LES analysis on the effects of fire source asymmetry on enhanced wind by fire

Investigation of aerodynamic characteristics of wind enhanced by bushfires is of great significance due to their destructive impacts on buildings located in bushfire-prone areas. Despite the abundance of studies in the fire-wind interaction domain, there have been limited studies concerning the effects of fire on wind aerodynamics. Fire source shape is one of the main factors affecting enhanced wind by fire. This study reports on the effects of fire source asymmetry on aerodynamic changes of wind by fire using a large eddy simulation analysis based on fireFoam solver of OpenFOAM platform. Wind aerodynamic analysis was performed by implementing a module to the solver to extract the corresponding components of fire-induced pressure gradient and acceleration. The results revealed that deviation from fire source symmetry results in asymmetric behaviour of counter-rotating vortices where the maximum cross-sectional wind enhancement occurs. Moreover, the concept of the first-moment area was used to quantify the level of fire source deviation from symmetry and it was shown that the higher first-moment area (about the equivalent symmetry axis) corresponds to a higher deviation from symmetry which delays the realignment of counter-rotating vortices toward the horizontal vortex line

Investigation of buoyant plume wind enhancement

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

Data mining analysis of an urban tunnel pressure drop based on CFD data

An accurate estimation of pressure drop due to vehicles inside an urban tunnel plays a pivotal role in tunnel ventilation issue. The main aim of the present study is to utilize computational intelligence technique for predicting pressure drop due to cars in traffic congestion in urban tunnels. A supervised feed forward back propagation neural network is utilized to estimate this pressure drop. The performance of the proposed network structure is examined on the dataset achieved from Computational Fluid Dynamic (CFD) simulation. The input data includes 2 variables, tunnel velocity and tunnel length, which are to be imported to the corresponding algorithm in order to predict presure drop. 10-fold Cross validation technique is utilized for three data mining methods, namely: multi-layer perceptron algorithm, support vector machine regression, and linear regression. A comparison is to be made to show the most accurate results. Simulation results illustrate that the Multi-layer perceptron algorithm is able to accurately estimate the pressure drop

LES analysis on the effects of baroclinic generation of vorticity on fire-wind enhancement

Substantial influence of fire flame on wind flow aerodynamic characteristics lead to enhancement of wind velocity that may cause considerable damages to buildings in bushfire-prone areas. Fire is associated with rotational vortical structures ranging from the small scales, sourced by baroclinic generation of vorticity (ωBGV), to large scale vortices arising from amalgamation mechanisms that can significantly affect wind flow characteristics during fire-wind interaction. This study aims to understand the extent to which baroclinic generation of vorticity affects wind enhanced by fire. Large eddy simulations (LES) of fire-wind interaction are conducted using fireFOAM solver of OpenFOAM platform for two different types of fuels with the aim to produce two different scenarios with similar heat release rate, but different flame temperatures. This will guarantee the production of different vortex structures in the two cases while flow expansion rate, which is proportional to the fire heat release rate, remains constant. FireFOAM solver was modified to extract fire-induced acceleration and vorticity components. The LES results show that under similar fire intensity and heat release rate conditions, wind enhancement is higher in the scenario with higher flame temperature along the centreline as well as the cross-sectional locations which are corresponding to the locations of counter-rotating vortices where the maximum wind enhancement appears. It was shown that in the case with higher flame temperature, baroclinic generation of vorticity is stronger, which causes stronger longitudinal fire-induced pressure gradient and consequently results in a higher wind enhancement in these cross-sectional locations. The distribution of maximum cross-sectional baroclinic generation of vorticity along longitudinal location are presented, confirming in both cases, baroclinic generation of vorticity reduces with an increase of distance from the fire source. However, in almost all distances, the scenario with a higher flame temperature generates stronger baroclinic generation of vorticity, so does the fire-induced pressure gradient and flow enhancement

