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FDS Background

Understanding Fire Dynamics Simulator (FDS) fundamentals for effective PyFDS usage.

What is FDS?

Fire Dynamics Simulator (FDS) is a computational fluid dynamics (CFD) model developed by the National Institute of Standards and Technology (NIST) for simulating fire-driven fluid flow.

Key Characteristics

Specialized for Fire

  • Focus on smoke and heat transport from fires
  • Low-speed, thermally-driven flows
  • Large Eddy Simulation (LES) turbulence model
  • Radiation transport solver

Open Source

  • Free to download and use
  • Active development by NIST
  • Large international user community
  • Extensive validation studies

Well Validated

  • 100+ validation studies
  • Compared against experimental data
  • Published in peer-reviewed literature
  • International collaborations

Physical Models

Governing Equations

FDS solves the Navier-Stokes equations for low-speed, thermally-driven flows:

Conservation of Mass

\[ \frac{\partial \rho}{\partial t} + \nabla \cdot \rho \mathbf{u} = 0 \]

Conservation of Momentum

\[ \frac{\partial}{\partial t}(\rho \mathbf{u}) + \nabla \cdot \rho \mathbf{u} \mathbf{u} + \nabla p = \rho \mathbf{g} + \mathbf{f} + \nabla \cdot \tau_{ij} \]

Conservation of Energy

\[ \frac{\partial}{\partial t}(\rho h_s) + \nabla \cdot \rho h_s \mathbf{u} = \frac{Dp}{Dt} + \dot{q}''' - \dot{q}_b''' - \nabla \cdot \dot{q}'' \]

Conservation of Species

\[ \frac{\partial}{\partial t}(\rho Y_\alpha) + \nabla \cdot \rho Y_\alpha \mathbf{u} = \nabla \cdot \rho D_\alpha \nabla Y_\alpha + \dot{m}_\alpha''' \]

Turbulence Modeling

Large Eddy Simulation (LES)

FDS uses LES to model turbulent flow:

  • Resolved scales: Computed directly on mesh
  • Sub-grid scales: Modeled using turbulence models
  • Deardorff model: Default turbulence closure

Mesh Requirements

For good LES results:

\[ D^* / \delta x \approx 4 \text{ to } 16 \]

Where:

  • \(D^*\) is the characteristic fire diameter
  • \(\delta x\) is the mesh cell size

Combustion

Mixture Fraction Model

FDS uses a mixture fraction combustion model:

  • Fuel and oxygen mix based on transport
  • Combustion is infinitely fast (mixing-controlled)
  • Heat release rate specified directly

Simple Chemistry

\[ \text{Fuel} + s \text{O}_2 \rightarrow \text{Products} \]

Parameters:

  • Heat of combustion
  • Soot yield
  • CO yield
  • Radiative fraction

Radiation

Radiation Transport Equation (RTE)

\[ \mathbf{s} \cdot \nabla I(\mathbf{x}, \mathbf{s}) = \kappa I_b(T) - \kappa I(\mathbf{x}, \mathbf{s}) \]

Finite Volume Method

  • Divides solid angle into ~100 angles
  • Solves RTE for each angle
  • Accounts for absorption by soot and gases

Gray Gas Assumption

  • Single absorption coefficient
  • Function of soot concentration
  • Simplifies multi-wavelength problem

Numerical Methods

Spatial Discretization

Structured Rectilinear Grid

  • Cartesian coordinate system
  • Rectangular cells (can be non-uniform)
  • Simplifies obstacle and boundary representation

Finite Difference Approximations

  • Second-order accurate in space
  • Conservative formulation
  • Preserves mass and energy

Time Integration

Predictor-Corrector Scheme

  1. Predictor: Explicit update
  2. Corrector: Implicit pressure solve
  3. Ensures divergence-free velocity field

Time Step Control

CFL condition limits time step:

\[ \Delta t \leq \frac{\delta x}{|\mathbf{u}| + c} \]

Where \(c\) is the speed of sound.

Pressure Solver

Poisson Equation

\[ \nabla^2 H = -\frac{\partial}{\partial t}(\nabla \cdot \mathbf{u}) - \nabla \cdot \mathbf{F} \]

Solved using:

  • Fast Fourier Transform (FFT) for single mesh
  • Iterative methods for multiple meshes

Application Areas

Building Fire Safety

Evacuation Analysis

  • Smoke spread in buildings
  • Tenability assessment
  • Available safe egress time (ASET)

Fire Protection Design

  • Sprinkler system effectiveness
  • Smoke control systems
  • Fire barrier performance

Code Compliance

  • Performance-based design
  • Alternative compliance methods
  • Engineering analysis

Wildland Fires

Wildfire Spread

  • Wind-driven fire behavior
  • Vegetation combustion
  • Ember transport

Wildland-Urban Interface

  • Structure ignition
  • Fire break effectiveness
  • Community protection

Industrial Applications

Process Safety

  • Flammable liquid fires
  • Chemical facility hazards
  • Explosion modeling

Nuclear Safety

  • Cable fire scenarios
  • Smoke transport in facilities
  • Equipment damage assessment

Special Applications

Aircraft Fires

  • Cabin fire scenarios
  • Cargo hold fires
  • Fuel spill fires

Tunnel Fires

  • Smoke control in tunnels
  • Critical velocity for smoke
  • Emergency egress

Marine Fires

  • Ship compartment fires
  • Offshore platform scenarios
  • Ferry evacuation

Mesh Design

Characteristic Fire Diameter

The characteristic fire diameter is:

\[ D^* = \left(\frac{\dot{Q}}{\rho_\infty c_p T_\infty \sqrt{g}}\right)^{2/5} \]

