Wildland-urban Interface Fire Dynamics Simulator Research Articles

Physics-based simulations of grassfire propagation on sloped terrain at field scale: motivations, model reliability, rate of spread and fire intensity†

This study focuses on physics-based modelling of grassfire behaviour over flat and sloped terrains through a set of field-scale simulations performed using the Wildland–urban Interface Fire Dynamics Simulator (WFDS), with varying wind speeds (12.5, 6 and 3 m s−1) and slope angles (−30° to +30°). To ensure the accuracy of this Large Eddy Simulation (LES), a sensitivity study was carried out to select the converged domain and grid sizes. Fire isochrones, locations of fire front, dynamic and quasi-steady rates of spread (RoS), and fire intensity results from the simulations are presented. Within the simulations conducted, the RoS and fire intensity were found to be higher with increasing slope angles, as well as with wind velocity. RoS comparisons are made with various empirical models. At different slope angles and driving wind velocities, different empirical quasi-steady RoS broadly match with particular dynamic maximum, minimum and averaged RoS values from this study. It appears that the ideal nature of grassfire propagation simulation and challenges related to measuring quasi-steady values in experimental studies are likely reasons for the observed differences. Additionally, for lower wind velocities, the RoS–fire intensity relationship (Byram’s) deviates from linearity for greater upslopes.

Modeling with WFDS Combustion Dynamics of Ornamental Vegetation Structures at WUI: Focus on the Burning of a Hedge at Laboratory Scale

ABSTRACT This study is part of an ongoing project on modeling vegetation fires and their impacts on dwellings at wildland-urban interfaces. As a first step, fire tests were conducted at laboratory scale on both wooden wool litter and rockrose non-natural hedge. To this end, a 0.5 m x 0.5 m x 1 m hedge reconstituted from rockrose branches was burned under a Large Scale Heat Release (LSHR) apparatus. A litter of wooden wool was used before and under the hedge to mimic an ignition from an underneath burning herbaceous layer. These fire tests provided experimental data for mass loss and heat release rate, which were used as a basis for comparison with the simulations performed by using the Wildland-urban interface Fire Dynamics Simulator (WFDS). To numerically model the rockrose hedge, the distribution of the different particle classes (leaves, twigs of various diameters) was determined as well as their physicochemical properties. The comparison of the experimental data and predicted quantities showed a good agreement, whether for wooden wool burnings alone or mixed wooden wool and rockrose hedge.

Analysis of Crown Fire Transition and Spread over Various Pine Trees Using Wildland–Urban Interface Fire Dynamic Simulator

The crown fire of various pine trees was investigated using a wildland–urban interface fire dynamics simulator (WFDS). The effects of wind speeds and the spatial distances between fuels on crown fire ignition and spread were investigated. The average 30-year values of atmospheric conditions in March and April were used as the reference conditions to represent the climatic conditions for the wildfire season. As the wind speed increases, crown fire initiation is promoted, and the intensity and spread rate of the crown fire increase. The effects of the spatial distance on the crown fire depend on the wind speed and fuel conditions. The results show that a computational fluid dynamics tool using physics-based models, such as the WFDS, can predict the crown fire ignition and spread behaviors for domestic pine trees. However, further studies are required for other vegetation and domestic atmospheric conditions to validate the applicability of the WFDS on domestic fuels.

Multifidelity prediction in wildfire spread simulation: Modeling, uncertainty quantification and sensitivity analysis

Wildfire behavior predictions typically suffer from significant uncertainty. However, wildfire modeling uncertainties remain largely unquantified in the literature, mainly due to computing constraints. New multifidelity techniques provide a promising opportunity to overcome these limitations. Therefore, this paper explores the applicability of multifidelity approaches to wildland fire spread prediction problems. Using a canonical simulation scenario, we assessed the performance of control variates Monte-Carlo (MC) and multilevel MC strategies, achieving speedups of up to 100x in comparison to a standard MC method. This improvement was leveraged to quantify aleatoric uncertainties and analyze the sensitivity of the fire rate of spread (RoS) to weather and fuel parameters using a full-physics fire model, namely the Wildland-Urban Interface Fire Dynamics Simulator (WFDS), at an affordable computation cost. The proposed methodology may also be used to analyze uncertainty in other relevant fire behavior metrics such as heat transfer, fuel consumption and smoke production indicators.