LES analysis of fire source aspect ratio effects on fire-wind enhancement

Enhancement of wind by bushfire, referred to as bushfire-wind enhancement phenomenon, causes damages to buildings located in bushfire-prone areas by increasing pressure load around the structures. This study focuses on the effects of point source aspect ratio (AR) on the wind enhanced by fire. FireFOAM solver of OpenFOAM platform is used to perform Large Eddy Simulation analysis for different fire source aspect ratios under two different fire source conditions: (i) identical fire intensity (fire heat release rate per unit area) and (ii) identical fire heat release rate conditions. Simulations were performed for three different fire source aspect ratios under these fire source boundary conditions. An appropriate normalization group based on fire source hydraulic diameter was introduced for fire-induced pressure gradient to explain the variation of wind enhancement with fire source aspect ratio. The results reveal that under a constant fire intensity condition, increasing the fire source aspect ratio causes a higher normalized fire-induced pressure gradient which leads to more intensified wind enhancement. In contrast, the increase of fire source aspect ratio while fire heat release rate is kept constant culminates in a reduction in the normalized fire-induced pressure gradient, reducing wind enhancement. Moreover, with the increase of the fire source aspect ratio, the area of counter-rotating vortices (CRV) where maximum wind enhancement occurs is expanded. The results also show that with the increase of fire source aspect ratio, the length of flame attachment to the ground immediately downstream of fire increases. In addition to the longitudinal wind enhancement, the effects of fire source aspect ratio on vertical velocity were also analyzed based on the Richardson number defined by hydraulic diameter and flow reference velocity. The effects of the aspect ratio on flame length were also studied. It was shown as a result of the increase of aspect ratio for one unit, flame length increases by approximately 14% and reduces by 7% under constant fire intensity and constant fire heat release rate condition, respectively

Investigation of fire-driven cross-wind velocity enhancement

Understanding the aerodynamics associated with the interaction of fire and cross-wind flow is of great importance because the consequence may have major implications in building design against bushfire (or wildland fire) attacks. However, a fundamental understanding of how the interaction of fire and wind can alter free stream flow aerodynamic properties has remained elusive. The scope of this study is to examine the pool fire and wind interaction under fixed wind velocity condition. This study dissects the fundamental mechanisms of how the interaction of horizontal momentum flow with a vertical buoyant plume leads to enhancement of wind velocity in the horizontal direction at a certain elevation. Changes in flow aerodynamics caused by the interaction of fire and wind were analysed using the computational fluid dynamics approach. The mechanisms causing the changes were explained. A module was developed and added to the FireFOAM solver to evaluate flow acceleration due to the pressure gradient, gravity, and viscous effects. The chosen computational model was validated against two sets of experimental data, namely, a buoyant diffusion fire plume in still air and the other in cross-wind condition. The numerical simulation revealed that due to the interaction of fire and wind, there is a negative longitudinal pressure gradient across the plume axis, causing the flow to accelerate and the velocity profile to alter. It was also shown that the distortion in velocity profile depends on the location downstream of the fire plume. The height of the distortion increases whilst the magnitude of the distortion diminishes as the longitudinal distance from the fire source increases. Investigation of the effects of heat release rate on wind enhancement further showed that fire with a higher heat release rate causes a greater pressure gradient and a lower density, culminating in higher flow acceleration and consequently increase of wind enhancement

LES simulation of terrain slope effects on wind enhancement by a point source fire

Fire-driven flows associated with wind intervention can dangerously threaten buildings in bushfire-prone areas by increasing pressure load on the structures through fire-wind enhancement phenomenon. This phenomenon through which wind is enhanced by interacting with fire is exacerbated when the affected terrain is located in a positive slope area. This study employs LES simulations using FireFOAM platform to investigate the extent to which the wind enhanced by a point source of fire is affected by terrain slope. A module was appended to the FireFOAM solver to extract and output fire-induced forces and acceleration components for the analysis. The effects of terrain slope on wind velocity enhancement as well as the location at which local maximum wind enhancement occurs were studied. The LES results showed that with the increase of terrain upslope angle, wind enhancement along the centerline is significantly intensified, whereas local maximum wind enhancement that occurs at each side of the centerline is less affected. It was also shown that global maximum wind enhancement occurs immediately downstream of the fire source for all upslope angles. Moreover, similar to the local maximum wind enhancement, the rate of increase in global maximum wind enhancement reduces with the increase of terrain upslope angle

