mesh2flac3d

Convert meshes (Gmsh .msh, VTK, and anything meshio reads) to Itasca FLAC3D .f3grid grids — preserving physical groups:

• Volume physical groups → ZGROUP (zone groups)
• Surface physical groups → FGROUP (face groups, for boundary conditions)
• Correct zone winding: every zone is reordered to FLAC3D's convention, so you never get negative-volume zone errors on import.

It reads meshes through meshio (MIT) and does not import gmsh, so it carries no GPL obligation — you can use it freely, including in commercial workflows.

Why not just use meshio?

meshio has a FLAC3D writer, but for a typical geomechanics mesh (volume and surface physical groups) it currently:

• crashes (TypeError in split_f_z) when both zone and face groups are present, and
• drops all 2D groups ("FLAC3D format only supports 3D cells. Skipping triangle…"), so you lose the face groups you need for boundary conditions, and
• does not guarantee zone orientation.

mesh2flac3d is a focused, correct writer built for that exact case.

Install

pip install mesh2flac3d          # once published
# or, from source:
pip install -e .

Command line

mesh2flac3d model.msh model.f3grid
mesh2flac3d model.msh                 # -> model.f3grid
mesh2flac3d model.msh out.f3grid --dat out.dat   # also write a FLAC3D command skeleton
mesh2flac3d model.msh --no-faces      # zones only

Example output:

[mesh2flac3d] model.msh -> model.f3grid
  points: 350  zones: 1218  faces: 522
  zone groups: Underburden(401), Salt(414), Overburden(403)
  face groups: Top(90), Bottom(90), Sides(342)

Python API

import mesh2flac3d as m2f

grid = m2f.convert("model.msh", "model.f3grid")
print(grid.zone_groups.keys())   # dict_keys(['Underburden', 'Salt', 'Overburden'])
print(grid.face_groups.keys())   # dict_keys(['Top', 'Bottom', 'Sides'])

Supported elements

High-order variants (tetra10, hexahedron20, …) are exported using their linear corner nodes.

In Gmsh, name your groups

Give your regions and boundaries Physical Groups with names — those names become the FLAC3D group names:

Physical Volume("Salt")  = {2};
Physical Surface("Top")  = {4};

Development

pip install -e ".[test]"
pytest -q

The test fixture is generated with Gmsh (tests/fixtures/make_testmesh.py); Gmsh is a test-only dependency, never used by the package at runtime.

License

MIT © AI SIM Engenharia Geotécnica. FLAC3D and Itasca are trademarks of Itasca Consulting Group; this project is independent and not affiliated with Itasca.