What this step does
Units. The coordinates are converted with a unit the user confirms (Model units). The unit in the file header is a hint only, because the CAD reader does not apply it.
Fluid domain. The model is either already the fluid volume ("the CAD is the fluid volume") or a solid shell whose inside carries the flow; for a shell, the program caps the open port ends and takes the enclosed space. A real muffler is often drawn as several overlapping fluid bodies (a chamber, inlet pipes, tubes inside the chamber). The bodies are then split along each other with Gmsh's OpenCASCADE kernel (Geuzaine and Remacle, 2009), so that neighbouring regions share conforming faces. For example, the five bodies of a test muffler become eight regions with 39 faces, 22 on the outer skin and 17 internal.
The surface table. Each face is listed with its area, hydraulic diameter, shape and whether it is internal (between two regions). Planar, round faces at an extremity of the model are proposed as inlet / outlet candidates. An internal face is one of:
- open: the fluid passes (a tube's open end, the mouth of a pipe into a chamber);
- an internal wall: a zero-thickness tube or baffle wall, no-slip on both sides in the flow solution and rigid on both sides in the acoustics;
- a perforated wall (Number of perforated walls).
The program proposes a wall for a curved face whose owning body is smaller than the body around it; flat baffles must be assigned by hand. Unassigned outer faces become the patch wall; unassigned internal faces remain open.
Choosing faces. A click in the 3-D view selects the front-most face under the cursor; clicking again at the same spot steps to the face behind it, through the whole stack (also listed as chips). A section plane cuts the model open. Selection in the table and in the view stay synchronised. The numbers of inlets and outlets (Number of inlets, Number of outlets) must match the patches that carry faces.
Packing regions. The Domains tab lists the fluid regions after the split. A region ticked as packing is filled with absorptive material, whose resistivity and acoustic model are set on the Boundary Conditions step under Absorptive backing (Boundary Conditions).
Surface triangulation. The exported STL surfaces are the geometry on which the flow mesh is built. The triangulation is refined until the estimated area error of every face is below 0.1 %, so that round ports keep their area (with a fixed element size, a 128 mm inlet pipe can become an octagon with 10 % less area). The result is stored with the run and reused when the same CAD model is opened again.
Replacing the model. "Replace CAD..." and "Reload from disk" load a changed model while keeping every setting:
1. Each face of the old model is described by its kind, area, position, normal, perimeter and hydraulic diameter. 2. The faces of the new model are paired with them by a minimum-cost assignment; split or merged faces are recognised. 3. Confident pairs are applied. Uncertain pairs wait for Accept / Reject / Pick in a review list; an inlet or outlet is never assigned without confirmation.
An inlet, outlet or perforated wall of the old model that no face carries any more blocks meshing and running until it is reassigned or its settings are removed.
Source
The 0.1 % triangulation rule and the octagon example come from the program's validation on case V2 (a two-inlet single-chamber muffler with internal inlet and outlet tubes). The fragmentation of overlapping bodies uses Gmsh's OpenCASCADE kernel (Geuzaine and Remacle, 2009).
Fields in this step 4
| Field | Symbol | Unit | Default |
|---|---|---|---|
| Model units | none: the unit must be confirmed; the suggestion comes from the model's size | ||
| Number of inlets | 1 | ||
| Number of outlets | 1 | ||
| Number of perforated walls | 0 (or the number the faces already use) |