BPS5234A Chap.5 Geometry Modelling and Clean-Up
Geometry Modelling and Clean-Up
The most time-consuming part of the job
The geometry lecture estimates that preparing and cleaning the model takes about seventy per cent of the time on a CFD study. A model must keep the features that govern flow and heat transfer, mesh robustly, and avoid detail that adds cells without adding accuracy.
Geometry can be built top-down from primitive solids, bottom-up from points, edges and faces, or imported from CAD and BIM, often through the open IFC exchange format.
Why imported models fail
BIM models are built for design and construction, not for simulation. They arrive with door handles and railings, window and door templates that leave small gaps, beams and slabs that clash, and faces that do not join.
The course lists the sources plainly: human error, including judging integrity by looking at the model, template defects in the authoring software, and meshers that differ in whether they tolerate small gaps. Collaboration between the BIM modeller and the simulation specialist is the long-term remedy.
From a clean solid to a fluid domain
The solver needs the air.
External studies draw an enclosure around the buildings and subtract them; internal studies extract the enclosed fluid volume. Shared topology makes meshes meet conformally where bodies touch, and named selections give every boundary surface a short, meaningful name so conditions can be assigned without guesswork.
The industry guest lecture adds practical rules for naming, including naming fans and air-handling units from the equipment’s own point of view.
What to be able to do
Expect to diagnose defects from symptoms, choose simplifications that keep the physics, and explain why a fluid domain must be extracted and named before meshing.
What this chapter covers
- 01
Three requirements for a CFD-ready geometry
- 02
Top-down, bottom-up and BIM-imported models
- 03
Defects in imported building models and their fixes
- 04
Defeaturing thin solids, gaps and small features
- 05
Volume extraction, enclosures and shared topology
- 06
Named selections that carry boundary conditions
Worked example · free
Naming the boundaries of a cross-ventilated room
- 2List every surface that will carry its own boundary condition.
- 1Explain the equipment naming rule.
- 1State how the closed door and the room volume are handled.
Key terms
- Defeaturing
- Removing or simplifying geometric features much smaller than the local mesh size that do not influence the flow.
- Watertight Model
- A geometry whose surfaces close completely, so a fluid volume can be extracted without leaks.
- Shared Topology
- Shared faces, edges and vertices between touching bodies, which allows a conformal mesh across their interface.
- Named Selection
- A labelled group of surfaces that carries a boundary condition from geometry through meshing to the solver.
- Enclosure
- A box, cylinder or sphere drawn around buildings to represent the surrounding air in an external study.
- Industry Foundation Classes
- An open BIM exchange standard through which building geometry can be passed to simulation tools.
Geometry Modelling and Clean-Up FAQ
Why not import the BIM model directly into the solver?
Because it carries construction-level detail, template gaps and clashing parts that either break meshing or create huge numbers of tiny cells. The course’s best practice is to start from the simplest geometry, add complexity incrementally, replace thin objects with surfaces or porous models, and verify watertightness before meshing.
How should a perforated screen or louvre bank be represented?
As a two-dimensional surface or a porous boundary that carries the right pressure drop, rather than as every blade or hole. That keeps the effect on room-scale flow while avoiding a cell count driven by small features.
What is the risk of fixing every geometry issue automatically at once?
Repair tools can create invalid patches while fixing many locations simultaneously. The lecture advises double-checking automatically fixed locations and validating the result with the geometry check tool.
Why does shared topology matter for a multi-zone model?
Only shared topology gives a conformal mesh where bodies meet, so that cells line up across the interface between, for example, a room and a ceiling void. Merging bodies into one solid is not equivalent and meshes differently.
What happens if the fluid domain is created without subtracting the buildings?
Both the solid and fluid regions are passed to the solver, so the buildings are meshed as air or as an unintended zone. The lecture lists this as a classic error to avoid.
Assessment move
Before each assignment, keep a geometry log: what was removed, what was simplified and why, and which surfaces were named. The log becomes the geometry section of the report and forces each simplification to be justified against the objective.