Fire behavior in multiple burning shrubs separated horizontally and vertically

Fire interactions between multiple 1 m tall, 0.7 m diameter chamise shrubs was studied utilizing the Wildland-Urban Interface Fire Dynamics Simulator (WFDS, Mell et al., 2009). Two shrub arrangements were investigated. First, nine shrubs were placed in a 3×3 horizontal region. The shrub separation distance and wind speed were varied. Two competing interaction mechanisms were identified: heat feedback enhancement, primarily due to thermal radiative heating, and air entrainment restriction. Shrub burning characteristics were examined and a global average burning rate was analyzed. For the no wind condition, the peak mass loss rate of the center shrub is 23% higher than the others, indicating that heat feedback enhancement is dominant. Air entrainment causes the surrounding shrubs to burn less intensely. At an imposed wind speed, air entrainment effects are dominant. Shrubs that are best shielded from the wind burn most intensely. Second, the vertical separation between two shrubs was varied under different wind conditions. With no ambient wind, nearly no interaction between the two shrubs was observed. At a wind speed of 1 m/s, significant interaction between shrubs occurred due to flame-tilting. The downwind shrubs burned the most vigorously for vertical separation distances between 0.2 and 0.8 m.

Fire spread upslope: Numerical simulation of laboratory experiments

Numerical simulations of laboratory-scale experiments, with no wind imposed, were performed for fuel bed slopes ranging from 0° to 45°. The implementation of a vertical symmetry plane (SP) placed, span-wise, along the middle of the computational domain, was assessed as an approach to reduce computational cost. The simulations were performed with Wildland-urban interface Fire Dynamics Simulator (WFDS). The experimental trends of the compared quantities were reproduced well by the simulations. Radiative heat transfer is the dominant mechanism of heating for slope angles between 0° and 22°, and convection heat transfer mechanism starts to be relevant and becomes more important than radiation for slopes of 31° and 45°. Similar to other studies, it was found a “critical” angle of ≈22o. For slopes angles greater than the critical value, there are significant changes in the wind conditions at the base of the flame which result in a more rapid increase in the rate of spread with increasing slope.

Examination of WFDS in Modeling Spreading Fires in a Furniture Calorimeter

Validation of physics-based models of fire behavior requires comparing systematically and objectively simulated results and experimental observations in different scenarios, conditions and scales. Heat Release Rate (HRR) is a key parameter for understanding combustion processes in vegetation fires and a main output data of physics-based models. This paper addresses the validation of the Wildland-urban interface Fire Dynamics Simulator (WFDS) through the comparison of predicted and measured values of HRR from spreading fires in a furniture calorimeter. Experimental fuel beds were made up of Pinus pinaster needles and three different fuel loadings (i.e. 0.6, 0.9 and 1.2 kg/m2) were tested under no-slope and up-slope conditions (20°). An Arrhenius type model for solid-phase degradation including char oxidation was implemented in WFDS. To ensure the same experimental and numerical conditions, sensitivity analyses were carried out in order to determine the grid resolution to capture the flow dynamics within the hood of the experimental device and to assess the grid resolution’s influence on the outputs of the model. The comparison of experimental and predicted HRR values showed that WFDS calculates accurately the mean HRR values during the steady-state of fire propagation. It also reproduces correctly the duration of the flaming combustion phase, which is directly tied to the fire rate of spread.

