Global Settings (MISC)¶

Configure global simulation parameters using the MISC namelist.

Overview¶

The MISC namelist controls global settings that affect the entire simulation:

Ambient conditions (temperature, pressure)
Turbulence models
Gravity and stratification
Special simulation modes
Output and logging options

# Set ambient conditions
sim.set_misc(
    tmpa=20.0,        # Ambient temperature (°C)
    humidity=50.0,    # Relative humidity (%)
    p_inf=101325.0    # Ambient pressure (Pa)
)

Ambient Conditions¶

Temperature and Pressure¶

# Standard conditions
sim.set_misc(
    tmpa=20.0,       # Ambient temperature (°C)
    p_inf=101325.0   # Ambient pressure (Pa, 1 atm)
)

# Hot day
sim.set_misc(
    tmpa=35.0,
    p_inf=101325.0
)

# High altitude (Denver, ~5000 ft)
sim.set_misc(
    tmpa=20.0,
    p_inf=84000.0    # Reduced pressure
)

Humidity¶

# Dry air
sim.set_misc(
    tmpa=20.0,
    humidity=10.0    # 10% relative humidity
)

# Humid conditions
sim.set_misc(
    tmpa=30.0,
    humidity=80.0    # 80% relative humidity
)

Gravity and Orientation¶

Standard Gravity¶

# Default: gravity in -Z direction (downward)
sim.set_misc(gvec=(0.0, 0.0, -9.81))  # m/s²

Modified Gravity¶

# Reduced gravity (simulating microgravity effects)
sim.set_misc(gvec=(0.0, 0.0, -1.0))

# Zero gravity
sim.set_misc(gvec=(0.0, 0.0, 0.0))

# Tilted domain (for slope simulations)
import math
angle = 10.0  # degrees
g = 9.81
sim.set_misc(gvec=(
    0.0,
    g * math.sin(math.radians(angle)),
    -g * math.cos(math.radians(angle))
))

Turbulence Models¶

Default LES (Large Eddy Simulation)¶

FDS uses LES by default:

# Default Deardorff model
sim.set_misc(turbulence_model='DEARDORFF')

DNS (Direct Numerical Simulation)¶

For very fine mesh simulations:

# Turn off turbulence model (DNS)
sim.set_misc(turbulence_model='DNS')

DNS Requirements

DNS requires extremely fine mesh resolution and is computationally expensive. Only use for small domains or academic studies.

Constant Smagorinsky¶

sim.set_misc(turbulence_model='CONSTANT SMAGORINSKY')

Simulation Control¶

Time Step Control¶

# Let FDS automatically determine time step (default)
sim.set_misc()  # CFL-based adaptive time step

# Limit maximum time step
sim.set_misc(dt_max=0.1)  # Maximum 0.1s time step

# Force constant time step (not recommended)
sim.set_misc(dt=0.01, constant_dt=True)

Restart Simulations¶

# Enable restart capability (save state every 100s)
sim.set_misc(restart=True, dt_restart=100.0)

# Continue from previous simulation
sim.set_misc(restart=True, restart_chid='previous_run')

Special Modes¶

Check Run (Geometry Only)¶

Validate geometry without running simulation:

sim.set_misc(check_geometry=True)

No Combustion¶

Disable combustion for heat transfer studies:

sim.set_misc(no_combustion=True)

Freeze Velocity Field¶

Fix velocity field (for testing):

sim.set_misc(freeze_velocity=True)

Stratification¶

Temperature Stratification¶

# Linear temperature profile
# T(z) = TMPA + DTDZ * z
sim.set_misc(
    tmpa=20.0,           # Temperature at z=0 (°C)
    stratification='T',   # Temperature stratification
    dtdz=0.5             # Temperature gradient (°C/m)
)

At z=0m: T = 20°C At z=10m: T = 20 + 0.5×10 = 25°C

Density Stratification¶

sim.set_misc(
    stratification='rho',  # Density stratification
    drhodz=-0.1            # Density gradient (kg/m³/m)
)

