OpenFOAM Cases

An OpenFOAM case is a directory, not a file: the mesh lives in <case>/constant/polyMesh/. The extension reads and writes one.

Opening a case

Open the case's .foam marker — the empty <case>/something.foam file that ParaView also uses. VS Code editors bind to files, not folders, so the marker is the handle:

cavity/
  cavity.foam          ← open this
  constant/polyMesh/
    points  faces  owner  neighbour  boundary

If your case has no marker, create an empty file with any name and a .foam extension next to constant/. Nothing reads its contents.

A case exported from this extension already has one.

What you get

• The volume cells as Elements.
• The boundary faces as Conditions.
• One SubModelPart per boundary patch, named as constant/polyMesh/boundary names it — inlet, outlet, movingWall. This is what lets you assign boundary conditions in the Problemtype section; without the names the boundary would be one anonymous surface.

points.gz, faces.gz and friends are decompressed automatically, so a case written with writeCompression on opens normally.

While the preview is open it watches constant/polyMesh/ for mesh changes and the time directories for new steps and field edits, so re-running blockMesh or advancing a solver refreshes the view in place.

Time-directory fields and the timeline

Numeric time directories (0, 0.5, 1e-3, …) are listed as the timeline: the preview's timeline bar scrubs them, Plot over time samples them, and mesh_field_series reads them headless. Each step shows the selected directory's fields over the selected mesh:

• volScalarField / volVectorField / volTensorField / volSymmTensorField arrive as Elemental fields; pointScalarField / pointVectorField as Nodal fields.
• internalField uniform ... and nonuniform List<...> are read (plain or .gz); an explicit uniform boundaryField patch value arrives as a Conditional field of the same variable.
• Binary fields, #include/coded/substituted content, surface fields and nonuniform patch values are skipped with a warning — the geometry still opens.
• A directory whose <time>/polyMesh exists overlays its files over constant/polyMesh for its own step (a moving mesh commonly overrides only points); otherwise every step shares the constant mesh.

Headless, mesh_info reports the selected step's fields plus every available time value, and timeStep selects one (0 is the first, negative counts back from the last).

Zones, regions and decomposed cases

• Zones. cellZones, faceZones and pointZones become SubModelParts, like the boundary patches.
• Multi-region cases. A case with no top-level constant/polyMesh but one constant/<region>/polyMesh per region opens with every region merged. Each region is a top-level SubModelPart named after it, with its patches as children. A field present in several regions (say T in both fluid and solid) arrives as one field spanning them. Headless, region selects a single region instead.
• Decomposed cases. A case written by decomposePar (processorN/constant/polyMesh plus the *ProcAddressing files) is reconstructed into one mesh, and patch names are taken from processor0's boundary. Fields under processorN/<time>/ are not reconstructed; the Information panel says so. Run reconstructPar first if you need them.

If constant/polyMesh/boundary is missing, the mesh loads with no boundary faces at all, because that file is what defines them. points, faces and owner are required; if one is missing, the open fails and names it.

Saving and exporting

Save is refused for a case, deliberately. The file you opened is an empty marker and the mesh is in sibling files, so saving "the file" would rewrite constant/polyMesh/ underneath the preview. A rewrite keeps patch names, zones and simple patch types (patch, wall, empty, symmetry, symmetryPlane), but other things would still be lost:

• Patch types that need extra keys, such as cyclic (its neighbourPatch). These are written as patch, and a warning names them.
• Time directories, regions and processor directories. The writer produces a single constant/polyMesh.

Use Export ▸ Solvers ▸ OpenFOAM or Save As… to write a new case directory instead. Exporting into the same directory under a different .foam name is refused too — it is the same constant/polyMesh.

An exported case also gets a 0/<VAR> file for each Elemental field that covers every volume cell (volScalarField, volVectorField or volTensorField by component count, with zeroGradient on every patch). A field that covers only part of the mesh is skipped and named in a warning. dimensions is written as [0 0 0 0 0 0 0], because the extension does not track units; set them before running a solver.

Exporting to any other format (.mdpa, .vtu, …) is unrestricted. A problem archive bundles the marker plus constant/polyMesh/, so File ▸ Save problem works on a case directly.

Headless

mesh_info, mesh_convert, mesh_transform and mesh_field_series all take a .foam marker path like any other mesh. For a multi-region case, mesh_info and mesh_convert take region to read one region instead of all of them. mesh_info reports the patch SubModelParts, the selected step's fields and every available time value, and re-reads correctly after blockMesh reruns or a solver rewrites a field — its freshness is keyed on the polyMesh and time-directory files, not on the marker, which never changes.