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Geometry

Learn how to create walls, obstructions, and solid objects in your FDS simulations.

Overview

Geometry in FDS is created using obstructions - solid rectangular blocks that can represent walls, furniture, equipment, and other objects.

from pyfds.core.namelists import Obstruction
from pyfds.core.geometry import Bounds3D

sim.add(Obstruction(
    xb=Bounds3D.of(0, 0.2, 0, 5, 0, 2.5),  # Coordinates
    surf_id='WALL'               # Surface properties
))

Creating Obstructions

Basic Obstruction

from pyfds import Simulation
from pyfds.core.namelists import Mesh, Obstruction
from pyfds.core.geometry import Bounds3D, Grid3D

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

# Create a wall (0.2m thick)
sim.add(Obstruction(
    xb=Bounds3D.of(0, 0.2, 0, 5, 0, 2.5),
    surf_ids=('CONCRETE_WALL', 'INERT', 'INERT')
)

Coordinate Specification

Coordinates define a rectangular box:

xb = (xmin, xmax, ymin, ymax, zmin, zmax)

Example - 1m × 1m × 0.5m box: 

sim.add(Obstruction(xb=Bounds3D.of(1.0, 2.0, 1.0, 2.0, 0.0, 0.5)))

Common Geometry Patterns

Walls

Four Room Walls

# 5m × 5m × 2.5m room with 0.2m thick walls
wall_thickness = 0.2

# West wall (low X)
sim.add(Obstruction(xb=Bounds3D.of(0, wall_thickness, 0, 5, 0, 2.5), surf_ids=('WALL', 'INERT', 'INERT')))

# East wall (high X)
sim.add(Obstruction(xb=Bounds3D.of(5-wall_thickness, 5, 0, 5, 0, 2.5), surf_ids=('WALL', 'INERT', 'INERT')))

# South wall (low Y)
sim.add(Obstruction(xb=Bounds3D.of(0, 5, 0, wall_thickness, 0, 2.5), surf_ids=('WALL', 'INERT', 'INERT')))

# North wall (high Y)
sim.add(Obstruction(xb=Bounds3D.of(0, 5, 5-wall_thickness, 5, 0, 2.5), surf_ids=('WALL', 'INERT', 'INERT')))

Wall with Door Opening

# Wall with 1m door centered at Y=2.5, 2.1m tall
wall_thickness = 0.2
door_width = 1.0
door_height = 2.1
door_center_y = 2.5

# Left part of wall (below door center - half width)
sim.add(Obstruction(
    xb=Bounds3D.of(5-wall_thickness, 5, 0, door_center_y - door_width/2, 0, 2.5),
    surf_ids=('WALL', 'INERT', 'INERT')
)

# Right part of wall (above door center + half width)
sim.add(Obstruction(
    xb=Bounds3D.of(5-wall_thickness, 5, door_center_y + door_width/2, 5, 0, 2.5),
    surf_ids=('WALL', 'INERT', 'INERT')
)

# Top part of wall (above door)
sim.add(Obstruction(
    xb=Bounds3D.of(5-wall_thickness, 5, door_center_y - door_width/2, door_center_y + door_width/2, door_height, 2.5),
    surf_ids=('WALL', 'INERT', 'INERT')
)

Floors and Ceilings

# Floor (thin obstruction at Z=0)
sim.add(Obstruction(xb=Bounds3D.of(0, 5, 0, 5, 0, 0.01), surf_ids=('FLOOR', 'INERT', 'INERT')))

# Ceiling (at top of domain)
sim.add(Obstruction(xb=Bounds3D.of(0, 5, 0, 5, 2.49, 2.5), surf_ids=('CEILING', 'INERT', 'INERT')))

Furniture and Objects

# Table: 1.5m × 0.8m × 0.8m tall
sim.add(Obstruction(xb=Bounds3D.of(2.0, 3.5, 1.5, 2.3, 0.0, 0.8), surf_ids=('WOOD', 'INERT', 'INERT')))

