FDS Road Map - firemodels/fds Wiki
This wiki describes research plans for enhancing the fire model FDS.
The Road Traveled
FDS and Smokeview were officially released in 2000. However, for two decades prior to 2000, various CFD codes using the basic FDS hydrodynamic framework were developed at NIST by Howard Baum, Ron Rehm and Kevin McGrattan for different applications and for research. In the mid 1990s, many of these different codes were consolidated into what eventually became FDS. Before FDS, the various models were referred to as LES (large-eddy simulation), NIST-LES, LES3D, IFS (Industrial Fire Simulator), and ALOFT (A Large Outdoor Fire Plume Trajectory).
The early NIST LES model described the transport of smoke and hot gases in an enclosure using the Boussinesq approximation, where it is assumed that the density and temperature variations in the flow are relatively small. Much of the early work with this form of the model was devoted to the formulation of the low Mach number form of the Navier-Stokes equations and the development of the basic numerical algorithm.
Eventually, the Boussinesq approximation was dropped and simulations began to include more fire-specific phenomena, such as fire plumes, ceiling jets, sprinkler activation, warehouse fires, and large oil fires. The early validation efforts were encouraging, but still pointed out the need to improve the hydrodynamic model. To address this, we introduced the Smagorinsky model to close the LES subgrid stress in 1998. This addition improved the stability of the model because of the relatively simple relation between the local strain rate and the turbulent viscosity.
The first official version of FDS, released in 2000, was aimed at large scale simulations of smoke movement from prescribed, well-ventilated fires, ideal for design work where the fire's heat release rate is not predicted by the model, but rather specified by the designer or Authority Having Jurisdiction (AHJ). Over the next few years, Jason Floyd, then a NIST post-doctoral fellow, and Simo Hostikka of VTT, Finland, as a guest researcher at NIST, developed the basic combustion model and the finite volume radiation transport solver, respectively. These improvements were implemented in version 2 (2001). Versions 3 (2002) and 4 (2004) saw gradual improvements in these routines, along with the introduction of parallel processing and various fire-specific features.
During the NIST Investigations of the World Trade Center collapse and the Station Nightclub fire, it became fairly obvious what needed to be done with FDS to make it an effective tool for reconstructing fires. Up to that point, FDS had been used by the FPE community for design applications, and to some extent forensic work, but the scope of the investigations pushed the model to its limits. By 2005, it was clear that FDS was going to need a major overhaul, so we set about creating a new version (FDS 5) that would dramatically increase the flexibility and functionality of the model. The work proceeded along two broad fronts - the gas phase and the solid phase. In short, better combustion and better pyrolysis. Version 5 was released in 2007. It included a major overhaul of the input parameters and constructs. Over the next three years improvements were gradually added, and in 2010 work began on FDS 6, which began beta testing in the fall of 2012. FDS 6 was officially released in the fall of 2013. Since then gradual improvements have been made to hydrodynamics, chemistry, and multi-mesh calculations. The HVAC solver, a key addition to the model, was also included in version 6 and has been steadily enhanced.
Plans for FDS 7 include the ability to handle complex, unstructured geometry via a high-order immersed boundary method. Scalar transport near the surface of an immersed object will be handled with a cutcell method.
Proposing New Construction
Before talking about the Road Ahead, a few words about how we decide on new areas of development. First and foremost, suggestions and requests by the user community are given a high priority, especially if the ideas come from different sectors of fire protection. Also, the individual developers, and the organizations they represent, have a specific research agenda. Wherever they come from, proposals for new features in FDS or for changing existing features must meet a number of criteria before they will be considered, which are explained below:
This Road was Improved?
There is a substantial cost in terms of time and effort to develop and test new source code. Proposed changes must make that effort worth it. In general this occurs by demonstrating that the proposed changes will demonstrably improve the prediction of FDS for a wide range of fire protection applications. For some computed quantities this is a very high bar. For example in well-ventilated fires FDS has been shown to make predictions of far-field gas temperatures and species concentrations within experimental uncertainty. This is an area where it would be difficult to demonstrate significant improvement, especially in light of the results of a recent US NRC V&V exercise (NUREG 1824) in which it was shown that FDS, for certain predicted quantities, is within the uncertainty of the experiments against which it was compared.
