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Coreform Cubit Mesh Export

A Python library for exporting mesh files from Coreform Cubit to various formats.

Installation

Install from PyPI (Recommended)

pip install --upgrade coreform-cubit-mesh-export

Install into Cubit's Python Environment

To use within Cubit's journal scripts (play "script.py") for Gmsh/VTK/Nastran export:

Windows:

"C:\Program Files\Coreform Cubit 2025.3\bin\python3\python.exe" -m pip install coreform-cubit-mesh-export

Linux/Mac:

/path/to/cubit/bin/python3/python -m pip install coreform-cubit-mesh-export

Install for Netgen/NGSolve Integration

For export_netgen() with high-order curving, install into your own Python environment (not Cubit's bundled Python):

# Install with ngsolve dependency
pip install coreform-cubit-mesh-export[netgen]

# Or install separately
pip install coreform-cubit-mesh-export ngsolve

Then add Cubit's bin path to your script:

import sys
sys.path.append("C:/Program Files/Coreform Cubit 2025.3/bin")
import cubit

Install Development Version from GitHub

pip install git+https://github.com/ksugahar/Coreform_Cubit_Mesh_Export.git

Supported File Formats

  • Exodus II ⭁ENEW
    • Cubit's native format (.exo, .e, .g)
    • All element types and orders
    • Nodesets and Sidesets support
    • Large model support (64-bit)
  • Netgen
    • Direct mesh conversion (no intermediate files)
    • High-order curving via mesh.Curve() (order 2, 3, 4, 5, ...)
    • All element types: Tet, Hex, Wedge, Pyramid
  • Gmsh Format
    • Version 4.1 (with $Entities section, 2D/3D auto-detect)
    • Version 2.2 (full support, 2nd-order elements)
  • Nastran Format
    • 2D meshes
    • 3D meshes
  • MEG Format (for ELF)
    • 2D meshes
    • 3D meshes
  • VTK Format
    • Legacy VTK (.vtk) - auto-detect element order
    • VTK XML (.vtu) - modern format with binary support

Usage

Netgen (High-Order Curving)

import cubit
import cubit_mesh_export
import ngsolve

# Create mesh in Cubit
cubit.cmd("create sphere radius 1")
cubit.cmd("volume 1 scheme tetmesh")
cubit.cmd("mesh volume 1")
cubit.cmd("block 1 add tet all in volume 1")
cubit.cmd("block 1 name 'domain'")
cubit.cmd("block 2 add tri all in surface all")
cubit.cmd("block 2 name 'boundary'")

# Export geometry for Curve() support
cubit.cmd('export step "geometry.step" overwrite')

# Convert to NGSolve mesh with high-order curving
ngmesh = cubit_mesh_export.export_netgen(cubit, geometry_file="geometry.step")
mesh = ngsolve.Mesh(ngmesh)
mesh.Curve(3)  # 3rd order curved elements

# Use in FEM analysis
print(mesh.GetMaterials())   # ('domain',)
print(mesh.GetBoundaries())  # ('boundary',)

See docs/export_NetgenMesh.md for detailed documentation.

Usage within Cubit

After generating a mesh in Cubit, execute a Python script as follows:

# Cubit command line
play "export_mesh.py"

Example content of export_mesh.py:

import cubit_mesh_export

# Export to Nastran format (3D)
FileName = 'output/model.nas'
cubit_mesh_export.export_nastran(cubit, FileName, DIM="3D")

# Export to Gmsh format (v2.2, supports 2nd-order elements)
FileName = 'output/model.msh'
cubit_mesh_export.export_gmsh_v2(cubit, FileName)

# Export to VTK format (auto-detects element order)
FileName = 'output/model.vtk'
cubit_mesh_export.export_vtk(cubit, FileName)

Function Reference

Naming Convention

Functions use snake_case naming (e.g., export_gmsh_v2). Legacy PascalCase names are available as aliases for backward compatibility.