# Chair: 0.5m × 0.5m × 0.5m
sim.add(Obstruction(xb=Bounds3D.of(1.5, 2.0, 1.5, 2.0, 0.0, 0.5), surf_ids=('PLASTIC', 'INERT', 'INERT')))

# Cabinet: 1m × 0.5m × 2m tall
sim.add(Obstruction(xb=Bounds3D.of(0.2, 1.2, 4.5, 5.0, 0.0, 2.0), surf_ids=('METAL', 'INERT', 'INERT')))

Columns

# Square column: 0.3m × 0.3m, full height
sim.add(Obstruction(xb=Bounds3D.of(2.35, 2.65, 2.35, 2.65, 0.0, 2.5), surf_ids=('CONCRETE', 'INERT', 'INERT')))

# Multiple columns in a row
for x in [1.0, 2.0, 3.0, 4.0]:
    sim.add(Obstruction(xb=Bounds3D.of(x-0.15, x+0.15, 2.35, 2.65, 0.0, 2.5), surf_ids=('CONCRETE', 'INERT', 'INERT')))

Advanced Geometry

Parametric Geometry

Use loops and functions for complex geometry:

def create_bookshelf(sim, x, y, z, width, depth, height, shelves):
    """Create a bookshelf with multiple shelves."""
    shelf_thickness = 0.02

    # Back and sides
    sim.add(Obstruction(xb=Bounds3D.of(x, x+width, y+depth-0.02, y+depth, z, z+height), surf_ids=('WOOD', 'INERT', 'INERT')))
    sim.add(Obstruction(xb=Bounds3D.of(x, x+0.02, y, y+depth, z, z+height), surf_ids=('WOOD', 'INERT', 'INERT')))
    sim.add(Obstruction(xb=Bounds3D.of(x+width-0.02, x+width, y, y+depth, z, z+height), surf_ids=('WOOD', 'INERT', 'INERT')))

    # Shelves
    shelf_spacing = height / shelves
    for i in range(shelves + 1):
        shelf_z = z + i * shelf_spacing
        sim.add(Obstruction(
            xb=Bounds3D.of(x, x+width, y, y+depth, shelf_z, shelf_z+shelf_thickness),
            surf_ids=('WOOD', 'INERT', 'INERT')
        )

# Create bookshelves
create_bookshelf(sim, x=0.5, y=4.5, z=0.0, width=1.0, depth=0.3, height=2.0, shelves=4)
create_bookshelf(sim, x=2.0, y=4.5, z=0.0, width=1.0, depth=0.3, height=2.0, shelves=4)

Arrays of Objects

# Grid of cubes
cube_size = 0.3
spacing = 0.5

for i in range(5):
    for j in range(5):
        x0 = 0.5 + i * spacing
        y0 = 0.5 + j * spacing
        sim.add(Obstruction(
            xb=Bounds3D.of(x0, x0+cube_size, y0, y0+cube_size, 0, cube_size),
            surf_ids=('BOX', 'INERT', 'INERT')
        )

Stepped Geometry

# Stairs: 10 steps, 0.3m wide, 0.2m rise
step_width = 0.3
step_rise = 0.2

for i in range(10):
    x0 = 1.0 + i * step_width
    z0 = i * step_rise
    sim.add(Obstruction(
        xb=Bounds3D.of(x0, x0+step_width, 1.0, 2.0, 0, z0+step_rise),
        surf_ids=('CONCRETE', 'INERT', 'INERT')
    )

Surface Assignment

Default Surface

All obstructions have a surface. If not specified, FDS uses 'INERT':

# Uses default 'INERT' surface
sim.add(Obstruction(xb=Bounds3D.of(0, 1, 0, 1, 0, 1)))

# Explicit surface
sim.add(Obstruction(xb=Bounds3D.of(0, 1, 0, 1, 0, 1), surf_ids=('CONCRETE', 'INERT', 'INERT')))

Different Surfaces on Each Face

Assign different surfaces to each face:

# Box with fire on top, concrete on other faces
sim.add(Obstruction(
    xb=Bounds3D.of(2, 3, 2, 3, 0, 0.5),
    surf_id='CONCRETE',      # Default for all faces
    surf_ids=['CONCRETE', 'CONCRETE', 'CONCRETE',
              'CONCRETE', 'CONCRETE', 'FIRE']  # Override top face
)

Face order: -X, +X, -Y, +Y, -Z, +Z (west, east, south, north, bottom, top)

Holes and Cutouts

Creating Holes

Use HOLE namelist (via obstruction with hole=True):

# Create wall
sim.add(Obstruction(xb=Bounds3D.of(5, 5.2, 0, 5, 0, 3), surf_ids=('WALL', 'INERT', 'INERT')))

# Cut window in wall
sim.add(Obstruction(xb=Bounds3D.of(5, 5.2, 1.5, 2.5, 1.5, 2.5), hole=True))

Holes

Holes remove geometry. They must overlap existing obstructions exactly.

Best Practices

1. Align with Mesh

Align geometry with cell boundaries when possible:

# Mesh has 0.1m cells
sim.add(Mesh(ijk=Grid3D.of(50, 50, 25), xb=Bounds3D.of(0, 5, 0, 5, 0, 2.5)))

# Good - aligns with cells
sim.add(Obstruction(xb=Bounds3D.of(1.0, 2.0, 1.0, 2.0, 0.0, 0.5)))

# Less ideal - doesn't align
sim.add(Obstruction(xb=Bounds3D.of(1.07, 2.13, 1.05, 1.98, 0.02, 0.47)))

2. Avoid Tiny Gaps

Don't leave tiny gaps between obstructions:

# Bad - tiny 0.01m gap
sim.add(Obstruction(xb=Bounds3D.of(0, 2.49, 0, 5, 0, 2.5)))
sim.add(Obstruction(xb=Bounds3D.of(2.51, 5, 0, 5, 0, 2.5)))

# Good - touching or overlapping
sim.add(Obstruction(xb=Bounds3D.of(0, 2.5, 0, 5, 0, 2.5)))
sim.add(Obstruction(xb=Bounds3D.of(2.5, 5, 0, 5, 0, 2.5)))

3. Organize by Type

Group related geometry:

# ============================================================
# WALLS
# ============================================================
sim.add(Obstruction(xb=Bounds3D.of(0, 0.2, 0, 5, 0, 2.5), surf_ids=('WALL', 'INERT', 'INERT')))
sim.add(Obstruction(xb=Bounds3D.of(4.8, 5, 0, 5, 0, 2.5), surf_ids=('WALL', 'INERT', 'INERT')))

# ============================================================
# FURNITURE
# ============================================================
sim.add(Obstruction(xb=Bounds3D.of(1, 2, 1, 2, 0, 0.8), surf_ids=('TABLE', 'INERT', 'INERT')))
sim.add(Obstruction(xb=Bounds3D.of(1, 1.5, 1, 1.5, 0, 0.5), surf_ids=('CHAIR', 'INERT', 'INERT')))

# ============================================================
# FIRE SOURCES
# ============================================================
sim.add(Obstruction(xb=Bounds3D.of(2.5, 3.5, 2.5, 3.5, 0, 0.1), surf_ids=('BURNER', 'INERT', 'INERT')))

4. Use Helper Functions

Create reusable functions for common patterns:

def add_rectangular_room(sim, xb, wall_thickness, surf_id='WALL'):
    """Add walls for a rectangular room."""
    xmin, xmax, ymin, ymax, zmin, zmax = xb
    t = wall_thickness