Obey the Speed Limit
One of the main advantages of FDS over other CFD models is its fast computational speed and relatively modest requirements in terms of computational hardware. FDS was developed for practicing fire protection engineers who typically cannot afford to have a computation take months to complete. Even if the application is not necessarily practical, like a combustion research application, we still insist that any change to FDS must be evaluated for its impact on computational speed. If the end result of a proposed change is to greatly increase the computational time with little added benefit, such a change will not be considered; however, a proposed change that has little benefit in terms of predictive outcome but does reduce computational time is likely to be considered. Along with this is the development concept that routines related to a particular phenomenon should consume computer time in proportion to the impact of that phenomenon (we call this "Baum's Rule" after NIST Fellow Howard Baum). For example, radiation from a fire plume typically accounts for around a third of the energy released by the fire. Thus, the various routines for radiation heat transfer should only consume about a third of the computer time. The basis for this is that if FDS spends a lot of time computing phenomena that have little impact on the overall uncertainty of the computation, then that time would be better spent improving the computation of a more critical phenomenon.
No Roads to Nowhere
We strive to maintain a high degree of flexibility within the FDS source code. This allows developers to experiment with adding new features or changing how existing features are done without having to overhaul large portions of the source code. A proposed change that restricts this flexibility is not desired.
Along with developmental flexibility is the broad applicability of FDS. Proposed changes that would restrict the use of FDS to specific types of fire protection problems are not desired.
No Limited Access Highways
One reason for the success of FDS is the ability for a fire protection engineer with modest training to use FDS for traditional applications such as smoke control, detector layout, etc. While there are and always will be applications that require a high level of knowledge of FDS, its numerics, and the physics and chemistry behind fire, the developers do not wish that for traditional applications. Changes to the code that would make it difficult for a typical user to use FDS for traditional applications are not desired.
The Road Ahead
The topics listed below are areas of potential research, listed in no particular order.
Large Outdoor Fire Modeling (LOFM)
In March of 2019, NIST held a workshop on Large Outdoor Fire Modeling (LOFM) with the goals of assessing the state-of-the-art in the field and identifying research gaps. The two-day workshop culminated with a list of prioritized grand-challenge problems designed to drive research forward in the coming decade. The research plan and list of objective target applications for the current FDS project cycle are derived from the LOFM workshop discussions.
THREE-YEAR RESEARCH OBJECTIVE: To demonstrate that physics-based fire models can be accurate, robust, and reliable tools for prescribed burns management and fire protection engineering at parcel to community scales.
To this end, below we list several research areas we feel to be critical to achieving our objective.
Atmospheric Flow Physics and Wind Boundary Conditions
While FDS is a variable density, low-Mach flow code, it was not designed with atmospheric flows in mind. The range of length and time scales, the variability in the heat fluxes from solar radiation, the uncertainty in the terrain boundaries, and the variability in the inflow and outflow conditions all combine to make atmospheric flows challenging for FDS. Over the past few years we have been building up atmospheric flow capabilities. However, there is plenty of room for more verification and validation of atmospheric flows.
Potential Research Topic: Add any standard atmospheric flow verification or validation cases not yet implemented in the FDS manuals.
The preferred method for simulating wind in FDS is via special OPEN wind boundaries. Vertical profiles of velocity and temperature may be generated using Monin-Obukhov similarity theory.
Potential Research Topic: The current methodology for generating wind a wind field works reasonably well for relatively flat terrain, but has not been rigorously studied for complex terrain.
There is little to no data for wind fields at the parcel or community scales around structures that can be used for FDS validation.
Potential Research Topic: Generate experimental data and perform model validation for wind fields around structures at the parcel and community scales.
Special OPEN wind boundaries generate a "ceiling effect" that does not allow thermal or smoke plumes to exit the top boundary freely. If the top boundary cannot be moved to sufficient elevation then the user can use a
PRESSURE_GRADIENT_FORCE with standard OPEN boundaries to drive the flow. While this method works well, it requires iteration to figure out the force needed to produce a certain wind speed.
Potential Research Topic: Eliminate the ceiling effect in wind OPEN boundaries.
