@conference {bnh-8333, title = {Improvement of drag model for non-burning firebrand transport in Fire Dynamics Simulator }, booktitle = {24th International Congress on Modelling and Simulation}, year = {2021}, month = {12/2021}, address = {Sydney, NSW}, abstract = {

Firebrands play a crucial role in increasing the severity of wildfires by driving fire growth, damaging structures, and starting new fires. Predicting the transport of firebrands and their propensity to ignite new fires is of significant interest to fire communities. Developing an operational firebrand transport sub-model from the field studies is cumbersome, expensive, and has significant associated risks to equipment, community and firefighters. Physics-based models have the potential to assist in the development of such firebrand transport sub-models which can be utilised to improve the efficacy of existing operational fire models. The present study showcases one of the initial works carried out in the development of such a physics-based firebrand model. The work utilises Fire Dynamics Simulator (FDS), a commonly used open-source physicsbased fire model. The Lagrangian particle sub-model of FDS is used to simulate the transport of firebrand particles. The Lagrangian sub-model is generally used to model the transport of droplets and mist and has been extensively validated. However, the validation of this sub-model for the transport of solid particles such as firebrands is limited. The issue is exacerbated when particles are of a non-spherical shape and can undergo complex reactions over their transport such as burning. In this work, we utilise a firebrand generator prototype that produces a uniform Lagrangian shower of nonburning idealised firebrands. A set of in-house experiments are conducted to study the transport of three isometric shapes of non-burning firebrands i.e. cubiform, cylindrical and square-disc. These sets of experiments are used to quantify the efficacy of the inbuilt particle drag model of FDS and suggest potential alternative drag models that can be employed without loss of computational speed, major amendment in the fire model, are applicable to a wide range of particles shapes, and potentially improved prediction. In general, it is found that the suggested alternative Haider and Levenspiel drag model improves the estimation of firebrand distribution in terms of peak location, maximum and minimum longitudinal distribution with exception to cubiform particles for peak location. The exception is mainly due to inherent error association with the alternative drag model in overestimating the drag coefficient. For other situations, Haider and Levenspiel drag model shows either an improvement or stays the same. However, the study found that the existing point particle assumption to represent particles in FDS is not suited to estimate the lateral spread of firebrands especially when the secondary motion of a particle on its axis is involved such as cylindrical and square-disc particles. Our studies found, the lateral spread is found to be in the range of ~5-15\% thinner compared to its experimental width for cylindrical particle distribution. For the square disc, it is not possible to quantify such differences due to the computational limit associated with our present study. It can be qualitatively suggested that it is found to be more than cylindrical particles. A further set of experiments and their numerical validation is required to ascertain the above finding, especially with different sizes, isometric and non-isometric shape, the speed of firebrand particles and burning process to establish the efficacy of a particular drag model for firebrand transport.

}, keywords = {drag models, Fire Dynamics Simulator (FDS), firebrand generator, firebrand particles, Lagrangian particle}, url = {https://www.researchgate.net/profile/Rahul-Wadhwani/publication/356914453_Improvement_of_drag_model_for_non-burning_firebrand_transport_in_Fire_Dynamics_Simulator/links/61c2c9e28bb20101842b52be/Improvement-of-drag-model-for-non-burning-firebrand-transport}, author = {Rahul Wadhwani and Duncan Sutherland and Graham Thorpe and Khalid Moinuddin} } @article {bnh-3917, title = {Verification of a Lagrangian particle model for short-range firebrand transport}, journal = {Fire Safety Journal}, volume = {91}, year = {2017}, month = {07/2017}, pages = {776-783}, chapter = {776}, abstract = {

Firebrands are a harbinger of damage to infrastructure; their effects cause a particularly important threat to people living within the wildland-urban-interface. Short-range firebrands travel with the wind with little or no lofting, and cause spotfires. In this work, the design of a novel firebrand generator prototype is discussed to achieve a uniform shower of firebrands. The transport of short-range firebrand is studied to verify the existing Lagrangian particle model of Fire Dynamics Simulator. Uniform, non-combusting cubiform and cylindrical firebrands are projected using the firebrand generator. The experimentally observed distribution of particles on the ground is compared with a simulated distribution using the fire dynamic simulator. The results show that the existing Lagrangian model gives a good agreement with the experimental data.