    # Four walls
    sim.add(Obstruction(xb=Bounds3D.of(xmin, xmin+t, ymin, ymax, zmin, zmax), surf_ids=(surf_id, 'INERT', 'INERT')))
    sim.add(Obstruction(xb=Bounds3D.of(xmax-t, xmax, ymin, ymax, zmin, zmax), surf_ids=(surf_id, 'INERT', 'INERT')))
    sim.add(Obstruction(xb=Bounds3D.of(xmin, xmax, ymin, ymin+t, zmin, zmax), surf_ids=(surf_id, 'INERT', 'INERT')))
    sim.add(Obstruction(xb=Bounds3D.of(xmin, xmax, ymax-t, ymax, zmin, zmax), surf_ids=(surf_id, 'INERT', 'INERT')))

# Use it
add_rectangular_room(sim, (0, 5, 0, 5, 0, 2.5), wall_thickness=0.2)

Common Issues

Obstruction outside mesh

Cause: Coordinates extend beyond mesh bounds

Solution: Check mesh and obstruction bounds 

# Mesh: 0 to 5
sim.add(Mesh(xb=Bounds3D.of(0, 5, 0, 5, 0, 2.5)))

# Bad: extends to 5.5
sim.add(Obstruction(xb=Bounds3D.of(4, 5.5, 0, 5, 0, 2.5)))

# Good: within bounds
sim.add(Obstruction(xb=Bounds3D.of(4, 5, 0, 5, 0, 2.5)))

Surface ID not found

Cause: Referenced surface before defining it

Solution: Define surfaces before use 

# Bad - surface not defined
sim.add(Obstruction(xb=Bounds3D.of(0, 1, 0, 1, 0, 1), surf_ids=('WOOD', 'INERT', 'INERT')))

# Good - define first
sim.add(Surface(id='WOOD', matl_id='PINE'))
sim.add(Obstruction(xb=Bounds3D.of(0, 1, 0, 1, 0, 1), surf_ids=('WOOD', 'INERT', 'INERT')))

Overlapping geometry

Cause: Multiple obstructions in same space

Solution: FDS handles overlaps, but avoid if unintentional 

# Overlapping (may be intentional or error)
sim.add(Obstruction(xb=Bounds3D.of(1, 2, 1, 2, 0, 1)))
sim.add(Obstruction(xb=Bounds3D.of(1.5, 2.5, 1, 2, 0, 1))  # Overlaps first

Complete Examples

Simple Room

from pyfds import Simulation
from pyfds.core.namelists import Mesh, Vent
from pyfds.core.geometry import Bounds3D, Grid3D

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

# Walls
add_rectangular_room(sim, (0, 5, 0, 5, 0, 2.5), wall_thickness=0.2)

# Door opening (use vent, covered in next guide)
sim.add(Vent(xb=Bounds3D.of(5, 5, 2, 3, 0, 2.1), surf_id='OPEN'))

Office Space

from pyfds import Simulation
from pyfds.core.namelists import Mesh, Obstruction
from pyfds.core.geometry import Bounds3D, Grid3D

sim = Simulation(chid='office')
sim.add(Mesh(ijk=Grid3D.of(80, 60, 30), xb=Bounds3D.of(0, 8, 0, 6, 0, 3)))

# Walls
add_rectangular_room(sim, (0, 8, 0, 6, 0, 3), wall_thickness=0.2)

# Desk
sim.add(Obstruction(xb=Bounds3D.of(1, 2.5, 1, 2, 0, 0.75), surf_ids=('WOOD', 'INERT', 'INERT')))

# Filing cabinet
sim.add(Obstruction(xb=Bounds3D.of(0.5, 1, 5, 5.5, 0, 1.2), surf_ids=('METAL', 'INERT', 'INERT')))

# Bookshelf
create_bookshelf(sim, x=7, y=5.5, z=0, width=0.8, depth=0.3, height=2, shelves=5)

Next Steps

Boundary Conditions → Back to Guide →