Wildland Fire Spread using the Level Set Method (FDS-LS)
FDS implements a simplified wildfire spread model based tracking the fire front with the level set method. This model uses a catalog of 13 fuel types described in the LANDFIRE database that is maintained by the U.S. Geological Survey (USGS). The fuels database is stored in a GIS format. The data can be loaded as a layer in dedicated GIS tools, such as QGIS (open source). The LANDFIRE database provides 30-meter spatial resolution “tiles” with a temporal resolution for means on the order of 10 years. A plug-in for QGIS known as QGIS2FDS developed by Emanuele Gissi of Corpo Nazionale dei Vigili del Fuoco (CNVVF, Italian Fire and Rescue Service) and Ruggero Poletto of CFD FEA Service (Verona, Italy) currently reads in the U.S. digital elevation map (DEM) and the LANDFIRE fuels database as GIS data layers. With this tool, one can quickly create FDS input files and run level set simulations of fire spread with a variety of options for coupling the fire and wind.
Potential Research Topic: Add to QGIS2FDS the ability to assimilate historical weather conditions for fire simulations.
Potential Research Topic: Compare FDS level set results to historical fire data in the United States and elsewhere.
Embers contribute to both short-range spotting and long-range spotting. They also play a major role in structure to structure ignition. General ember models are broken down into three categories: (1) ember generation, (2) ember transport, and (3) ember ignition (of another surface or fuel package). FDS currently has a very simple ember generation model based on prescribed threshold density and velocity difference with the local wind field. Ember transport is done in the same way as water droplet transport for sprinklers. And there is currently no ember ignition model in FDS.
Potential Research Topic: Determine ember yields for various vegetation types and fire conditions.
Potential Research Topic: Determine the effect of turbulent transport on ember transport. Currently, embers are transported via filtered velocity fields without considering subgrid velocity fluctuations. How important are subgrid fluctuations and random Magnus effects to predicability for long-range spotting?
Potential Research Topic: Formulate and test a stochastic model for ember ignition and long-range spotting.
Physics-based Flame Spread
A branch of FDS, named WFDS (Wildland FDS), was developed by Ruddy Mell of the U.S. Forest Service to simulate fires in the Wildland-Urban Interface (WUI). This branch has been merged back into FDS---going forward WFDS is deprecated.
Two key application areas for physics-based fire models are forensic reconstructions and predicting fire behavior of prescribed burns. Both rely on robust and accurate models. In this phase of the LOFM project the objective is to ensure reliability of the physics-based model results. This can only be achieved through a rigorous verification and validation process.
Additionally, FDS now has three different flame spread models that may be used. Ostensibly, these models increase in fidelity from level set (coarsest level, 5 m to 100 m resolution), to the boundary fuel method (1 m to 10 m resolution), to Lagrangian particle methods (highest level, 1 cm to 2 m resolution). Theoretically, the model results should improve. However, at present there is no guarantee of convergence of the methods (which would require numerical consistency at overlapping scales), nor is there any reason to expect the highest fidelity models to give more accurate results for flame spread rates.
Potential Research Topic: Reconstructions of well-documented major fire events using LS, boundary fuel, and Lagrangian particle methods in FDS.
Potential Research Topic: Investigations of well-documented prescribed burns using LS, boundary fuel, and Lagrangian particle methods in FDS.
Smoke Plume Injection Height
The National Oceanic and Atmospheric Administration (NOAA) in collaboration with the U.S. Forest Service maintains a Smoke Forecasting System that is responsible for 48 hour forecasts of air quality with the contiguous U.S. Satellite detection of fires is accomplished through image analysis and the fire power is estimated based on pixel intensity. As is well known in fire science, the power of a fire determines its plume height. In the NOAA system, the smoke plume is modeled by the BlueSky model developed by Sim Larkin at the U.S. Forest Service. BlueSky determines the smoke plume injection height, which is fed to the HYSPLIT code which is effectively a 2D transport model for the emissions. The accuracy of the forecasts has been shown to be most sensitive to the accuracy of the smoke plume injection height.
Potential Research Topic: Use FDS to refine BlueSky smoke plume injection models.
High Performance Computing and Data Visualization
Over the past several years FDS has developed into a successful HPC code. It scales almost linearly up to 10,000 cores (done on Titan at Oak Ridge National Laboratories). We routinely run validation cases in the 100s of cores range. And we expect to expand our linux cluster to around 2,000 cores in the next few years.
Outdoor flows are prime candidates for HPC codes. The domains tend to be rectangular and simple domain decomposition with uniform block size is a natural way to set up the problem, leading to good load balancing and scaling of the code.
We are fast approaching, and in some cases have already surpassed, the point where data visualization is a bottleneck in our computing capabilities. We are in the process of analyzing Smokeview for areas where we can make use of parallel computing algorithms (in space or time), areas where we can improve data compression and data transfer rates, and reduce Smokeview's dependency on outdated (and in some cases completely unsupported) libraries.
