Boundary Conditions¶

Configure vents, openings, and boundary conditions for your FDS simulations.

Overview¶

Boundary conditions are specified using the VENT namelist. Vents define openings, HVAC systems, external boundaries, and pressure zones.

from pyfds import Vent
from pyfds.core.geometry import Bounds3D

# Open door
sim.add(Vent(xb=Bounds3D.of(5, 5, 2, 3, 0, 2.1), surf_id='OPEN'))

# HVAC supply vent
sim.add(Vent(xb=Bounds3D.of(2, 2.5, 2, 2.5, 3, 3), surf_id='HVAC', volume_flow=0.5))

Open Boundaries¶

Doors and Windows¶

Open boundaries to ambient conditions:

# Door: 1m wide × 2.1m tall at X=5
sim.add(Vent(xb=Bounds3D.of(5, 5, 2, 3, 0, 2.1), surf_id='OPEN'))

# Window: 1.5m wide × 1m tall
sim.add(Vent(xb=Bounds3D.of(0, 0, 1, 2.5, 1.5, 2.5), surf_id='OPEN'))

Zero-Thickness Vents

Vents have zero thickness - one coordinate dimension has equal min/max values.

Open Mesh Boundaries¶

Open the edges of the computational domain:

# Open all horizontal boundaries
sim.add(Vent(mb='XMIN', surf_id='OPEN')  # -X face
sim.add(Vent(mb='XMAX', surf_id='OPEN')  # +X face
sim.add(Vent(mb='YMIN', surf_id='OPEN')  # -Y face
sim.add(Vent(mb='YMAX', surf_id='OPEN')  # +Y face

# Top open to atmosphere
sim.add(Vent(mb='ZMAX', surf_id='OPEN'))

Mesh boundary options: XMIN, XMAX, YMIN, YMAX, ZMIN, ZMAX

HVAC Systems¶

Supply and Exhaust Vents¶

# Set ambient conditions
sim.set_misc(tmpa=22.0, humidity=50.0)

# Supply vent: 0.5 m³/s inflow
sim.add(Vent(
    xb=Bounds3D.of(2, 2.5, 2, 2.5, 3, 3),
    surf_id='HVAC',
    volume_flow=0.5  # Positive = supply
)

# Exhaust vent: -0.4 m³/s outflow
sim.add(Vent(
    xb=Bounds3D.of(4, 4.5, 4, 4.5, 3, 3),
    surf_id='HVAC',
    volume_flow=-0.4  # Negative = exhaust
)

Velocity-Specified Vents¶

# Supply vent with specific velocity
sim.add(Surface(id='SUPPLY', vel=2.0, tmp_front=15.0)  # 2 m/s, 15°C
sim.add(Vent(xb=Bounds3D.of(2, 2.5, 2, 2.5, 3, 3), surf_id='SUPPLY'))

Circular and Annular Vents¶

Circular Burner or Vent¶

# 1m diameter circular burner
sim.add(Surface(id='FIRE', hrrpua=1000.0))
sim.add(Vent(
    xb=Bounds3D.of(-2, 2, -2, 2, 0, 0),    # Bounding box
    surf_id='FIRE',
    xyz=Point3D.of(0, 0, 0),               # Center point
    radius=0.5                   # Radius in meters
)

The xb defines the bounding box, but only the circular area defined by xyz and radius is used.

Annular (Ring-Shaped) Vent¶

# Ring burner: outer radius 1.5m, inner radius 0.5m
sim.add(Vent(
    xb=Bounds3D.of(-2, 2, -2, 2, 0, 0),
    surf_id='FIRE',
    xyz=Point3D.of(0, 0, 0),
    radius=1.5,
    radius_inner=0.5  # Creates a ring
)

Special Boundary Types¶

Mirror Boundaries (Symmetry)¶

Use symmetry to model half or quarter domains:

# Mirror at X=0 plane (model half domain)
sim.add(Vent(mb='XMIN', surf_id='MIRROR'))

# Quarter domain
sim.add(Vent(mb='XMIN', surf_id='MIRROR'))
sim.add(Vent(mb='YMIN', surf_id='MIRROR'))

Periodic Boundaries¶

For repeating patterns:

# Periodic in X direction
sim.add(Vent(mb='XMIN', surf_id='PERIODIC'))
sim.add(Vent(mb='XMAX', surf_id='PERIODIC'))

Solid Walls¶

Default boundaries are solid walls. Make explicit if needed:

sim.add(Vent(mb='ZMIN', surf_id='INERT')  # Solid floor

Pressure Zones¶

Leak Areas¶

Model pressure zones with leak areas:

# Compartment with leak area
sim.add(Vent(
    xb=Bounds3D.of(2, 3, 2, 3, 2.5, 2.5),
    surf_id='OPEN',
    leak_area=0.05  # m²
)

Pressure Zones¶

Define pressure zones for compartments:

# Create pressure zone
sim.add(Vent(
    xb=Bounds3D.of(0, 5, 0, 5, 2.5, 2.5),
    surf_id='OPEN',
    pressure_zone='ROOM_1'
)

Temperature Boundary Conditions¶

Fixed Temperature Surface¶

# Hot wall at 200°C
sim.add(Surface(id='HOT_WALL', tmp_front=200.0))
sim.add(Vent(xb=Bounds3D.of(0, 0, 0, 5, 0, 2.5), surf_id='HOT_WALL'))

# Cold surface at 5°C
sim.add(Surface(id='COLD', tmp_front=5.0))
sim.add(Vent(xb=Bounds3D.of(5, 5, 0, 5, 0, 2.5), surf_id='COLD'))

