@mastersthesis {bnh-6632, title = {Physics-based simulation of short-range spotting in wildfires}, year = {2020}, month = {01/2020}, school = {Victoria University}, address = {Melbourne}, abstract = {

Firebrands play a vital role in the propagation of fire fronts and starting new fires called spotfires ahead of fire fronts during wildfire progression. Firebrands are a harbinger of damage to infrastructure; their effects cause a particularly important threat to people living within the wildland-urban-interface, hampers the suppression of the wildfire or even blocking the evacuation routes for communities and emergency services. Short-range firebrands (\<750m) which travel along with the wind with little or no lofting are particularly crucial in increasing the fire front propagation and damaging structures situated closed to wildland-urban interface. In the Daylesford fire of 1962, massive short-range spotting (the process of spot fire ignition and merging of spots caused by firebrands) occurred in eucalyptus forest and increased the rate of fire spread by roughly three times more than the computed using empiricial correlation used by operational fire model. Despite the massive importance of short-range firebrands, little research has been conducted because of the safety risks and challenges of fire to emergency service personnel and to the remote equipment like collection boxes, IR cameras, UAVs, which could be used by researchers to quantify and measure fire properties. An operational model to represent the transport of short-range firebrand and their likelihood to ignite the surface fuel like forest litter could be developed from a numerical model. This study first attempts to validate a numerical model of firebrand transport with a set of benchmark experiments. The validation of numerical model is carried out using idealised regular shaped firebrand. Fire Dynamic Simulator (FDS) is an open-source Computational Fluid Dynamics (CFD) based fire model which is used in this study. The validation of the numerical model is split into two parts focusing on validation of (1) transport, and (2) ignition potential of firebrands. Transport of short range firebrands are modelled in FDS using a lagrangian particle sub-model. The model was validated using two firebrand generators (a plastic pipe-based prototype and stainless steel based main firebrand generator) constructed at our facility as a part of this study. The firebrand generator is equipment which generates a repeatable firebrand shower in a confined space. There are few firebrand dragons built around the world. However, our firebrand generators produce a uniform flow field which simplifies the transport of short-range firebrand to be validated. The set of experiments conducted is used to validate the Lagrangian particle model available in FDS used in the transport of short-range firebrands. The validation is carried out on cubiform, cylindrical, and square disc-shaped firebrands. As the default drag model in FDS was not suitable for shapes of firebrands, the drag model is improved to account for a generic shape of firebrand particle. The results show a reasonable agreement with the experiments for all three shapes over a range of particle Reynolds number. A set of laboratory scale equipment is used to study the ignition likelihood from a short-range firebrand in the numerical model. The boundary fuel vegetation model of FDS is validated. The pyrolysis of vegetation is first tested using thermogravimetric analyser and then with cone calorimeter to estimate mass loss rate, heat-release rate, and time to sustained flaming ignition of three forest litter (pine, eucalyptus, and hay) fuels. Further, a set of thermo-physical properties (thermal conductivity, heat capacity, the heat of pyrolysis, the heat of combustion) of the material tested are also measured using in-house equipment required in the above numerical model. The result showed that the simple linear pyrolysis model is good enough for different forest litter tested with thermogravimetric analyser and cone calorimeter. Finally, a parametric study of short-range firebrand transport inside an open woodland forest canopy is carried out using the validated Lagrangian particle sub-model. The work focuses on understanding how firebrand distribution varies with a set of variable firebrand characteristics in a wildfire and set a stepping stone for the future study. The results are found to be qualitatively similar to the literature.

}, keywords = {Fire, Firebrands, fluid dynamics, simulation, spotting, transport, Wildfire}, url = {http://vuir.vu.edu.au/40025/}, author = {Rahul Wadhwani} } @conference {bnh-6292, title = {Incorporating firebrands and spot fires into vorticity-driven wildfire behaviour models}, booktitle = {23rd International Congress on Modelling and Simulation}, year = {2019}, month = {12/2019}, abstract = {

Complex modes of fire behaviour resulting from local coupling between the fire and the atmosphere are a significant challenge for rapid operational wildfire spread simulations. While threedimensional fully coupled fire-atmosphere models are able to account for many types of fire behaviour, their computational demands are prohibitive in an operational context. Two-dimensional fire spread models have much lower computational overhead, but are generally not able to account for complex local coupling effects and cannot provide a three-dimensional flow structure suitable for modelling the transport of firebrands. In this paper we investigate extending two-dimensional fire spread simulations to model local coupling effects resulting from wind flow over a ridge that can result in a number of non-intuitive modes of fire behaviour. These include fire propagation opposite to the direction of the prevailing wind on the lee slope of ridges caused by re-circulation on the lee slope, called vorticity-driven lateral spread (VLS). Furthermore we develop extensions of these two-dimensional models to incorporate three-dimensional firebrand transport and show that enhanced downwind spot fire formation can result under certain VLS conditions.

The spread of fires under VLS conditions is driven by vortices in the ground plane. A model for the production and effects of these vortices was incorporated into computational simulations using a vector potential formu-lation in similar manner to a scalar {\textquoteleft}pyrogenic potential{\textquoteright} model, detailed in earlier studies. Firebrands were incorporated using a Lagrangian scheme to model transport through the atmosphere and a sub-scale model for spot fire creation and growth. The firebrand transport took factors such as drag, gravity and buoyancy into account. As effect of plume buoyancy on firebrands under real-world conditions for this scenario is currently unknown, the plume buoyancy was parameterised using a exponential decay model. The sensitivity of the decay parameter in this model was then examined in relation to the resulting spot fire distribution and area burnt. All simulations were carried out using Spark, a wildfire prediction framework.

The coupled VLS and firebrand transport simulations indicated that a higher value of decay parameter, rep-resenting a higher cooling rate of the plume, acted to enhance the lateral spread as firebrands were lofted for shorter times and were caught in the vortices at the edge of the lateral spread region. In contrast, a lower value of decay parameter, representing a lower cooling of the plume, resulted in widespread downwind spot fires and larger burnt areas. This appeared to be due to longer lofting times resulting in firebrands being transported further downwind and away from the vortices within the lateral spread region. The model appears, at least qualitatively, to match observed lateral spread and {\textquoteright}deep flaming{\textquoteright} fire behaviour although many of the parame-ters in the model require further research and experimental calibration. Further development of the model may allow these complex modes

}, keywords = {dynamic fire propagation, embers, Firebrands, spotting, wildfire simulation}, doi = {https://doi.org/10.36334/modsim.2019.H7.hilton2}, url = {https://mssanz.org.au/modsim2019/H7/hilton2.pdf}, author = {James Hilton and Garg, N and Jason J. Sharples} } @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.} }