Potential Research Topic: Write VTK output format for FDS, which is a standard format for HPC data visualization.
Potential Research Topic: Develop a proof-of-concept application in Qt (or similar) to read and visualize FDS geometry, 3D smoke, and slice files.
Potential Research Topic: Demonstrate improved data compression and visualization for massive FDS cases (100s of cores, millions of particles) in Smokeview.
The Pressure Solver
We still need to improve the coupling of the pressure solver across mesh boundaries in a multi-mesh (often called "multi-block") simulation. Discontinuities in the pressure solution across these boundaries is currently the biggest speed bump to broader, more reliable use of the multi-mesh approach. The hope is to determine a computationally efficient way to perform this coupling that still permits the use of the direct pressure solver (CRAYFISHPAK) which is one of the primary reasons why FDS is fast. FDS currently includes a way to tighten the tolerance of the pressure solution across multiple meshes by iterating the single mesh solutions, but this method is relatively slow.
Potential Research Topic: Improve efficiency of parallel version of FDS on very large computing clusters (100+ processors). Document scalability using conventional parallel computing terminology.
A key component of the FDS algorithm is the projection scheme which enforces the velocity divergence constraint implied by combining the continuity and energy equations. The projection requires the solution of an elliptic PDE and this constraint poses problems for efficient distributed memory parallel computing. We are working in collaboration with Susanne Kilian of hhpberlin to develop a fast global pressure solver. We are also pursuing research in OpenMP parallelization on shared memory machines. Initial OpenMP efforts were led by Christian Rogsch of the University of Wuppertal. Later, Daniel Haarhoff of Juelich Supercomputing Centre added OpenMP for FDS 6. As we move forward both these approaches will be extremely important in establishing a robust algorithm with optimal performance.
Potential Research Topic: Adaptive Mesh Refinement (AMR). Explore strategies for structured grid refinement in a low-Mach LES solver and develop refinement criteria for fire applications. A key aspect of this research will focus on load balancing for parallel calculations.
The lumped species framework and eddy dissipation model have proven robust and accurate over the course of FDS 6 development. However, improvements to the methods to allow for better in-flame and post-flame prediction of CO and soot concentrations is desired.
The FDS Validation Guide contains examples of fire scenarios that include a two-step reaction that generates soot and CO in the first step and oxidation to CO2 in the second. Both reactions are fast, but the first reaction has precedence over the second. Examples include Smyth Burner, Waterloo Methanol, UMD Line Burner, NIST RSE 1994 and 2007, and NIST FSE 2008.
Potential Research Topic: The two-step reaction scheme has the potential to predict CO and soot concentrations within the flame envelop and within oxygen vitiated gas layers. However, it is not clear from the simple mechanism how to determine the relative amounts of soot and CO produced in the first step.
Recent large scale experiments conducted at NIST under US NRC sponsorship included measurements of smoke concentration that were substantially lower than predictions of both FDS and CFAST, the NIST zone fire model (US NRC, NUREG-1824). A possible explanation has been suggested by researchers at Jensen Hughes, who have noted that significant amounts of smoke can be deposited on walls, an effect that is usually neglected by fire models. Starting in FDS 6, there is an optional soot deposition model implemented by Jason Floyd and Kris Overholt that uses empirical correlations to predict the rate of soot deposition onto walls as well as an optional model to predict the agglomeration of soot over time.
Potential Research Topic: Reduced and full-scale experiments quantifying the mass of soot depositing onto surfaces. Experiments should isolate the various deposition mechanisms (thermophoresis, gravitational, and turbulent) to enable independent V&V of the specific mechanisms.
Potential Research Topic: Measurement of post-flame soot size distributions from a range of fuels. Measurement of soot size distribution over time in compartment fires with and without ventilation openings.
Potential Research Topic: Algorithms for the oxidation of soot deposited onto surfaces.
Potential Research Topic: Visualization techniques to shade surfaces based upon the amount of soot deposited.
Modeling gas phase suppression (flame extinction) is also a long term goal. FDS 5 has a simple, empirically based model of flame extinction based on the concept of the lower oxygen limit applied locally, grid cell by grid cell. Also available is the same concept for the fuel stream using the lower flammability limit. Validation work is on-going to test the simple model, and it is planned to also look at gaseous suppression agents and water mist.