Recommended (snake_case) Legacy (PascalCase)
export_exodus (already snake_case)
export_gmsh_v2 export_Gmsh_ver2
export_gmsh_v4 export_Gmsh_ver4
export_nastran export_Nastran
export_netgen export_NetgenMesh
export_meg (already snake_case)
export_vtk (already snake_case)
export_vtu (already snake_case)

Exodus II Format

  • export_exodus(cubit, filename, overwrite=True, large_model=False) - Export to Exodus II format
    • Cubit's native format (.exo, .e, .g)
    • Supports all element types and orders
    • large_model=True for meshes with >2 billion elements/nodes

Netgen Format

  • export_netgen(cubit, geometry_file=None) - Export to netgen.meshing.Mesh object
    • Returns mesh ready for ngsolve.Mesh() conversion
    • With geometry_file: enables mesh.Curve(order) for high-order elements
    • Supports: Tet, Hex, Wedge, Pyramid, Tri, Quad

Gmsh Format

  • export_gmsh_v4(cubit, filename, DIM="auto") - Export to Gmsh v4.1 format with $Entities section
    • DIM: "auto"/"2D"/"3D"
  • export_gmsh_v2(cubit, filename) - Export to Gmsh v2.2 format with 2nd-order element support

Nastran Format

  • export_nastran(cubit, filename, DIM="2D|3D", PYRAM=True|False) - Export to Nastran BDF format
    • Convert pyramids to degenerate hex with PYRAM=False (for JMAG compatibility)

MEG Format (for ELF)

  • export_meg(cubit, filename, DIM="T|R|K", MGR2=[]) - T: 3D, R: axisymmetric, K: 2D

VTK Format

  • export_vtk(cubit, filename) - Export to Legacy VTK format, auto-detects element order
  • export_vtu(cubit, filename, binary=False) - Export to VTK XML format (.vtu)

High-Order Element Support

Format 1st Order 2nd Order 3rd+ Order
Exodus II ✁E ✁E ✁E
NGSolve ✁E ✁E(via Curve) ✁E(via Curve)
Gmsh v4.1 ✁E ✁E ❁E
Gmsh v2.2 ✁E ✁E ❁E
VTK/VTU ✁E ✁E ❁E
Nastran ✁E ❁E ❁E

Note: Exodus II is Cubit's native format and supports all element orders. NGSolve's mesh.Curve(order) generates high-order nodes from geometry.

Block Registration

This module uses blocks only for mesh export (not nodesets or sidesets).

Why? In Cubit, only blocks support element order control via block X element type tetra10. Nodesets and sidesets lack this capability. By standardizing on blocks, we ensure consistent 2nd order element support across all export formats.

Boundary conditions can still be defined using separate blocks:

block 1 add tet all in volume 1    # Domain
block 1 name "domain"
block 2 add tri all in surface 1   # Boundary
block 2 name "boundary"
block 1 element type tetra10       # Convert to 2nd order

Geometry vs Mesh Elements in Blocks

Blocks can contain either mesh elements or geometry. Both are now supported:

# Method 1: Register mesh elements directly (recommended)
block 1 add tet all
block 1 add hex all

# Method 2: Register geometry (volume, surface, curve, vertex)
block 1 add volume 1      # Exports tet/hex/wedge/pyramid in the volume
block 2 add surface 1     # Exports tri/quad on the surface
block 3 add curve 1       # Exports edges on the curve

Behavior by geometry type:

Block Contains Elements Exported
Volume tet, hex, wedge, pyramid (3D elements only)
Surface tri, quad (2D elements only)
Curve edge (1D elements only)
Vertex node (0D elements only)

Mixed Element Types Warning

When a block contains multiple 3D element types (e.g., tet + hex), a warning is displayed with instructions for 2nd order conversion:

WARNING: Block 1 ('mixed') contains multiple 3D element types: hex, tet
  To convert all elements to 2nd order, you need to issue separate commands:
    block 1 element type hex20
    block 1 element type tetra10

This warning helps users understand that each element type requires its own conversion command.

Requirements

  • Coreform Cubit 2024.3 or later (provides Python environment and cubit module)
  • NGSolve/Netgen (optional, for export_NetgenMesh() function)

License

LGPL-2.1 (GNU Lesser General Public License version 2.1)

Author

Kengo Sugahara (ksugahar@kindai.ac.jp)

Repository

Bug Reports & Feature Requests

Please submit via GitHub Issues.

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