Where:

  • \(\dot{Q}\) is the heat release rate (kW)
  • \(\rho_\infty\) is ambient density (kg/m³)
  • \(c_p\) is specific heat (kJ/kg·K)
  • \(T_\infty\) is ambient temperature (K)
  • \(g\) is gravity (m/s²)

Mesh Resolution Guidelines

Application \(D^*/\delta x\) Quality
Coarse 2-4 Rough estimates
Moderate 4-10 Typical design
Fine 10-16 High accuracy
Very Fine >16 Research quality

Aspect Ratio

Cell aspect ratios should be reasonable:

  • Ideal: 1:1:1 (cubic cells)
  • Acceptable: Up to 2:1 aspect ratio
  • Maximum: Generally avoid >4:1

Boundary Conditions

Wall Functions

Heat Transfer at Walls

\[ \dot{q}'' = h(T_w - T_g) + \epsilon \sigma (T_w^4 - T_\infty^4) \]

Components:

  • Convective heat transfer
  • Radiative heat transfer
  • 1D heat conduction into solid

Material Properties

Required for solid boundaries:

  • Thermal conductivity
  • Specific heat
  • Density
  • Emissivity

Open Boundaries

Pressure Boundary

  • Atmospheric pressure specified
  • Flow in/out determined by simulation
  • Common for room openings

Velocity Boundary

  • Velocity specified
  • Used for forced ventilation
  • Supply/exhaust ducts

Devices and Measurements

Point Measurements

Measure quantities at specific locations:

  • Temperature
  • Velocity
  • Species concentration
  • Pressure

Surface Measurements

Measure quantities on surfaces:

  • Heat flux
  • Wall temperature
  • Radiative flux
  • Convective flux

Special Devices

Sprinklers

  • RTI-based activation
  • Spray pattern modeling
  • Cooling effects

Heat Detectors

  • RTI-based response
  • Activation temperature
  • Plume correlation

Validation

NIST Approach

FDS validation follows scientific method:

  1. Identify scenario
  2. Select experiments
  3. Setup simulation
  4. Compare results
  5. Document uncertainty

Validation Studies

Published comparisons include:

  • Room fire experiments
  • Plume correlations
  • Detector activation
  • Sprinkler suppression
  • Tunnel fires
  • Wildland fires

Uncertainty

FDS predictions have typical uncertainties:

Quantity Uncertainty
Gas temperature ±15%
Heat flux ±25%
Velocity ±20%
Species ±20-40%

Best Practices

Model Setup

Define Objectives

  • What questions need answers?
  • What accuracy is required?
  • What resources are available?

Start Simple

  • Begin with coarse mesh
  • Add complexity incrementally
  • Validate at each step

Document Assumptions

  • Material properties
  • Fire characteristics
  • Boundary conditions
  • Mesh resolution

Quality Assurance

Grid Sensitivity

Run multiple mesh resolutions:

  • Coarse, medium, fine
  • Compare key results
  • Demonstrate convergence

Verification

Check simulation health:

  • Mass conservation
  • Energy balance
  • CFL condition
  • Pressure iterations

Validation

Compare against:

  • Experimental data
  • Analytical solutions
  • Engineering correlations

Reporting Results

Include Context

  • Simulation objectives
  • Key assumptions
  • Limitations
  • Uncertainties

Show Sensitivity

  • Parameter variations
  • Mesh convergence
  • Scenario variations

Visual Presentation

  • Clear plots
  • Smokeview images
  • Comparison charts
  • Tables of results

Limitations

Model Limitations

Mixture Fraction Combustion

  • Assumes fast chemistry
  • Cannot predict ignition
  • Limited for non-standard fuels

Gray Gas Radiation

  • Single absorption coefficient
  • May not capture spectral effects
  • Simplified compared to reality

LES Turbulence

  • Requires adequate mesh resolution
  • Not RANS (time-averaged)
  • Computationally expensive

Application Limits

Not Suitable For

  • Backdraft/flashover prediction
  • Detailed ignition processes
  • Slow pyrolysis
  • Supersonic flows
  • Strongly stratified flows with sharp gradients

Use with Caution

  • Very large domains (>100 m)
  • Very long times (>hours)
  • Complex chemical kinetics
  • Multi-phase flows

Resources

Official Documentation

Community

Publications

Key papers:

  • McGrattan et al., "Fire Dynamics Simulator Technical Reference Guide"
  • McGrattan et al., "Fire Dynamics Simulator User's Guide"
  • Validation study compilations

Using FDS with PyFDS

PyFDS Advantages

Programmatic Control

from pyfds import Simulation

# Build simulation in Python
sim = Simulation(chid='example')
sim.add(Time(t_end=600.0))
sim.add(Mesh(ijk=Grid3D.of(50, 50, 25), xb=Bounds3D.of(0, 5, 0, 5, 0, 2.5)))

# Write FDS file
sim.write('example.fds')

Reproducibility

  • Version control with git
  • Parameterized cases
  • Automated workflows

Integration

  • Pre-processing in Python
  • Run FDS simulations
  • Post-process results
  • Generate reports

Workflow

graph LR
    A[Define Parameters] --> B[Build Simulation]
    B --> C[Validate]
    C --> D[Run FDS]
    D --> E[Analyze Results]
    E --> F[Report]

    style A fill:#ff6b35
    style D fill:#004e89
    style F fill:#00a878

Next Steps

Namelist Reference →