Current Activities: Led by Jason Floyd of Jensen Hughes, we are working on development of a consistent extinction model for multi-step fast reaction schemes. It has been shown that a simple set of fast reactions of the form Fuel + Air => CO + Other Products followed by CO + (1/2) O2 => CO2 can do a reasonable job of predicting global CO concentrations for under-ventilated compartment fires. However, the original thermal extinction model (Vaari et al., IAFSS, 2011) was designed to work with only a single reaction.
Potential Research Topic: Identify and perform simulations of full-scale or reduced-scale experiments where the fire self-extinguished or was extinguished by a diluent or suppressing agent. The results could be added to a new chapter on Suppression in the FDS Validation Guide.
Potential Research Topic: Generalize the extinction model to a Damköhler number (Da) approach to capture effects of strain on flame extinction.
Modeling of ignition or re-ignition (deflagrations and backdrafts) is also a long term goal. The new lumped species approach will make possible development into these areas. It is noted that the low-Mach number limitations of FDS means that computations on flows that approach a detonation event will not be possible.
It should also be noted that spurious re-ignition is currently still an issue in FDS because of the Eddy Dissipation Concept (EDC) combustion modeling approach where fuel instantly burns upon mixing (rates are limited by rate of turbulent mixing). When unburnt fuel hits fresh air it will burn numerically unless the AIT parameter has been set so to a temperature above the ambient air temperature. The current version of FDS allows for a small "pilot" zone near a fuel surface where AIT can be set to 0 K (default) and AIT can be set higher elsewhere to prevent spurious re-ignition. This feature is obviously an ad hoc approach that is nevertheless practically necessary.
Potential Research Topic: Explore reduced mechanisms for the prediction of CO and soot formation and oxidation (identified as a need by the International Forum of Fire Research Directors (FORUM)). These mechanisms must be computationally efficient and capable of being used for the room and building scale modeling efforts typically undertaking by practicing engineers.
Potential Research Topic: Algorithms for the re-ignition of combustible gases that have undergone extinction. Currently, FDS will burn fuel if it encounters sufficient oxygen whether or not some form of ignition source is present.
Potential Research Topic: Modeling backdraft experiments, noting that FDS currently lacks certain necessary physical mechanisms that would need to be developed in cooperation with us.
Potential Research Topic: Treatment of turbulence/chemistry interactions (TCI). TCI refers to the nonlinear effects of subgrid heterogeneity in species concentration and temperature on the production rates of chemical species. In fire modeling, we are interested in the impacts of TCI on the formation and oxidation of CO and soot and on flame suppression.
Potential Research Topic: Treatment of turbulence/radiation interactions (TRI). TRI refers to the nonlinear effects of subgrid heterogeneity in species concentration and temperature on the rates of emission and absorption of radiation. In fire modeling, we are interested in accurate prediction of the radiant fraction and the thermal feedback to a pyrolyzing surface.
Pyrolysis and the Solid Phase
Simo Hostikka of Aalto University, Finland, developed the basic framework for describing heat conduction and pyrolysis within bounding solids. The algorithm includes 1-D heat transfer through multiple layers of materials, and a generalized pyrolysis algorithm to allow for multiple materials undergoing multiple reactions.
The major hurdle in this area is still the availability of material property data. After 30 years of model evolution, there is now a fairly broad consensus on the basic approach for modeling solids within a fire model, but there remains the major hurdle of measuring the necessary material properties.
In addition to material properties, the problem of two-way coupling between the gas flame and solid phase pyrolysis has received very limited attention.
Potential Research Topic: Model flame spread (upward, lateral, and downward) for well-characterized materials.
Potential Research Topic: Demonstrate accurate flame spread calculations on relatively coarse, engineering-level grids. This will likely involve developing near-wall subgrid-scale models for flame surface interactions involving convective and radiative heat transfer.
Recently, we have added a 3D heat transfer model to the solid phase. One of the potential applications of this new method would be to model downward flame spread. Several challenges remain for making the 3D model useful for practical engineering problems.
Potential Research Topic: Develop internal refinement of solid OBSTructions.
Potential Research Topic: Implement an implicit time integration scheme for 3D heat transfer.
Potential Research Topic: The current code requires at least one cell thickness in the solid phase. Overhaul the transport scheme to enable 2D lateral thermal transport while using shell elements (the usual 1D approach) in the wall normal direction.
