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

Quickstart

A ready-to-run example mesh lives in examples/ — install, then:

mesh2flac3d examples/three_layer_box.msh out.f3grid

See examples/README.md for the full 30-second walkthrough, or examples/dean/ for the Dean (2006) SPE-79709 reservoir benchmark meshes (problems 1–4, SI units).

Slots (overlapping groups)

A cell often belongs to several groups at once — a well cell is also part of a reservoir and of a geological layer. FLAC3D handles this with slots (one group per slot per zone). Name a physical group name@slot and mesh2flac3d writes it to that slot:

Physical Volume("well@Well")        = { ... };   // ZGROUP "well"      SLOT "Well"
Physical Volume("reservoir@Region") = { ... };   // ZGROUP "reservoir" SLOT "Region"

The well cells then stay part of the reservoir. A name without @ uses the default slot.

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
mesh2flac3d model.msh --check         # exit non-zero if any zone has bad volume
mesh2flac3d model.msh --json          # machine-readable grid summary

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)
  total volume: 900000   negative-volume zones: 0 [OK]

negative-volume zones: 0 certifies the grid imports into FLAC3D without a volume error — computed here, so you don't have to open FLAC3D to find out. Use --check to turn that into an exit code for CI/pipelines.

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'])

s = grid.summary()
print(s["total_volume"], s["negative_volume_zones"])  # 900000.0 0

Supported elements

Family	Gmsh / meshio	FLAC3D
Zones	tetra	T4
	pyramid	P5
	wedge / prism	W6
	hexahedron	B8
Faces	triangle	T3
	quad	Q4

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};

Entities you don't assign to a group are imported anyway — they just land in no ZGROUP/FGROUP. (meshio alone aborts a Gmsh 4.1 file when only some element blocks carry a Physical Group — Incompatible cell data 'gmsh:physical'; mesh2flac3d falls back to a built-in tolerant Gmsh 4.1 ASCII reader in that case. If your .msh is binary or an older version, re-export it as "Version 4.1 ASCII" — the Gmsh default.)

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.

Reporting a bug

Open an issue: https://github.com/Jeanbjoseph/mesh2flac3d/issues/new/choose. The guided form asks for your command, the full output, versions, and — most useful of all — the input .msh that reproduces the problem (zip it if GitHub blocks the extension). A small reproducing mesh gets things fixed fastest.

License

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