}, doi = {10.1016/j.firesaf.2017.03.019}, url = {http://www.sciencedirect.com/science/article/pii/S0379711217301121}, author = {Rahul Wadhwani and Duncan Sutherland and Andrew Ooi and Khalid Moinuddin and Graham Thorpe} } @article {bnh-3049, title = {Fire spread prediction across fuel types: Annual project report 2015-2016}, number = {214}, year = {2016}, month = {09/2016}, institution = {Bushfire and Natural Hazards CRC}, address = {Melbourne}, abstract = {

Operational models that are used to predict fire behaviour can be implemented easily and rapidly. However, the operational models are only truly valid in the range of experimental conditions used to build the model. This leads to a number of difficulties when using the existing operational models to predict real-world wildfires. Physics-based modelling, that is simulating the fire behaviour from the basic equations of atmospheric fluid flow, combustion, and thermal degradation of fuel materials offers considerable insight into the dynamics of wildfire. However, physics-based simulations are computationally intensive and, at present, can only be applied to small, idealised cases. Nevertheless, the aim of this project is to use physics based models to gain insight into wildfire behaviour and use that insight to improve the current operational models for fire behaviour prediction. With this in mind, a number of investigations are underway.

The effect of a tree canopy on the near surface wind speed is investigated using Large Eddy Simulation with a view to modelling the wind reduction factor due to the canopy. The wind reduction factor is used in operational fire prediction models such as the McArthur model, to account for the reduction in wind velocity due to a tree canopy. A set of full three-dimensional simulations over idealised rectangular canopies, where the length and leaf area density of the canopy are varied, were conducted. The flow over the canopy is characterised and the potential effects on fire spread of complicated flow structures that develop at the leading and trailing edges are assessed. The simulated wind speed in the fully-developed canopy flow and the wind speed far from canopy region is used to assess the constant wind-reduction factor modelling approach.

The physics of firebrand, or ember, transport is not well understood. The distance and dispersion of the firebrands depends greatly on the turbulent fluid flow which transports the firebrands. The physics-based modelling of firebrand transport is at a preliminary stage. We seek to validate a Lagrangian particle approach for firebrand transport modelling by comparing the results from an experimental firebrand generator with simulations of same scenario. Three particle shapes, cubical, cylindrical, and disc shaped particles, representing idealised firebrands have been studied. Qualitative features of the landing distribution of the firebrands have been identified. Simulations are in progress to compare with the experimental results. So far the results are encouraging. In a related study, the thermos-kinetic properties of firebrand materials, such as bark, twigs, and leaves have been measured. This data will then be used for simulations of burning firebrands.\ 

Fires in grasslands are prevalent in Australia, and are relatively simple to model computationally due to the uniform fuel and flat simple terrain. In the present study, the CSIRO grassland experiments are used as validation cases for the physics-based simulations.\  A parametric study has been conducted where the background windspeed and the grass height have been varied independently.\  The rate-of-spread was found to be linear with windspeed in the parameter range considered. Two simulations were conducted at different heights and correspondingly different bulk density, representing grass which had been cut and left on the ground.\  The fire in the taller grassland was found to have a higher rate-of-spread. Three simulations were conducted where the bulk density was kept constant as height varied. In these simulations no systematic dependence on grass height was observed.

Direct Numerical Simulation (DNS) is a numerical technique to faithfully study fluid flows by resolving all the turbulent motions instead of resorting to modelling small scale turbulence. DNS provides great insight into the physics of flows but a limited to highly idealised and numerically tractable geometries such as channels. Nonetheless, such simulations can be used to gain insight into flows which have relevance to wildfire modelling. Three flows are studied: pressure driven flow over sinusoidal roughness, mixed convective flow (flow driven by both a temperature difference and a pressure gradient), and a buoyant line plume in a confined region.

In the future, the DNS work will contribute to improved turbulence and near-wall modelling used in the physics-based wildfire models. The results of simulations from physics-based wildfire models will in turn improve operational models. We eventually aim to produce models for the wind-reduction factor, improve knowledge of firespread in grasslands, and provide a model of firebrand transport, all of which can be used operationally.\ 

}, author = {Khalid Moinuddin and Duncan Sutherland and Graham Thorpe} } @article {bnh-1552, title = {Fire Spread Across Fuel Types Annual Report 2014}, year = {2015}, abstract = {

This is an annual report for the year 2013/14 for the Fire Spread Prediction Across Fuel Types project. The principal aim of the project is to develop an accurate physics-based mathematical model that is computationally efficient.

