@mastersthesis {bnh-6164, title = {Investigation of spotting and intrinsic fire dynamics using a coupled atmosphere-fire modelling framework}, volume = {Doctor of Philosophy}, year = {2019}, month = {10/2019}, school = {University of New South Wales}, type = {Doctorate}, address = {Sydney}, abstract = {

Large plume-driven wild res are among the most destructive and unpredictable of all natural hazards. A prerequisite for the development of the deep convection characteristic of these res is the existence of a large area of active aming, also known as deep aming. There are a number of processes associated with the development of deep aming; many involve some form of dynamic re behaviour, in which dramatic changes in re behaviour can occur with little or no change in ambient conditions. Another important driver of deep aming is intense spotting and spot- re coalescence, which itself involves dynamic re behaviour. It is difficult to model dynamic re behaviour in a computationally efficient way; it cannot be modelled with existing operational re-spread models. This thesis is concerned with the modelling of dynamic re behaviour, and the modelling of ember transport in turbulent plumes. A coupled atmosphere framework is used to model junction res (the merging of two separate relines at an acute angle), and the fundamental processes causing the asociated dynamic behaviour are identi ed. The idea that reline curvature can act as a proxy for some of the processes underlying dynamic re behaviour is critically examined, and rejected. A recently-developed simple coupled model, the pyrogenic-potential model, is discussed. It is found to produce results comparable with that of a coupled atmosphere\ model in simple test cases involving the ignition of res along circular arcs. The\ pyrogenic-potential model can capture some forms of dynamic behaviour, and is\ efficient enough to be used operationally.\ To study ember transport in turbulent plumes, a large eddy model is used to simulate\ the plume from a static heat source, and the resulting wind eld is used to\ model the transport of embers under various assumptions. It is shown that the\ terminal-velocity assumption, in which embers are assumed to always move at their\ terminal velocity with respect to the wind eld, leads to an overestimate of emberlanding\ densities at medium to long ranges. This has important implications for\ the stochastic modelling of spot- re development.

}, keywords = {bushfire dynamics, bushfires, Fire, fire modelling, framework, spotting}, author = {Thomas, C. M.} } @conference {bnh-5024, title = {Pyroconvective interactions and dynamic fire propagation}, booktitle = {AFAC18}, year = {2018}, month = {09/2018}, address = {Perth}, abstract = {

Modelling the dynamic propagation of wildfires remains a significant challenge. Pyroconvective interactions between the fire and the atmosphere, or between different parts of the fire itself, can produce distinctly non-steady modes of fire propagation that cannot be accounted for using current operational models.


While sophisticated three-dimensional models (e.g. computational fluid dynamics (CFD) models or coupled fire-atmosphere models) have been successfully applied to wildfires, their computational requirements render them impractical for operational usage.


Here we discuss a computationally efficient two-dimensional propagation model, which can accurately replicate dynamic features of fire spread that cannot be simulated using existing two-dimensional models. These features include the development of a wind-driven fire line into a parabolic shape, attraction between nearby fires and the observed closing behaviour of junction fires. The model is compared to experimental results with good agreement.
The model incorporates a simple sub-model to account for the inflow of air generated by a fire, which allows the model to run orders of magnitude faster than full physical models, while still capturing many of the essential features of dynamic fire propagation. We argue that such a model could lead to significant improvements in operational wildfire prediction.
In addition, we will highlight some recent insights in to how the geometry of a fire line and the flaming zone influences development of the pyroconvective plume above a fire. In particular, we present evidence that the geometry of the burning region can affect plume development in a way that is comparable to the effect of total energy release.

}, author = {James Hilton and Badlan, R and Sullivan, Andrew and Swedosh, W and Thomas, C. M. and Jason J. Sharples} } @article {bnh-3925, title = {Modelling the dynamic behaviour of junction fires with a coupled fire-atmosphere model}, journal = {International Journal of Wildland Fire}, volume = {26}, year = {2017}, month = {04/2017}, pages = {331-344}, chapter = {331}, abstract = {

Dynamic fire behaviour involves rapid changes in fire behaviour without significant changes in ambient conditions, and can compromise firefighter and community safety. Dynamic fire behaviour cannot be captured using spatial implementations of empirical fire-spread models predicated on the assumption of an equilibrium, or quasi-steady, rate of spread. In this study, a coupled atmosphere{\textendash}fire model is used to model the dynamic propagation of junction fires, i.e. when two firelines merge at an oblique angle. This involves very rapid initial rates of spread, even with no ambient wind. The simulations are in good qualitative agreement with a previous experimental study, and indicate that pyro-convective interaction between the fire and the atmosphere is the key mechanism driving the dynamic fire propagation. An examination of the vertical vorticity in the simulations, and its relationship to the fireline geometry, gives insight into this mechanism. Junction fires have been modelled previously using curvature-dependent rates of spread. In this study, however, although fireline geometry clearly influences rate of spread, no relationship is found between local fireline curvature and the simulated instantaneous local rate of spread. It is possible that such a relationship may be found at larger scales.

}, doi = {10.1071/WF16079}, url = {http://www.publish.csiro.au/wf/WF16079}, author = {Thomas, C. M. and Jason J. Sharples and Jason P. Evans} }