Radiation¶

Radiation Solution¶

# Enable radiation (default)
sim.set_misc(radiation=True)

# Increase radiation solver frequency
sim.set_misc(number_radiation_angles=100)

# Wide-band radiation model
sim.set_misc(wide_band_model=True)

Radiative Fraction¶

# Default radiative fraction for all fires
sim.set_misc(radiative_fraction=0.35)  # 35% radiated

Wind and Outdoor Simulations¶

Wind Speed and Direction¶

# 5 m/s wind in +X direction
sim.set_misc(wind_speed=5.0, wind_direction=0.0)

# Wind from the west (blowing toward +X)
sim.set_misc(wind_speed=3.0, wind_direction=270.0)

Atmospheric Profile¶

# Logarithmic wind profile
sim.set_misc(
    wind_speed=5.0,
    lapse_rate=0.01,      # Temperature lapse rate (K/m)
    roughness_length=0.1   # Surface roughness (m)
)

Complete Examples¶

Standard Indoor Fire¶

from pyfds import Simulation

sim = Simulation(chid='indoor_fire')
sim.add(Time(t_end=600.0))
sim.add(Mesh(ijk=Grid3D.of(60, 50, 25), xb=Bounds3D.of(0, 6, 0, 5, 0, 2.5)))

# Standard indoor conditions
sim.set_misc(
    tmpa=20.0,           # 20°C ambient
    humidity=50.0,       # 50% RH
    p_inf=101325.0,      # 1 atm
    radiation=True       # Include radiation
)

# Fire
sim.add(Surface(id='FIRE', hrrpua=1000.0))
sim.add(Obstruction(xb=Bounds3D.of(2.5, 3.5, 2, 3, 0, 0.1), surf_id='FIRE'))

sim.write('indoor_fire.fds')

Hot Environment¶

sim = Simulation(chid='hot_environment')
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)))

# Hot, dry conditions
sim.set_misc(
    tmpa=40.0,      # 40°C ambient (104°F)
    humidity=15.0,  # Low humidity
    p_inf=101325.0
)

# Fire
sim.add(Surface(id='FIRE', hrrpua=800.0))
sim.add(Obstruction(xb=Bounds3D.of(2, 3, 2, 3, 0, 0.1), surf_id='FIRE'))

sim.write('hot_environment.fds')

Outdoor Fire with Wind¶

sim = Simulation(chid='outdoor_wind')
sim.add(Time(t_end=600.0))
sim.add(Mesh(ijk=Grid3D.of(100, 100, 40), xb=Bounds3D.of(0, 50, 0, 50, 0, 20)))

# Outdoor conditions with wind
sim.set_misc(
    tmpa=25.0,
    humidity=60.0,
    wind_speed=5.0,        # 5 m/s wind
    wind_direction=270.0,  # From west
    roughness_length=0.1   # Grassland
)

# Open all boundaries
sim.add(Vent(mb='XMIN', surf_id='OPEN'))
sim.add(Vent(mb='XMAX', surf_id='OPEN'))
sim.add(Vent(mb='YMIN', surf_id='OPEN'))
sim.add(Vent(mb='YMAX', surf_id='OPEN'))
sim.add(Vent(mb='ZMAX', surf_id='OPEN'))

# Fire
sim.add(Surface(id='FIRE', hrrpua=500.0))
sim.add(Obstruction(xb=Bounds3D.of(22, 28, 22, 28, 0, 0.5), surf_id='FIRE'))

sim.write('outdoor_wind.fds')

Stratified Environment¶

sim = Simulation(chid='stratified')
sim.add(Time(t_end=600.0))
sim.add(Mesh(ijk=Grid3D.of(50, 50, 50), xb=Bounds3D.of(0, 5, 0, 5, 0, 5)))

# Temperature increases with height (stable stratification)
sim.set_misc(
    tmpa=20.0,            # 20°C at floor
    stratification='T',
    dtdz=2.0              # +2°C per meter
)
# At z=5m: T = 20 + 2*5 = 30°C

# Fire at floor level
sim.add(Surface(id='FIRE', hrrpua=1000.0))
sim.add(Obstruction(xb=Bounds3D.of(2, 3, 2, 3, 0, 0.1), surf_id='FIRE'))

sim.write('stratified.fds')