Spatially explicit measurements of forest structure and fire behavior following restoration treatments in dry forests

Restoration treatments in dry forests of the western US often attempt silvicultural practices to restore the historical characteristics of forest structure and fire behavior. However, it is suggested that a reliance on non-spatial metrics of forest stand structure, along with the use of wildland fire behavior models that lack the ability to handle complex structures, may lead to uncharacteristically homogeneous rather than heterogeneous forest structures following restoration. In our study, we used spatially explicit forest inventory data and a physics based fire behavior model to investigate the effects of restoration driven, variable retention harvests on structural complexity, both of horizontal and vertical dimensions, and potential fire behavior. Structural complexity was assessed at stand and patch scales using a combination of point pattern analyses, a patch detection algorithm, and nearest-neighbor and tree patch indices of height variation. The potential fire behavior before and after treatment was simulated across a range of open wind speeds using a 3-D physics based fire behavior model, the Wildland-urban interface Fire Dynamics Simulator (WFDS). Our results show that treatments resulted in an aggregated spatial pattern of trees consisting of a matrix of individual trees, clumps and openings similar to descriptions of historical dry forests. Treatments had inconsistent effects on vertical complexity across sites likely due to differences in treatment of ladder fuels; lack of reference conditions hinder evaluation of this structural aspect. Simulation modeling using WFDS suggest that treatments moderated fire rate of spread, fireline intensity and canopy consumption across all wind speeds tested and shifted potential fire behavior towards historical ranges. Our findings suggest that current restoration-based variable retention harvests can simultaneously fulfill objectives of altering structural complexity and of reducing fire behavior, though we recommend further research on desired ranges of vertical complexity to inform treatment design.

Exploring tree crown spacing and slope interaction effects on fire behavior with a physics-based fire model

ABSTRACTA substantial proportion of forests in the Republic of Korea are characterized by dense, young-growth pine stands which are prone to burn, particularly in the dry spring and fall seasons. Forest fuel reduction by thinning is needed to reduce the risk of catastrophic forest fire in Korea, but thinning guidelines to effectively reduce crown fire potential are not well established, particularly in areas with steep slopes. We describe an exploration of the interaction of different tree spacings, forest age classes, and slopes using the Wildland–urban interface Fire Dynamics Simulator (WFDS), a physics-based fire behavior model. A total of 36 simulations were performed using two terrain slopes, three stand ages and six tree spacings. Our results suggest that tree spacing significantly affects how fires propagate through forest canopies. We observed large reductions in fire size and intensity when tree spacing approached crown width. Also, most crown fires in our simulation stopped spreading through the canopy in less than 100 m when tree spacing was at least 6 m. We found that a different range of tree spacings was required to limit fire size, intensity, and fire spread distance according to its age class and the terrain slope. Although this exploratory study is only a first step in assessing the potential effectiveness of thinning treatments on fire behavior in steep terrain, this study demonstrates a potential use of WFDS for testing fuel reduction thinning guidelines. Future studies should assess sensitivity to different wind speeds and a broader range of fuel characteristics.

A comparison of level set and marker methods for the simulation of wildland fire front propagation

Simulating an advancing fire front may be achieved within a Lagrangian or Eulerian framework. In the former, independently moving markers are connected to form a fire front, whereas in the latter, values representing the moving front are calculated at points within a fixed grid. Despite a mathematical equivalence between the two methods, it is not clear that both will produce the same results when implemented numerically. Here, we describe simulations of fire spread created using a level set Eulerian approach (as implemented in the wildland–urban interface fire dynamics simulator, WFDS) and a marker method (as implemented in FARSITE). Simulations of surface fire spread, in two different fuels and over domains of increasing topographical complexity, are compared to evaluate the difference in outcomes between the two models. The differences between the results of the two models are minor, especially compared with the uncertainties inherent in the modelling of fire spread.

