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Cuboid components

Cuboid components describe axis-aligned boxes directly in the computational domain’s coordinates. They can occupy a solid volume or form a zero-thickness surface. Both use the same rectilinear mesh as the surrounding fluid and solids.

Position and dimensions

bbox.origin gives the minimum [x, y, z] corner and bbox.size gives the dimensions in meters. The opposite corner is origin + size. Faces are named x_min, x_max, y_min, y_max, z_min, and z_max in the same coordinate system.

A cuboid has no rotation parameter. For more detailed geometry and rotated components use CAD voxelization.

Volumes and surfaces

A volumetric component has three positive dimensions. Its entire interior becomes solid and displaces fluid, regardless of the thermal model. A single cuboid therefore cannot represent a hollow enclosure; separate walls must surround the space that remains fluid.

A surface component has exactly one zero dimension and two positive dimensions. It occupies mesh faces without occupying volume cells. A surface can act as a thin airflow baffle or add thermal resistance at a material interface without requiring cells through its thickness.

Volumetric and surface cuboids in a rectilinear mesh Two mesh cross-sections compare a volumetric cuboid, which makes a central group of cells solid, with a zero-thickness surface, which occupies a line of shared cell faces while retaining cells on both sides. Volumetric cuboid Surface cuboid Solid Fluid Fluid Fluid Interior cells become solid Cells remain on both sides
Cross-sections through the shared mesh. A volumetric cuboid assigns solid cells; a surface cuboid acts on the highlighted faces while retaining cells on both sides.

For a surface with zero size along the z-axis, z_min and z_max identify its lower and upper sides at the same z coordinate. The other four faces have zero area and cannot carry boundary conditions.

Thermal behavior

The thermal model determines how the component participates in heat transfer:

GeometryThermal modelThermal representation
VolumeinsulatingInterior excluded from the temperature solve; interfaces default to adiabatic.
VolumeconductingTemperature solved throughout the solid, including heat spreading and any specified power.
Volumetwo_resistor_ctm or star_resistor_ctmInterior replaced by a junction node and thermal-resistance paths to selected faces.
SurfaceinsulatingHeat transfer blocked across the sheet by default.
Surfaceconducting_surfaceResistance added to heat flow normal to the sheet.

Both surface models form impermeable, no-slip walls wherever they touch fluid. A conducting surface provides a resistive thermal connection or modifies an existing thermal boundary. Surfaces do not support volumetric conducting or compact thermal models.

Boundary conditions

The component’s boundary_conditions list assigns conditions to selected faces or patches. Each entry selects a side and a full, rectangle, circle, or annulus region; see Select a boundary region. Multiple patches can share a face, but their areas must not overlap. Conditions act on the cells immediately outside the component.

The permitted conditions depend on the component model:

  • Insulating volumes accept flow and thermal conditions. Flow patches must face fluid throughout; thermal patches must face fluid or conducting cells. Unconfigured areas remain no-slip walls beside fluid and adiabatic beside thermally active cells.
  • Insulating surfaces accept only fixed_temp, heat_flux, and fixed_htc, on their two faces with nonzero area. Each patch must face fluid or conducting cells. Unconfigured areas remain adiabatic. Inlets, outlets, fans, and radiative conditions are not permitted; both sides remain impermeable, no-slip walls beside fluid.
  • Conducting volumes and compact thermal models do not accept explicit boundary_conditions; their thermal model determines the interface behavior.
  • Conducting surfaces add resistance to an existing thermal boundary. Define that condition on the underlying domain face or insulating component; see Surfaces over thermal boundaries.

Contacts and mesh representation

Cuboid faces contribute fixed coordinates to the rectilinear mesh, fitting the mesh to their geometry. Nearby coordinates can still move when mesh planes are fused, and small components can be culled. Body fitting therefore does not remove the need to check the final mesh.

Volumetric components may touch but cannot overlap in volume. Surface components may meet along edges or corners and may cross orthogonally, but coplanar surfaces cannot overlap over an area. The overlap check uses their meshed rather than user-defined positions, which includes changes caused by fusing. A surface also cannot cover an existing fluid inlet, outlet, or other non-wall fluid boundary.

Surface components have volumetric representation in the simulation outputs. A conducting surface’s model thickness changes its thermal resistance without changing the bounding box or creating a geometric gap.

See also