Potential Research Topic: With 3D transport comes the possibility of more accurate treatment of mass transport of chemical species, which can be important for char oxidation or smoldering peat fires. Implement a porous media model for transport of gaseous species in the 3D solid.
Liquid Pool Evaporation
The capability to predict liquid pool burning rates has been based on the use of the solid pyrolysis solver for 1-D heat conduction and simple models for the pool surface evaporation. Early versions of the evaporation routine were based on equilibrium gas pressure, but since FDS 6, a mass transfer number approach has been used.
An important aspect of predicting the temperature of liquid layers (or transparent solids) is the calculation of internal radiation. Recent works have shown that it is very difficult to specify a single mean absorption coefficient that would yield good predictions of radiation absorption both near-the-surface and deeper in the pool. Implementation of Full-Spectrum K-distribution (FSK) method for the internal 1D radiation is being implemented.
Potential Research Topic: Generalize the two-flux model based on the Schuster-Schwarzschild approximation for cylindrical and spherical coordinates. (Has been tried, but turns out to be difficult.)
Potential Research Topic: Use of FSK method requires a tool to compute the k-values and quadrature weights from spectral data. Generation of such model parameters from a spectral data will require simple and well-documented support programs that travel and are maintained within the FDS repository.
Heat and Mass Transfer Coefficients
In LES with near-wall modeling, which is the formulation used by FDS, we require "wall functions" to provide the correct wall fluxes for momentum (wall stress), energy (convective heat transfer), and species transport (mass transfer). Most of the simple wall functions are derived and tested for forced convection flows using wind tunnel data. In fires, the Rayleigh numbers can be very high and we are therefore often in a situation where we have mixed forced and natural convection phenomena to consider.
Potential Research Topic: Development and testing of mixed convection heat and mass transfer models over curvilinear surfaces in the presence of blowing (mass transfer from the surface).
FDS uses Lagrangian particles to represent unresolved subgrid objects for applications such as electrical cable fires and for burning vegetation is wildland fires. The form of the heat and mass transfer models used for these applications can have a leading order effect on the results. To date, there is very little data on heat and mass transfer coefficients for burning objects in buoyant or forced convection flows. Experimental techniques to directly measure heat transfer coefficients of complex vegetation have not been developed. To further extend these methods to the case where the vegetation is burning would be a significant contribution to the field.
Potential Research Topic: Carry out wind tunnel tests on different types of vegetation to determine both heat and mass transfer coefficients under cold flow and burning conditions.
Droplets, Particles, and the Dispersed Second Phase
Code development in the areas of spray dynamics, spray heat transfer, and other areas related to dispersed second phases has been modest in comparison to the gas and solid phase efforts. Future areas of work include improved tracking of droplets and particles, improved radiative heat transfer to and from droplets and particles, and improved interaction between droplets and particles and the gas phase and solid phase. These areas would encompass the ability to have combusting particles (fire brands) and to better account for surface wetting, surface suppression, and surface penetration by droplets.
Potential Research Topic: Validation of droplet evaporation rates at temperatures ranging from ambient to flame.
Potential Research Topic: Simulate standard ADD (Actual Delivered Density) and RDD (Required Delivered Density) sprinkler approval tests. Add to FDS Validation Guide chapter on Suppression.
Potential Research Topic: Improve modeling of dense water spray. Currently, water droplets do not interact with each other.
Potential Research Topic: Computationally efficient methods to model droplet and film flow over solids.
Potential Research Topic: Improve modeling of heat transfer from droplets and films to solids.
FDS has always been tied to structured Cartesian block geometries. Given that the zeroth-order specification of the heat release is often the best that can be expected from a fire model, the block geometries have not typically been viewed as a limiting factor. However, at a minimum, it is often a cumbersome process to input geometric information for something as simple as a cylinder. As we begin to tackle more challenging problems with FDS (fire spread, atmospheric flows) we are gradually being forced to account for second-order physical processes and we therefore require second-order numerical treatments at solid interfaces. For example, to first order the spread of wildfires is dictated by surface winds. In mountainous regions where these fires occur, the terrain is complex and simulations using stair-stepped grids lead to spurious fluctuations in the wind field.
For upcoming FDS 7 we have developed a new geometry module which creates a solid surface interface based on triangular facets as the primitive geometry element. The research needs in this area mainly involve ensuring consistency with the 3D Cartesian solver (OBST) and invariance of the numerical solutions (to second-order) under rotations of the domain.
