Simple geometries

ℹ️ Updated to BFDS 7.0.x

This wiki page explains how the geometries of a Blender Object are exported to the FDS geometric namelists.

• The Blender Object and its shape
• The traditional FDS geometric namelists

The Blender Object and its shape

Each Blender Object represents a geometric entity in the Blender Scene 3d space.

While the object data-block acts as a local reference system and contains the item location, rotation, and scale, the Object shape is described by a related Blender Mesh data-block. The mesh is a collection of connected vertices, edges, and faces.

The traditional FDS geometric namelists

The so called geometric FDS namelists extend their effects to volumes, planes, faces, segments, or points in the FDS domain.

For example, this namelist generates a box-shaped obstacle in the fluid domain:

&OBST ID='Example obstacle' XB=-1.0,1.0,-1.0,1.0,-1.0,1.0, /

See the FDS User's Guide for further details.

The more common geometric FDS namelists are: DEVC, HOLE, INIT, MESH, OBST, PROF, SLCF, VENT, and ZONE. Those namelists always require one or more of the following geometric parameters: XB, XYZ, PBX, PBY, or PBZ.

BFDS manages those geometric parameters in the panel related to the active Object in the Properties editor, and takes care of exporting/importing the related namelists to FDS as shown in the following paragraphs.

The XB parameter

In FDS, the XB parameter is able to represent a parallelepiped parallel to the reference axis, a flat face perpendicular to one of the reference axis, or any segment in space.

You can select one of the following transformations for the active Object shape to the XB parameter of the exported namelist.

XB	Exported as	Example
Bounding Box	Export one namelist, a parallelepiped corresponding to the bounding box of the shape
Voxels	Export many namelists, stair-stepping approximation of the shape volume
Faces	Export many namelists, one per face of the shape, set parallel to reference planes
Edges	Export many namelists, one per edge of the shape.

If your shape is not an axis oriented box but something else, eg. a cylinder, you can choose the Voxels transformation, that approximates the round shape with many smaller axis oriented boxes.

When stair-stepping the shape volume, the size of the voxel is taken from the Voxel/Pixel Size setting on the FDS Case Config panel. The resolution can be also customized for each object by filling the appropriate setting Custom Voxel/Pixel Size under the XB field.

The Center Voxels/Pixels setting centers the stair-stepping algorithm to the object, instead of the default alignment with other objects voxels. The default alignment prevents the formation of holes between the stair-stepped shapes. The voxel centering improves the fidelity of the stair-stepped geometry to the original shape, eg. for creating a symmetric stair-stepped sphere.

The IDs Suffix setting adds different kinds of suffixes to the object ID parameter, when generating multiple namelists from the same object.

The XYZ parameter

In FDS, the XYZ parameter is able to represent a point in space. BFDS proposes two different transformations for exporting Object shapes to the XYZ parameter.

XYZ	Exported as	Example
Vertices	Export many namelists, one per shape vertex
Center	Export one namelist, center point of the object	None

For example, if you wish to create a line of thermocouples:

• add a new an object,
• set its namelist to DEVC and XYZ parameter to Vertices,
• then create the vertices in the appropriate locations.

By setting the IDs Suffix to z you would obtain these DEVC namelists with nice ID parameters:

! XYZ Vertices: 7
&DEVC ID='TC_z+0.500' XYZ=1.500000,1.500000,0.500000 QUANTITY='THERMOCOUPLE' /
&DEVC ID='TC_z+0.700' XYZ=1.500000,1.500000,0.700000 QUANTITY='THERMOCOUPLE' /
&DEVC ID='TC_z+0.900' XYZ=1.500000,1.500000,0.900000 QUANTITY='THERMOCOUPLE' /

The PBX, PBY, and PBZ parameters

In FDS, the PB* parameters represents a plane perpendicular to one of the reference axis. BFDS proposes one only transformation of the Object shape to the PB* parameters.

PB*	Exported as	Example
Planes	Export many namelists, one per face of the shape, set parallel to reference planes

For example, if you need to generate several SLCF slice files:

• add a new an object,
• set its namelist to SLCF and PB* parameter to Planes,
• then create the faces in the appropriate locations.

The faces you create in the object are transformed into planes, parallel to reference planes.

Here is an example of the SLCF namelists you could obtain:

! PB* Planes: 2
&SLCF ID='Temp slice_x+0.000' PBX=0.000000 QUANTITY='TEMPERATURE' VECTOR=T CELL_CENTERED=T /
&SLCF ID='Temp slice_y-0.000' PBY=-0.000000 QUANTITY='TEMPERATURE' VECTOR=T CELL_CENTERED=T /