@article {bnh-8376, title = {Determining firebrand generation rate using physics-based modelling from experimental studies through inverse analysis}, journal = {Fire}, volume = {5}, year = {2022}, month = {01/2022}, abstract = {

Firebrand spotting is a potential threat to people and infrastructure, which is difficult to predict and becomes more significant when the size of a fire and intensity increases. To conduct realistic physics-based modeling with firebrand transport, the firebrand generation data such as numbers, size, and shape of the firebrands are needed. Broadly, the firebrand generation depends on atmospheric conditions, wind velocity and vegetation species. However, there is no experimental study that has considered all these factors although they are available separately in some experimental studies. Moreover, the experimental studies have firebrand collection data, not generation data. In this study, we have conducted a series of physics-based simulations on a trial-and-error basis to reproduce the experimental collection data, which is called an inverse analysis. Once the generation data was determined from the simulation, we applied the interpolation technique to calibrate the effects of wind velocity, relative humidity, and vegetation species. First, we simulated Douglas-fir (Pseudotsuga menziesii) tree-burning and quantified firebrand generation against the tree burning experiment conducted at the National Institute of Standards and Technology (NIST). Then, we applied the same technique to a prescribed forest fire experiment conducted in the Pinelands National Reserve (PNR) of New Jersey, the USA. The simulations were conducted with the experimental data of fuel load, humidity, temperature, and wind velocity to ensure that the field conditions are replicated in the experiments. The firebrand generation rate was found to be 3.22 pcs/MW/s (pcs-number of firebrands pieces) from the single tree burning and 4.18 pcs/MW/s in the forest fire model. This finding was complemented with the effects of wind, vegetation type, and fuel moisture content to quantify the firebrand generation rate.

}, keywords = {FDS, Firebrands, physics-based modeling, wildland fire, Wildland-urban interface}, doi = {https://doi.org/10.3390/fire5010006}, url = {https://www.mdpi.com/2571-6255/5/1/6}, author = {Amila Wickramasinghe and Nazmul Khan and Khalid Moinuddin} } @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} } @article {bnh-6037, title = {Particle tracking and detection software for firebrands characterization in wildland fires}, journal = {Fire Technology}, volume = {55}, year = {2018}, month = {12/2018}, pages = { 817-836}, abstract = {

Detection and analysis of the objects in a frame or a sequence of frames (video) can be used to solve a number of problems in various fields, including the field of fire behaviour and risk. A quantitative understanding of the short distance spotting dynamics, namely the firebrand density distribution within a distance from the fire front and how distinct fires coalesce in a highly turbulent environment, is still lacking. To address this, a custom software was developed in order to detect the location and the number of flying firebrands in a thermal image then determine the temperature and sizes of each firebrand. The software consists of two modules, the detector and the tracker. The detector determines the location of the firebrands in the frame, and the tracker compares the firebrand in different frames and determines the identification number of each firebrand. Comparison of the calculated results with the data obtained by the independent experts and experimental data showed that the maximum relative error does not exceed 12\% for the low and medium number of firebrands in the frame (less than 30) and software agrees well with experimental observations for firebrands \> 20 {\texttimes} 10-5\ m. It was found that fireline intensity below 12,590\ kW\ m-1\ does not change significantly 2D firebrand flux for firebrands bigger than 20 {\texttimes} 10-5\ m, while occasional crowning can increase the firebrand flux in several times. The developed software allowed us to analyse the thermograms obtained during the field experiments and to measure the velocities, sizes and temperatures of the firebrands. It will help to better understand of how the firebrands can ignite the surrounding fuel beds and could be an important tool in investigating fire propagation in communities.

}, keywords = {Firebrand detection, Firebrand tracking, Firebrands, Structural, Wildland}, doi = {https://doi.org/10.1007/s10694-018-0805-0}, url = {https://link.springer.com/article/10.1007/s10694-018-0805-0}, author = {Alex Filkov and Sergey Prohanov} }