End User representatives
Bushfires occur on a scale that may be measured in kilometres. However, a challenge faced in developing next generation bushfire models is to capture the significant contributions that small scale phenomena make to the spread of bushfires.
This project is applying physics-based approaches to fire scenarios. It attempts to simulate fire with unprecedented detail and in the process obtain useful application tools for end-users. To address existing gaps in the mathematical and computational modelling of bushfire dynamics, an ideal fire scenario is subdivided into four parts.
Modelling wind speed through tree canopies
The rate of spread of a bushfire depends both on fuel types and the wind velocity profiles at ground and tree canopy levels. Ground cover, tree trunks, branches and leaves all affect the velocity profile. This particular aspect of the project aims to understand the velocity profile within the tree canopy in order to predict the wind reduction factor, which is present in some empirical models of fire spread. This work will improve the modelling of wind-driven fire behaviour as it enters, traverses and leaves a wooded area. The project will develop a set of user-friendly tools to calculate wind reduction factor (WRF) and improved wind field generating software. WRF will be computed as a function of existing forest parameters and prevailing weather conditions and will assist fire behaviour analysts to utilise WRF to predict the rate-of-spread and intensity of a fire.
Spread and distribution of firebrands
Embers and firebrands carried ahead of the main fire front often dominate the rate of spread of bushfires. The team is harnessing its expertise in aerodynamics to design, construct and operate a firebrand generator to accurately quantify how embers disperse. Along with wind speed, bushfire spread rates strongly depend on the physical and chemical properties of vegetative materials, such as grasses, wood and leaves. To prepare for experiments using the generator, the team invested in equipment and training for measuring properties such as thermal conductivity, specific heat, density, heat of pyrolysis, heat of combustion and reaction rate constants. These studies will assist in understanding the propensity of grasses and litter fuels to ignite from firebrands.
Improving computational methods
Physics-based models of bushfires must consider phenomena that occur on length scales that range from a fraction of a millimetre (e.g. flame thickness) up to several hundred metres (e.g. in terrain). The researchers have addressed this challenge by considering how the average of the small-scale phenomena would affect large-scale phenomena, such as the length and intensity of flames.
Bushfire-driven airflow over surface features
This aspect of the study applies the principles of engineering science to calculate bushfire-generated airflows above buildings, structures and forests. The aim is to quantify the behaviour of airflow and heat transfer in order to calculate how the wind profiles above the surface features of variable heights changes. The approach is to calculate details of the flow and heat transfer to produce highly accurate solutions, from which simple-to-use equations are extracted for operational use.
Bushfires occur on a scale that may be measured in kilometers. However, a challenge faced in developing next generation bushfire models is to capture the significant contributions that small scale phenomena make to the propagation of bushfires.
Firebrands are burning pieces of, for example, bark, leaf litter, and twigs. Firebrands can be transported by wind from metres to kilometres from the head fire. Firebrands are responsible for causing spot fires during the spread of bushfire. Firebrands are the primary factor in house loss during bushfire.
A simple model of flow through a tree canopy and comparison with large-eddy simulations.
Operational fire models rely on wind reduction factors to relate the standard meteorological measured or forecast wind speed to the flame-height wind speeds within a tree canopy.
- Simulations of a fire entering, propagating under and leaving a tree canopy are conducted using FDS , a physics-based model.
- The presence of a tree canopy effects the wind speed, which in turn effects the rate-of-spread of a fire.
- From the simulated data we extract the average sub-canopy wind speeds in the absence of a fire and measure the rate-of-spread of a fire.
- This is the first step to testing the wind-reduction factor approach used in current operational models.
|Mapping bushfire hazard and impacts||Dr Marta Yebra||Australian National University|
|Fire surveillance and hazard mapping||Prof Simon Jones||RMIT University|
|Improved predictions of severe weather to reduce community impact||Dr Jeff Kepert||Bureau of Meteorology|
|Optimisation of fuel reduction burning regimes||A/Prof Tina Bell||University of Sydney|