Potential Research Topic: Develop verification tests using GEOM lines.
Potential Research Topic: Add validation cases for complex geometry.
Understanding structural response to fire is critical for building design. The finite-element models used for structural analysis require thermal boundary conditions. Often those boundary conditions are prescribed by time-temperature curves---the original fire model. More and more, there is a desire to consider the thermal conditions from a fire simulation such as FDS. At present, only a one-way couple between FDS and the FEM simulation is possible. The Structural Interaction scripts developed by Julio Caesar Goncalves da Silva represent the state of the art in this field.
Potential Research Topic: Write a script to utilize a boundary file (BNDF) instead of devices (DEVC) to output temperatures for one-way coupling to FEM models.
Potential Research Topic: Develop two-way coupling with a FEM model.
Computational Wind Engineering
As FDS is being developed to handle wildland fire simulations, and specifically to accurately predict the behavior of fires at the parcel and community scales, it will necessarily need to handle the high winds typical of design parameters in computational wind engineering of large buildings. For example, FDS must be capable of accurate prediction of pressure coefficients on the surfaces of structures in order to predict where low pressure zones will increase the likelihood of ember accumulation and ignition.
FDS has demonstrated that with sufficient grid resolution it can capture mean, max, and rms (root mean square) pressure coefficients for rectilinear structures that align with the Cartesian mesh.
Potential Research Topic: Using complex geometry, conduct a thorough validation study using the University of Western Ontario wind tunnel data.
Reference: T.C.E. Ho, D. Surry, and D. Morrish. NIST/TTU Cooperative Agreement—Windstorm Mitigation Initiative: Wind Tunnel Experiments on Generic Low Buildings. Technical Report BLWT-SS20- 2003, The Boundary Layer Wind Tunnel Laboratory, University of Western Ontario, London, On- tario, Canada, May 2003.
Active Fire Protection Systems
In the 2006 workshop of The International Forum of Fire Research Directors (FORUM), the improvement of the ability to predict the impact of active fire protection systems on fire growth and fate of combustion products was identified as the most important research topic of the fire research community. Many of the future developments in FDS will focus on this topic. The issues affecting the modeling of fire suppression systems have already been addressed in the sections concerning combustion (modeling gas phase suppression) and Droplets, Particles and Dispersed Second Phase (description of water sprays). The modeling of water mist systems in particular should be addressed because they are rapidly becoming common. In addition to the basic physical submodels of FDS, improvements are needed in the description of the suppression systems. For example, the simultaneous discharge from several sprinkler or water mist nozzles have effects on the pressure of the pipe system, and therefore on the mass flow of suppressant.
Potential Research Topic: In cooperation with VTT, Finland, better characterize the spray of fine mist and validate FDS for mist suppression.
Potential Research Topic: Develop and validate methodologies (both for defining inputs and for assessing extinction) for non-aqueous suppression systems such as inert gas, aerosol generators, CO2, and fluorocarbons.
FDS version 1 used heat emitting particles to represent a fire. Radiation transport consisted of Monte-Carlo ray tracing from the particles to surfaces, essentially painting the radiative fraction of the fire on surfaces. Hot surfaces and hot gas layers were not emitters.
Early in the development of FDS 2 it was recognized that FDS was being used for conditions where the participation of surfaces and gases was important, and thus, a new radiation model was needed. The model that was eventually developed consisted of using RadCal (added by Jason Floyd while a NIST post-doc) to generate look-up tables of absorption coefficients and a Finite Volume Method radiation transport model (developed by Simo Hostikka, then at VTT in Finland, now a professor at Aalto University). This approach consumes around 20% of the computational time and for the simpler versions of the combustion model generally performs well. The original implementation generated look-up tables for mixtures, and the absorption coefficient values were sought using the local mixture fraction. Generalizing the mixture fraction model into the lumped species method required simplification in the tabulation. In FDS 6, the different gas contributions are added up by simple superposition.
Ideally, we would prefer to compute absorptivity during the actual simulation rather than attempting to use look-up tables which do not readily support the needs of the more complex combustion models being developed. However, current validation efforts indicate that errors resulting from the use of the look-up tables are not large (indeed it is not clear if the errors are any larger than those in the experimental data we use for validation) so any new approach to generating absorptivities or radiation source terms should not consume significantly more time than currently used by the radiation transport. For example, RadCal could be called for each grid cell each time radiation transport is computed, but that would consume a tremendous quantity of resources.