Evaluating Crown Fire Rate of Spread Predictions from Physics-Based Models

Modeling the behavior of crown fires is challenging due to the complex set of coupled processes that drive the characteristics of a spreading wildfire and the large range of spatial and temporal scales over which these processes occur. Detailed physics-based modeling approaches such as FIRETEC and the Wildland Urban Interface Fire Dynamics Simulator (WFDS) simulate fire behavior using computational fluid dynamics based methods to numerically solve the three-dimensional, time dependent, model equations that govern, to some approximation, the component physical processes and their interactions that drive fire behavior. Both of these models have had limited evaluation and have not been assessed for predicting crown fire behavior. In this paper, we utilized a published set of field-scale measured crown fire rate of spread (ROS) data to provide a coarse assessment of crown fire ROS predictions from previously published studies that have utilized WFDS or FIRETEC. Overall, 86% of all simulated ROS values using WFDS or FIRETEC fell within the 95% prediction interval of the empirical data, which was above the goal of 75% for dynamic ecological modeling. However, scarcity of available empirical data is a bottleneck for further assessment of model performance.

Large eddy simulation of forest canopy flow for wildland fire modeling

Large eddy simulation (LES) based computational fluid dynamics (CFD) simulators have obtained increasing attention in the wildland fire research community, as these tools allow the inclusion of important driving physics. However, due to the complexity of the models, individual aspects must be isolated and tested rigorously to ensure meaningful results. As wind is a driving force that can significantly dictate the behavior of a wildfire, the simulation of wind is studied in the context of a particular LES CFD model, the Wildland–urban interface Fire Dynamics Simulator (WFDS). As WFDS has yet to be tested extensively with regard to wind flow within and above forest canopies, a study of its ability to do so is carried out. First, three simulations are conducted using periodic boundary conditions. Two of these assume a spatially heterogeneous forest and one models wind downstream of a canopy edge. Second, two simulations are conducted with specified “inflow” conditions using two inflow profiles: one static and one dynamic (driven by a precursor simulation). Using periodic boundary conditions, the model is found to generate profiles of mean velocity and turbulent statistics that are representative of experimental measurements. The dynamic inflow scenario is found to perform better than the static case.

Fire behavior and heat fluxes for lab-scale burning of little bluestem grass

This paper discusses the physics and fire behavior of grassland fuel using experimental and modeling results. Experimental characterization included intermediate-scale tests to determine the mass loss rates, heat release rates (HRRs), and flame heat fluxes of burning little bluestem (Schizachyrium scoparium) grass plants at various fuel moisture contents and external flow conditions. The experiments included single-plant tests, multiple-plant tests with no forced flow/wind, and multiple-plant tests in which a forced flow was directed over the plants to simulate wind. The burning characteristics of single plants and fire spread between multiple plants under various conditions are discussed. The computational tool, Wildland–Urban Interface Fire Dynamics Simulator (WFDS), was then used to model the experiments using both a prescribed HRR and the particle-based fuel element model (predicted HRR). Comparisons are made between the experimentally measured quantities and the results predicted by WFDS. The results of the WFDS simulations with a prescribed HRR are in good agreement with the measured heat fluxes for the multiple-plant tests with no wind. The results of the particle-based WFDS fuel element model are in good agreement with the experimentally measured mass loss rates and HRRs of the single-plant tests. The WFDS fuel element model effectively captures the different stages of burning of the little bluestem plant. For the prediction of heat fluxes in the wind tests, there are limitations in the use of the prescribed HRR model.

