris carried by air currents) have been identified as a particularly important aspect of how bushfires propagate and have been shown to cause a substantial proportion of structure fires at the wildland-urban interface. This research develops a parameterized model of firebrand landing distribution via analysis of simulated wildfires, using a physics-based model Fire Dynamics Simulator (FDS) which includes a low Mach number hydrodynamic model, a Large Eddy Simulation (LES) turbulence model, mixing controlled combustion model, heat transfer models, and a Lagrangian particle model. FDS has been experimentally validated as a close approximation of real firebrand transport behaviour at laboratory scale. By simulating the distribution of firebrands at field-scale under different physical circumstances and flame dimensions, we aim to develop an appropriate statistical model of firebrand behaviour based on these parameters. A diverse set of simulations under a variety of wind speeds, HRRs, flame dimensions (ratio of width to depth), and forest conditions have been completed and analysed. The results obtained from these simulations show that the shape of the distribution of firebrands throughout the domain varies significantly, with a distinct shift in the shape of the distribution occurring as the Width to Depth ratio (W:D ratio) of the fire increases. Two distinct patterns of firebrand distribution have been observed from these simulations. In fires with a width to depth ratio of 2 or greater, a characteristic bimodal distribution is produced. A region of low firebrand landing density lies along the centre line of the domain, with a region of high landing density on either side. This two-peaked shape begins to form a very short distance away from the edge of the fire and persists for several hundred meters. The peaks gradually become less defined and sometimes can be seen to merge together at very long distances. In fires with a W:D ratio of 1, there is usually only one clearly defined peak. A statistical model of firebrand landing has been developed that can capture this behaviour based on a set of probability density functions that can gradually shift between a single-peaked and a double-peaked shape. Trends in the parameters of the functions that describe the landing density in both the downwind and crosswind directions have been analysed. This model has been shown to produce a good quality of fit for the simulated data with a small number of input parameters. The physics of fluid motion and particle transport around the fire have also been investigated to provide an understanding of the model from a physics perspective.

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