DNS Simulation (Small Domain)¶

sim = Simulation(chid='dns_test')
sim.add(Time(t_end=10.0))  # Short duration for DNS

# Very fine mesh for DNS
sim.add(Mesh(ijk=Grid3D.of(100, 100, 100), xb=Bounds3D.of(0, 0.5, 0, 0.5, 0, 0.5)))

# Direct numerical simulation (no turbulence model)
sim.set_misc(
    turbulence_model='DNS',
    tmpa=20.0
)

# Small fire
sim.add(Surface(id='FIRE', hrrpua=500.0))
sim.add(Obstruction(xb=Bounds3D.of(0.2, 0.3, 0.2, 0.3, 0, 0.01), surf_id='FIRE'))

sim.write('dns_test.fds')

Geometry Check Only¶

sim = Simulation(chid='geometry_check')
sim.add(Mesh(ijk=Grid3D.of(50, 40, 25), xb=Bounds3D.of(0, 5, 0, 4, 0, 2.5)))

# Check geometry without running simulation
sim.set_misc(check_geometry=True)

# Add complex geometry
sim.add(Obstruction(xb=Bounds3D.of(0, 0.2, 0, 4, 0, 2.5), surf_id='INERT'))
sim.add(Obstruction(xb=Bounds3D.of(2, 3, 1.5, 2.5, 0, 0.5), surf_id='INERT'))

sim.write('geometry_check.fds')

Common MISC Parameters¶

Parameter	Default	Units	Description
`tmpa`	20.0	°C	Ambient temperature
`p_inf`	101325.0	Pa	Ambient pressure
`humidity`	40.0	%	Relative humidity
`gvec`	(0, 0, -9.81)	m/s²	Gravity vector
`turbulence_model`	'DEARDORFF'	-	LES turbulence model
`radiation`	True	-	Enable radiation
`radiative_fraction`	varies	-	Default χ_r for fires
`wind_speed`	0.0	m/s	Wind speed
`wind_direction`	0.0	degrees	Wind direction
`stratification`	None	-	'T' or 'rho'
`dt_max`	varies	s	Maximum time step

Best Practices¶

1. Use Realistic Ambient Conditions¶

# Good: Realistic conditions
sim.set_misc(tmpa=20.0, humidity=50.0, p_inf=101325.0)

# Questionable: Extreme conditions without justification
sim.set_misc(tmpa=100.0, humidity=0.0)

2. Let FDS Control Time Step¶

# Preferred: Automatic adaptive time step
sim.set_misc()  # Uses CFL condition

# Avoid: Forcing constant time step
sim.set_misc(dt=0.01, constant_dt=True)  # Usually unnecessary

3. Document Special Modes¶

# Clear documentation for non-standard settings
# Simulating reduced gravity environment for research
sim.set_misc(
    gvec=(0.0, 0.0, -3.71),  # Mars gravity
    tmpa=-50.0                # Mars temperature
)

4. Check Geometry First¶

# For complex geometries, check before full run
sim.set_misc(check_geometry=True)
sim.write('check.fds')

# Then run full simulation
sim.set_misc(check_geometry=False)
sim.write('full_run.fds')

Common Issues¶

Simulation unstable

Cause: Time step too large or inappropriate settings

Solution: Limit maximum time step

sim.set_misc(dt_max=0.05)

Wind not affecting fire

Cause: Closed boundaries or insufficient domain size

Solution: Open boundaries and larger domain

# Open boundaries for wind flow
sim.add(Vent(mb='XMIN', surf_id='OPEN'))
sim.add(Vent(mb='XMAX', surf_id='OPEN'))
sim.set_misc(wind_speed=5.0, wind_direction=0.0)

Stratification not working

Cause: Wrong stratification parameter

Solution: Use correct stratification type

# Correct: Temperature stratification
sim.set_misc(stratification='T', dtdz=1.0)

# Wrong: Missing stratification type
sim.set_misc(dtdz=1.0)  # Won't work

Next Steps¶

Fire Sources - Configure fires with global radiative fraction
Boundaries - Wind boundary conditions
Initial Conditions - Set initial fields
Combustion - Fuel and reaction parameters

Initial Conditions →