Investigation of buoyant plume wind enhancement

Abstract

Bushfires are a natural disaster that has a devasting effect on nature and mankind. The vulnerability of buildings to bushfires has caused enormous loss of property and in extreme conditions, loss of life. It is well known that bushfires invade building structures via three mechanisms, namely embers, thermal radiation, and flame contact. Based on recent bushfire field surveys and numerical simulations, bushfire enhanced wind has also been identified to be a major contributor to building damage. Wind enhancement by bushfires can have a destructive impact on buildings arising from the increasing pressure load on structures downstream of the bushfire front as well as the increasing velocity of embers carried by wind during bushfire attacks. However, the mechanisms involved in this phenomenon are not yet fully understood. This study aims to (1) fundamentally understand the interaction of longitudinal wind velocity with vertical buoyant plume that leads to enhancement of wind velocity downstream of the buoyant source; (2) quantify the effects of fire intensity, wind velocity, terrain slope, and different fire sources on wind enhancement by fire; and (3) develop correlations between the enhanced wind flow characteristics and these contributing factors. This study used FireFOAM, an open-source computational fluid dynamics solver, to numerically solve thermo-fluid governing equations based on Large Eddy Simulation (LES). A module has been developed and implemented within the FireFOAM solver to compute and extract the identified parameters to help explain the phenomenon of wind enhancement by fire. To determine the effects of each contributing factor, the stepwise method in which one parameter is subjected to change while the others are maintained constant was used. The numerical model was validated against two sets of experimental data, namely, a buoyant diffusion fire plume in still air and a buoyant diffusion fire plume in cross-wind conditions. The reliability of the FireFOAM LES was checked by LES uncertainty analysis which includes the resolved fraction of the kinetic energy of turbulence, the ratio of the grid spacing to the Kolmogorov scale, and turbulent spectra at characteristic locations. The numerical analysis commenced with simulation of the interaction of wind and a dimensionally finite source of fire, called a point source fire. Results revealed that when wind interacts with fire, a longitudinal negative pressure gradient is generated within the fire plume region downstream of the fire source where the flow density is lower than that of ambient condition. This fire-induced pressure gradient causes flow acceleration and consequently results in enhancement of wind in longitudinal direction (parallel to the wind direction). The results generated in this thesis substantiated that this generation of the fire-induced pressure gradient is the main reason why wind enhancement occurs during fire-wind interaction. It was also found that with the increase of fire intensity corresponding to the fire heat release rate per unit area for a point source fire, the fire-induced pressure gradient and consequently wind enhancement increases. In addition to the impacts of fire intensity, the effects of free-stream wind velocity on the enhancement of wind by fire were also studied. To this end, a number of simulations were performed under constant point source fire intensity but different free-stream wind velocities. An appropriate normalization approach was developed based on the free-stream dynamic pressure. Consequently, the fire-induced pressure gradient was normalized to describe the effects of free-stream wind velocity on wind enhancement by fire. Results showed that with an increase of free-stream wind velocity under constant fire intensity, the normalized fire-induced pressure gradient decreases, which causes a comparative reduction in wind enhancement by fire. The effect of fire source configuration on wind enhancement by fire is another parameter studied in this thesis. The width of the bushfire front can be assumed as infinite and as such, can be treated as a line fire source. Hence the computational domain approximates a truncated section of an infinitely wide bushfire front. A study was carried out to compare wind enhancements by fires of point and line sources. Simulations were performed under the same free-stream wind velocity and fire heat release rate per unit area for both line and point source fires. It was found that the longitudinal fire-induced pressure force induced by a line fire source is much greater, hence resulting in a stronger wind enhancement, than a point source. Vertical flow distribution analysis was also performed for the two simulated cases. The results reveal that in contrast to the longitudinal flow enhancement, vertical flow enhancement by a point fire source is higher than that for a line fire source. This finding is attributed to the more intensified vertical fire-induced pressure gradient and buoyancy forces in the point source configuration than the line source case. Developing correlations for wind enhancement by fire based on the main contributing factors corresponding to fire intensity and wind velocity is one of the main practical findings of this research study. In this regard, a series of simulations with different combinations of free-stream wind velocity and line fire intensity was performed to develop correlations for wind enhancement. Two relevant non-dimensional groups, namely, Froude number and normalized fire intensity, were utilized to respectively quantify the impacts of free-stream wind velocity and fire intensity on wind enhancement. A correlation was developed to determine the maximum wind enhancement and the corresponding location as a function of Froude number and normalized fire intensity. Furthermore, the concept of wind enhancement plume line was defined as a line along which the local wind enhancement occurs at a given longitudinal location downstream of the fire source. A correlation was also developed for this case. It was also found that after wind hits the maximum value at a certain location downstream of the fire source, it undergoes a gradual decay along the wind enhancement plume line for which a correlation was also developed as a function of normalized longitudinal direction. In this thesis, the effect of terrain slope on wind enhancement caused by a line source fire has been presented. A number of simulation scenarios were performed for practical values of terrain upslope and downslope. It was observed that upslope terrain intensifies wind enhancement whereas downslope terrain reduces wind enhancement. The simulation results revealed that in upslope terrain cases, the buoyancy force component parallel to the sloped surface amplifies the fire-induced pressure force and consequently intensifies wind flow. However, in the downslope cases, the component of buoyancy parallel to the sloped surface opposes the wind flow and consequently mitigates the wind velocity. It was also found that a steeper gradient in upslope and downslope terrain respectively causes an increase and a reduction in wind enhancement by fire. In summary, this research provides a fundamental explanation for enhancement of horizontal wind with a vertical buoyant plume by the development of a theoretical framework based on fire-induced force and acceleration analysis. The developed fire-induced force analysis and acceleration theory were employed and the effects of wind velocity, fire intensity, fire-source configuration, and terrain slope on the enhanced wind by fire were studied. Trends between the studied contributing factors were analyzed and correlations were developed for fire-wind enhancement flow characteristics.

Date of Award	2019
Original language	English