Complete Examples¶

Room with Door¶

from pyfds import Simulation

sim = Simulation(chid='room_door')
sim.add(Time(ime(t_end=300.0)))
sim.add(Mesh(ijk=Grid3D.of(50, 40, 25), xb=Bounds3D.of(0, 5, 0, 4, 0, 2.5)))

# Fire
sim.add(Surface(id='FIRE', hrrpua=1000.0))
sim.add(Obstruction(xb=Bounds3D.of(2, 3, 1.5, 2.5, 0, 0.1), surf_id='FIRE'))

# Door opening (1m wide × 2.1m tall)
sim.add(Vent(xb=Bounds3D.of(5, 5, 1.5, 2.5, 0, 2.1), surf_id='OPEN'))

sim.write('room_door.fds')

HVAC Room¶

sim = Simulation(chid='hvac_room')
sim.add(Time(ime(t_end=600.0)))
sim.add(Mesh(ijk=Grid3D.of(60, 60, 30), xb=Bounds3D.of(0, 6, 0, 6, 0, 3)))

# Ambient conditions
sim.set_misc(tmpa=22.0, humidity=50.0)

# Fire
sim.add(Surface(id='FIRE', hrrpua=800.0))
sim.add(Obstruction(xb=Bounds3D.of(2.5, 3.5, 2.5, 3.5, 0, 0.1), surf_id='FIRE'))

# Supply vent at ceiling (0.5 m³/s)
sim.add(Vent(xb=Bounds3D.of(1, 1.5, 1, 1.5, 3, 3), surf_id='HVAC', volume_flow=0.5))

# Exhaust vent at ceiling (-0.4 m³/s)
sim.add(Vent(xb=Bounds3D.of(4.5, 5, 4.5, 5, 3, 3), surf_id='HVAC', volume_flow=-0.4))

# Door to outside
sim.add(Vent(xb=Bounds3D.of(6, 6, 2.5, 3.5, 0, 2.1), surf_id='OPEN'))

sim.write('hvac_room.fds')

Outdoor Fire with Open Boundaries¶

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

# Open all sides to atmosphere
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 source
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_fire.fds')

Circular Burner¶

sim = Simulation(chid='circular_burner')
sim.add(Time(ime(t_end=300.0)))
sim.add(Mesh(ijk=Grid3D.of(60, 60, 30), xb=Bounds3D.of(-3, 3, -3, 3, 0, 3)))

# 1m diameter circular fire
sim.add(Surface(id='BURNER', hrrpua=1500.0))
sim.add(Vent(
    xb=Bounds3D.of(-2, 2, -2, 2, 0, 0),
    surf_id='BURNER',
    xyz=Point3D.of(0, 0, 0),
    radius=0.5  # 0.5m radius = 1m diameter
)

sim.write('circular_burner.fds')

Best Practices¶

1. Zero-Thickness Requirement¶

Vents must be zero-thickness (one dimension has min=max):

# Correct: X dimension has equal values (5, 5)
sim.add(Vent(xb=Bounds3D.of(5, 5, 2, 3, 0, 2.1), surf_id='OPEN'))

# Incorrect: All dimensions have different min/max
sim.add(Vent(xb=Bounds3D.of(5, 5.1, 2, 3, 0, 2.1), surf_id='OPEN')  # Wrong!

2. Align with Mesh Faces¶

Align vents with mesh cell faces when possible:

# Mesh with 0.1m cells
sim.add(Mesh(xb=Bounds3D.of(0, 5, 0, 5, 0, 2.5)))

# Good: Aligned with mesh edge
sim.add(Vent(xb=Bounds3D.of(5, 5, 2, 3, 0, 2.1), surf_id='OPEN'))

# Less ideal: Not aligned
sim.add(Vent(xb=Bounds3D.of(4.87, 4.87, 2, 3, 0, 2.1), surf_id='OPEN'))

3. HVAC Volume Flow Conservation¶

Total inflow should approximately equal total outflow:

# Supply: 0.5 m³/s in
sim.add(Vent(xb=Bounds3D.of(1, 1.5, 1, 1.5, 3, 3), surf_id='HVAC', volume_flow=0.5))

# Exhaust: -0.4 m³/s out
sim.add(Vent(xb=Bounds3D.of(4, 4.5, 4, 4.5, 3, 3), surf_id='HVAC', volume_flow=-0.4))

# Door: makes up difference
sim.add(Vent(xb=Bounds3D.of(5, 5, 2, 3, 0, 2.1), surf_id='OPEN'))

4. Document Boundary Conditions¶

# Clear documentation
# Door: 1m wide × 2.1m tall at east wall
# Provides primary ventilation path
sim.add(Vent(xb=Bounds3D.of(5, 5, 2, 3, 0, 2.1), surf_id='OPEN'))

Common Issues¶

Vent not opening

Cause: Wrong SURF_ID or geometry issue

Solution: Check surface and coordinates

# Ensure OPEN surface or define custom
sim.add(Vent(xb=Bounds3D.of(5, 5, 2, 3, 0, 2.1), surf_id='OPEN'))

HVAC not working

Cause: Missing HVAC surface or wrong volume_flow

Solution: Verify HVAC surface and flow direction

# Positive = inflow, Negative = outflow
sim.add(Vent(xb=Bounds3D.of(2, 2.5, 2, 2.5, 3, 3), surf_id='HVAC', volume_flow=0.5))

Circular vent not circular

Cause: Missing xyz or radius parameters

Solution: Include center and radius

sim.add(Vent(
    xb=Bounds3D.of(-1, 1, -1, 1, 0, 0),
    surf_id='FIRE',
    xyz=Point3D.of(0, 0, 0),      # Required for circular
    radius=0.5          # Required for circular
)

Next Steps¶

Materials & Surfaces - Surface properties
Global Settings - MISC parameters
Examples - HVAC and ventilation examples

Materials & Surfaces →