One particular challenge of using the gray gas approximation is the need to specify a characteristic path length of radiation. The path length is an important parameter for gases with strongly non-uniform absorption spectra, such as water vapor. If the scenario includes many different geometrical length scales (such as a tunnel), defining a single value for the path length becomes cumbersome. Methods that avoid the use of path length are currently being investigated, including the Full-spectrum k-distribution method. Such methods should enable resolving the spectral content of the radiation with a relatively low cost, but the feasibility for FDS remains to be shown.
Potential Research Topic: Examine the means to reduce the ray-effect. This may include further verification and validation efforts to support the default ANGLE_INCREMENT and TIME_INCREMENT used in the finite volume solver. It is possible that in-flame radiation (and hence radiation feedback to pyrolyzing surfaces) is over-resolved in angular space, but under-resolved in time. Conversely, the angular resolution for radiation to distant targets is too low, while the time resolution is too high. It would be interesting to explore an adaptive radiation solver that would be optimal for both in-flame radiation and radiation at a distance.
Potential Research Topic: Determine a better way to define the radiation length scale or another way to calculate the 'path-mean' absorption coefficients from the narrow-band data.
Potential Research Topic: Implementation of alternative methods for participating medium, including the k-distribution methods and WSGG methods.
The International Forum of Fire Research Directors (FORUM) has identified the ability to determine the relationship between aspects of a buildings design and the safety of building occupants as one of the top five research priorities. For whole building analysis, environmental control systems are a major contributor to the movement of toxic combustion products through a building. FDS 6 was released with a first generation HVAC model as a first step in assessing the impact of HVAC systems on smoke movement. Further development will be needed to account for control system behavior, transport delay of species, combustion within HVAC systems, deposition of aerosols, and to support modeling of in-duct, fire detection systems.
Potential Research Topic: Improved stability of the coupling between the HVAC solver and the FDS pressure solver.
Potential Research Topic: Algorithms for determining the flow loss through filters exposed to soot, simple approaches to model duct heat transfer, computationally efficient approaches to model mixing and combustion processes within HVAC duct networks, accounting for transport delays and mass storage in ducts.
Potential Tool Development: User interfaces for developing the inputs to model complex HVAC networks and visualization tools to help understand the complex connectivity of the network system based on the user inputs (to help better diagnose problems).
In the context of FDS+Evac development we use the Fractional Effective Dose methods presented in the SFPE Handbook of FIRE Protection Engineering (Purser, 2008a). For these computations, we need a capability to define arbitrary toxic species as products of combustion. The concentrations of these species are usually so small, that they need not to be considered in gas transport computation (with possible exceptions).
Since some gases are produced in well-ventilated conditions and some in under-ventilated, it might be reasonable to let the user define if the gas appears in fixed proportion to major carbon carrying species, as discussed in Combustion section above. Assuming the CO model works, this would introduce under-ventilated products.
In FDS6, the FED/FEC algorithm will recognize the following species:
Asphyxiants: CO, HCN, Low O2, CO2
Irritants: HCL, HBr, HF, SO2, NO2, CH2CHO (acrolein), CH2O (formaldehyde), X(user defined)
Potential Research Topic: Yields of these species under different conditions, in relation to CO2 or CO production.
Current Activities: FDS is developed and maintained using fairly common software development tools. The source code and manuals are stored in an external repository and can be accessed by all members of the team. User support is provided via a Discussion Group hosted by Google Groups and an Issues page hosted by GitHub.
Going Our Way?
FDS has benefited from the contributions of many people from various sectors of the fire protection community. We encourage collaboration, even though direct funding from NIST and other sources is very limited. Graduate students interested in working with FDS should contact us before embarking on a project so that we can discuss whether or not it meets some of the criterion for improvement that we set out above. Some of the potholes students have encountered in the past are:
- working with old versions of the source code
- trying to improve parts of FDS that validation work has shown to be working well (e.g. US NRC and EPRI, NUREG-1824)
- obsessing over turbulence models
- developing unnecessarily detailed heat transfer and boundary layer algorithms
The Discussion Group and Issue Tracker should be the first line of communication between someone working on FDS and the user community. We ourselves spend a significant amount of time everyday participating the discussions, answering questions, fixing bugs, and so on. Any student who is working with FDS should be monitoring this traffic, and participating as well.