Beginner’s Guide to Gmsh: Mesh Generation Made Simple

Advanced Gmsh Techniques: Custom Fields, Plugins, and Post-ProcessingGmsh is a flexible open-source mesh generator widely used in finite element analysis, computational fluid dynamics, and computational geometry. This article covers advanced techniques to extend Gmsh’s capabilities: creating custom mesh size and background fields, writing and using plugins and external scripts, and performing efficient post-processing to prepare meshes and results for analysis.

Overview of Advanced Workflows

Advanced Gmsh usage typically combines:

Custom fields to control element sizes and grading,
Scripting (native .geo or Python API) to automate geometry, mesh, and meshing decisions,
Plugins or external tools to extend functionality (e.g., custom geometry importers, converters),
Post-processing to convert meshes, tag regions/boundaries, and export usable data for solvers or visualization.

This article assumes familiarity with basic Gmsh concepts: geometry entities, physical groups, meshing algorithms, and the .geo scripting language or the Python API.

Custom Fields

Custom fields let you define spatially varying mesh size, which is essential for capturing features like boundary layers, high-gradient regions, or embedding refined regions without global refinement.

Built-in field types

Gmsh supports several field types; most useful are:

MathEval — evaluate a mathematical expression to control size.
Distance — size based on distance to points, curves, or surfaces.
Threshold — map a Distance output into a smooth size transition.
Box, Cylinder, Sphere — region-based constant or variable sizes.
Harmonic — solves a Laplace equation to smoothly interpolate sizes.

Example: combine Distance and Threshold to refine near a curve

Field[1] = Distance; Field[1].NodesList = {1, 2, 3}; // point or curve tags Field[2] = Threshold; Field[2].IField = 1; Field[2].LcMin = 0.01; Field[2].LcMax = 0.5; Field[2].DistMin = 0.0; Field[2].DistMax = 0.2; Background Field = 2;

Tips:

Use Harmonic for globally smooth transitions when multiple local refinements interact.
Combine fields with Compose or Min/Max fields to blend strategies (e.g., Min to honor finest requirement).
For boundary layers, generate anisotropic meshes using transfinite or extruded structured layers where possible; otherwise control near-wall sizes strongly with Distance+Threshold.

Scripting and Automation

Automation yields reproducible meshes and integrates Gmsh into solver pipelines.

.geo scripting

Parametrize geometry with variables; change mesh density or geometry from the command line using gmsh -setnumber or -setstring.
Use For, If, and While constructs to generate repeated features (arrays of holes, patterned domains).
Create physical groups programmatically to ensure correct boundary condition labeling.

Example snippet:

// parameterized rectangle with holes L = 1.0; nx = 4; For i In {0:nx-1}   Point(10+i) = {0.2 + i*0.15, 0.5, 0, 0.01}; EndFor

Python API

Use gmsh Python module to build geometry, set fields, generate mesh, and read/write mesh formats in the same script.
Python makes complex logic, external data import (CSV, netCDF), and post-processing simple.

Simple Python workflow:

import gmsh gmsh.initialize() gmsh.model.add("example") # build geometry, fields... gmsh.model.mesh.generate(2) gmsh.write("mesh.msh") gmsh.finalize()

Integrations:

Call meshers (TetGen, Netgen) or solver pre-processors in the same Python script.
Use packages like meshio to convert between formats programmatically.

Plugins and Extending Gmsh

Gmsh supports plugins and has an API for extending behavior, though writing compiled plugins requires C++ and familiarity with Gmsh internals.

When to write a plugin

You need a custom geometry kernel or importer (special CAD formats).
You must implement a new mesh optimization or element type.
Performance-critical pre/post-processing should run inside Gmsh.

Plugin types and examples

Geometry plugins: add new CAD importers or primitives.
Mesh plugins: custom algorithms, quality optimizers.
GUI plugins: custom panels and dialogs.

Development workflow:

Study Gmsh’s src/plugins structure and examples in the source tree.
Build Gmsh from source with your plugin source included; use CMake to configure.
Register plugin factory classes with Gmsh’s plugin manager.

If C++ development isn’t desired, prefer Python scripting or external tools — many tasks performed by plugins can be achieved by scripting or calling external libraries.

Post-Processing

Post-processing prepares the mesh for solvers and visualizes results. Gmsh offers built-in post-processing plus export options.

Tagging and physical groups

Ensure volumes, surfaces, and lines have Physical Groups to map BCs and materials.
Use gmsh.model.getEntities(dim) and getBoundingBox in Python to auto-detect and tag faces/regions.

Example: assign physical groups by bounding boxes in Python

for dim, tag in gmsh.model.getEntities():     x1,y1,z1,x2,y2,z2 = gmsh.model.getBoundingBox(dim, tag)     if abs(x1 - 0.0) < 1e-6 and abs(x2 - 0.0) < 1e-6:         gmsh.model.addPhysicalGroup(dim, [tag], name="leftBoundary")

Mesh quality and optimization

Check element quality with built-in statistics; use gmsh.option.setNumber("Mesh.Optimize", 1) and smoother options.
Use Recombine for quadrangles/hexahedra where appropriate, then Optimize and Merge operations to improve element shapes.

Export formats and solver integration

Gmsh can write native .msh (v2/v4), UNV, STL, VTK, and more. Use meshio for additional conversion options.
For multiphysics, ensure consistent region IDs and store physical names when exporting to formats that support them (e.g., .msh v4 preserves names).

Example command to produce a v4 .msh:

gmsh -3 geometry.geo -o mesh.v4.msh -format msh2

(Note: adjust format flags per desired version; check your installed Gmsh version’s options.)

Result visualization and field output

Use Gmsh’s post-processing to load solver results and visualize scalar/vector fields.
Export results to XDMF/HDF5 (via external tools) for scalable visualization with ParaView when datasets are large.

Advanced Examples

Import airfoil coordinates.
Create Distance field from airfoil curve.
Use Threshold to set extremely small LcMin near the airfoil and larger LcMax away.
Optionally extrude the near-wall surface to create prismatic layers.

2) Multi-region mesh with conformal interfaces

Build adjacent volumes with shared surfaces.
Assign matching mesh constraints (transfinite where possible) and shared physical surfaces to ensure interface conformity.

3) Automated labeling for solver BCs

Use Python to detect surfaces by normal or bounding box and assign solver-specific IDs (e.g., in a Fluent .msh or SU2 format).

Performance and Practical Tips

Start with a coarse mesh and progressively refine fields to debug geometry and physical group assignments.
Profile Python automation scripts; minimize repeated calls to heavy operations (e.g., repeated mesh generation inside loops).
Use Background Field sparingly for complex 3D domains; harmonic fields are slower but produce smoother transitions.
Keep physical group naming consistent and documented for solver integration.

Further Resources

Gmsh manual and API docs (consult your installed Gmsh version for exact function names).
Source examples from Gmsh distribution for fields and plugins.
meshio for file conversion; ParaView for large-scale visualization.

If you want, I can:

Provide a ready-to-run .geo and Python example for any of the advanced examples above.
Help convert a specific CAD file or solver format.