}, author = {Graham Thorpe} } @article {bnh-2335, title = {Fire spread prediction across fuel types: Annual project report 2014-2015}, number = {124}, year = {2015}, month = {02/11/2015}, institution = {Bushfire and Natural Hazards CRC}, address = {Melbourne}, abstract = {

The rate at which grass fires spread over flat terrain can be predicted with reasonable accuracy.\  However, estimating the rate of fire-spread through other vegetative types is often more problematic.\  In wooded areas, say, the localized wind speed is determined in part by ground cover, bushes, tree trunks and the forest canopy. This presents a significant challenge to the development of physical models of bushfires.

At first sight, forests appear to be quite random. However, the geometries of individual trees and forests share a remarkable similarity. Not only are the diameters of the branches of an individual tree distributed in fixed ratios, but the ratios of the tree trunks in a forest share the same distribution.\  This uniformity has important implications for the modelling of bushfires. For example, in this work, we have generated fractal geometries of trees, and these have been used to estimate the velocity profiles of the wind as it flows through a stand of trees. The self-similarity between individual trees and forests provides an exciting new intellectual framework in which to develop the next generation of bushfire models. \ Results obtained to date offer considerable promise.

The rate of spread of bushfires can be dominated by embers and firebrands that are conveyed ahead of a fire front. We need to understand how embers are generated and conveyed, and how they ultimately ignite fires. We have designed and constructed an ember generator to help elucidate these mechanisms. A feature of its design is that the firebrands, and the air that conveys them, exit the generator with velocities that are close to being uniform and this helps in our analysis. Results obtained to date clearly demonstrate the uniformity of the air velocity, and methods of measuring the dispersion of embers are underway.

If the rate of ignition of bushfires is to be modeled accurately we need to know the thermo-physical properties of vegetative materials. An extensive battery of experiments aimed at measuring the properties has been developed, and requirements for further testing will be established with end-users.

A conflict exists between modelling the physical details that govern the rate of spread of bushfires and the availability of computing power. As a result, one strand of our research into the next-generation models of bushfires is to develop improved computational methods. One such method has enabled us to model buoyancy-driven flows with great accuracy {\textendash} it is known as explicit filtering of the discretised Navier-Stokes equation. Having proven the concept, the next stage of our work is to apply it specifically to the rate of spread of bushfires.

Infrastructure must be not only bushfire-resistant, but also aesthetically pleasing and economical to build. To be truly creative, designers benefit from having access to a deep understanding of the mechanisms that determine the rate of heat transfer between a bushfire and structures. Hence, a further strand of our integrated research program aims to develop simple-to-use formulae that will help designers of infrastructure at the urban-bushfire interface.

}, issn = {124}, author = {Graham Thorpe} } @article {bnh-2393, title = {The spread of fires in landscapes}, year = {2015}, author = {Daniel Chung and Khalid Moinuddin and Andrew Ooi and Duncan Sutherland and Graham Thorpe and Rahul Wadhwani} } @conference {BF-4221, title = {Large Eddy Simulation of Flow over a Backward Facing Step using Fire Dynamics Simulator}, booktitle = {The 14th Asian Congress of Fluid Mechanics}, year = {2013}, abstract = {Flow over a backward-facing step is a widely used benchmark case in the field of Computational Fluid Dynamics (CFD). This paper presents the numerical simulation of backward-facing step using Fire Dynamics Simulator (FDS), open source software CFD developed by National Institute of Standards and Technology (NIST), US. Large Eddy Simulation (LES) is the default mode of its operation. In this paper, the latest version, FDS 6, is used for the numerical simulation of turbulent flow over a backward-facing step. This recent version of FDS incorporates four different eddy viscosity models namely a Constant Coefficient Smagorinsky model, a Dynamic Smagorinsky model, Deardroff{\textquoteright}s and Vreman{\textquoteright}s Models. The principal objective of this paper is to compare these turbulence models with a proposed benchmark case. Moreover, these simulated results are compared with standard experimental results of Jovic and Driver to assess the accuracies of the various models. }, author = {Sarwar, M and Khalid Moinuddin and Graham Thorpe} }