CFD investigation of cross-flow effects on fire-wind enhancement

Investigation of fire-wind (fire cross-wind) interaction is highly instrumental in dissecting the potential effects of bushfire attacks on buildings. The increase of free-stream wind velocity downstream of the fire source due to the interaction of wind and fire is referred as fire-wind enhancement which has been recognized as one of the destructive consequences of bushfirewind interaction. Although occurrence of the fire-wind enhancement phenomenon has been reported in previous studies, the mechanisms and contributing factors affecting the phenomenon have not been reported in the literature. This study applies Computational Fluid Dynamics (CFD) technique to fundamentally investigate the effects of cross-wind on fire-wind enhancement. Fire-FOAM solver which is based on OpenFOAM platform was used to solve thermo-fluid governing equations. A module has been added to the solver to extract different components of flow acceleration and the corresponding fireinduced flow momentum. Experimental data of buoyant diffusion flame was used to validate the numerical model. A selected range of simulation scenarios with different free-stream wind velocities under constant fire intensity has been performed to identify the effects of free-stream wind velocity on fire-wind enhancement. The outcome of the research indicated that as a result of interaction of cross-wind and fire, a longitudinal (horizontal) favorable pressure gradient is generated which leads to enhancement of wind downstream of the fire. It was also shown that the normalized fire-induced pressure gradient decreases when free-stream wind velocity increases. Therefore, for constant fire intensity, the flow field with a higher free-stream wind velocity undergoes a lower enhancement

Correlations for fire-wind enhancement flow characteristics based on LES simulations

Unraveling the physics of fire-wind interaction has long been a subject of interest. Among all the physics involved, enhancement of wind by fire deserves great attention due to its potential effects on building structures downstream of the fire source in bushfire attack events. Predominantly, two contributing factors determine the extent to which wind is enhanced by fire: freestream wind velocity and fire intensity. This study employs Large-Eddy Simulation (LES) to fundamentally investigate the combined effects of freestream wind velocity and fire intensity on fire-wind enhancement. An added module was implemented to an open-source transient fire solver in order to analyze the effects of freestream wind velocity and fire intensity based on the analysis of interactions between momentum and fire-induced buoyancy forces. Simulations are performed for parametric combinations of wind velocity and fire intensity. The LES results demonstrate that the normalized maximum wind enhancement increases with a reduction of freestream wind velocity and an increase in fire intensity. The non-dimensional Froude number, Fr, and normalized fire intensity, I*, were employed to quantify the effects of freestream wind velocity and fire intensity, respectively. A correlation was developed to determine the maximum wind enhancement as a function of Fr and I*. The location corresponding to maximum wind enhancement occurs further downstream of the fire source as freestream wind velocity or fire intensity increases. A correlation based on the Fr number and I* was developed for the location at which maximum wind enhancement occurs. 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, for which a correlation was also developed. Moreover, a gradual decaying trend is observed in wind enhancement after reaching a peak along the wind enhancement plume line in all simulation scenarios for which a correlation was also developed as a function normalized longitudinal direction

Numerical simulation of the effect of terrain slope on fire-wind enhancement

Fire-wind enhancement is a phenomenon associated with the increase of free-stream wind velocity due to the interaction of fire and free stream flow. The potential adverse effects of enhanced wind on the pressure loads around buildings highlight the necessity of investigating the phenomenon. The interaction of wind and fire has long been a subject of interest however, investigation of the factors affecting the phenomenon has not received due attention. One of the factors that affect this phenomenon is terrain slope. This paper used a Computational Fluid Dynamic solver called FireFoam to evaluate the effect of terrain slope on fire-wind enhancement. The results revealed that the enhancement of wind velocity due to fire increases with an increase in terrain slope. This is because fire-wind interaction leads to the generation of a favourable longitudinal pressure gradient whose magnitude increases with the increase of terrain slope