Surface Fire Intensity Influences Simulated Crown Fire Behavior in Lodgepole Pine Forests with Recent Mountain Pine Beetle-Caused Tree Mortality

Recent bark beetle outbreaks have had a significant impact on forests throughout western North America and have generated concerns about interactions and feedbacks between beetle attacks and fire. However, research has been hindered by a lack of experimental studies and the use of fire behavior models incapable of accounting for the heterogeneous fuel complexes. We populated the Wildland-Urban Interface Fire Dynamics Simulator with data from 11 field sites to investigate the effect of mountain pine beetle (MPB)-caused tree mortality on simulated crown fire behavior across a range of surface fire intensities. Simulations addressed fire behavior during a 1- to 2-year period after the initiation of the outbreak in which some proportion of the trees have been killed but no foliage has yet fallen. The effect of MPB-caused tree mortality on simulated crown fire behavior significantly changed as a function of surface fire intensity. The largest effects of mortality on crown fire behavior occurred at moderate levels of surface fire intensity, whereas diminished effects occurred at low and high levels of surface fire intensities. Our results suggest that increased crown fire potential immediately after bark beetle infestations is dependent on the fire intensity generated by the preoutbreak surface fuels complex. FOR .S CI. 59(4):390 -399.

Characterization of Fuel Properties and Fire Spread Rates for Little Bluestem Grass

Rapid urban sprawl and population decentralization in recent decades have increased the size of the wildland-urban interface and resulted in higher community risk and vulnerability to wildfire. This paper primarily focuses on understanding grass-fueled fires common to Texas and improving the understanding of the physics and fire dynamics that are inherent in the grassland and prairie flame spread problem. Little bluestem (Schizachyrium scoparium) grass was chosen as the grassland fuel due to its prevalent coverage in the Texas area and its relevance to grassland fires in Texas. The methodology in this study relies on a framework to characterize fuel properties of little bluestem grass using small- and intermediate-scale experiments to better predict full-scale fire behavior. An intermediate-scale numerical flame spread model was developed for grass fuels that accounts for fuel moisture content to calculate the mass versus time of a burning little bluestem plant. The results of the small- and intermediate-scale experiments were used to develop input parameters for a field-scale numerical simulation of a grass field using a physics-based computational fire model, Wildland-urban interface Fire Dynamics Simulator (WFDS). A sensitivity analysis was performed to determine the effect of varying WFDS input parameters on the fire spread rate. The results indicate that the fuel moisture content had the most significant impact on the fire spread rate.

Numerical Simulation of Crown Fire Hazard Immediately after Bark Beetle-Caused Mortality in Lodgepole Pine Forests

Quantifying the effects of mountain pine beetle (MPB)-caused tree mortality on potential crown fire hazard has been challenging partly because of limitations in current operational fire behavior models. Such models are not capable of accounting for fuel heterogeneity resulting from an outbreak. Further, the coupled interactions between fuel, fire, and atmosphere are not modeled. To overcome these limitations, we used the Wildland-Urban Interface Fire Dynamics Simulator (WFDS) to investigate the influences of tree spatial arrangement and magnitude of MPB-caused tree mortality on simulated fire hazard. Field-collected, tree-level data from 11 sites were used to populate WFDS simulation domains representing a range of lodgepole pine forest structures for the postoutbreak period of time when dead needles are still present in the tree crowns. We found increases in the amount of crown fuel consumption and the intensity of crown fires as the percentage of MPB-caused tree mortality increased. In addition, we found complex interactions between the level of mortality, stand structure, and spatial arrangement of trees. These results suggest that preoutbreak forest structure and percent tree mortality influence crown fire behavior while dead needles are in the crown, and that the effect varies with spatial heterogeneity among trees.

Analysis of Forest Fire Behaviour by Advanced Computer Fire Simulators

Computer modelling of dangerous phenomena related to fire in forest or in wildland-urban interface is an important tool to support emergency planning for such complicated events. In this paper, the use of FARSITE (Fire ARea SImulaTor), adapted for real conditions in Slovak Paradise National Park (Slovakia), and WFDS (Wildland-urban interface Fire Dynamics Simulator) for simulation of forest fires and fires in wildland-urban interface is demonstrated. Computer reconstruction of an especially tragic destructive forest fire happened in 2000 as well as simple examples of automobile fire in wildland-urban interface are illustrated.