Format

Stellarmesh consumes Gmsh .msh v4.1 ASCII files annotated with physical groups using a URL-style key=value encoding.

Physical-group encoding

Per-entity metadata is stored as the name of a Gmsh physical group covering exactly one Gmsh entity. The name encodes a URL-style query string of key/value pairs, parsed by stellarmesh as follows.

Volume groups

tag=<integer>&material=<string>

  • tag — the volume’s Gmsh tag (matches the discrete entity tag in the same file).

  • material — a slug identifying the volume’s material region. Slugs are bounded to 28 characters (a hard MOAB limit downstream in the DAGMC pipeline).

Every volume entity in the file must carry exactly one such physical group.

Surface groups

tag=<integer>&forward_volume=<integer>&reverse_volume=<integer>

  • tag — the surface’s Gmsh tag.

  • forward_volume — the volume tag on the surface’s outward-normal side.

  • reverse_volume — the volume tag on the other side. Use 0 for exterior surfaces (boundary of the model with vacuum).

Edges and vertices

Discrete entities only; no physical groups. Stellarmesh ignores edge and vertex annotations if present.

Consumer behaviour

When stellarmesh reads a conforming .msh file:

  • Each volume’s material slug becomes a DAGMC mat:<slug> group in the output .h5m file.

  • Surface forward_volume / reverse_volume populate DAGMC’s surface-sense relationships.

  • Downstream tooling (e.g. OpenMC) maps the mat:<slug> groups to openmc.Material instances. Stellarmesh does not perform that mapping itself.