E.5 Mesh module

1. What should I do when the 2D unstructured algorithm fails?

   Verify that the curves in the model do not self-intersect. If ‘Mesh.RandomFactor*size(triangle)/size(model)’ approaches machine accuracy, increase Mesh.RandomFactor.

   If everything fails file a bug report with the version of your operating system and the full geometry.

2. What should I do when the 3D unstructured algorithm fails?

   Verify that the surfaces in your model do not self-intersect or partially overlap. If they don’t, try the other 3D algorithms (‘Tool->Options->Mesh->General->3D algorithm’) or try to adapt the mesh element sizes in your input file so that the surface mesh better matches the geometrical details of the model.

   If nothing works, file a bug report with the version of your operating system and and the full geometry.

3. My 2D meshes of IGES files present gaps between surfaces

   IGES files do not contain the topology of the model, and tolerance problems can thus appear when the OpenCASCADE importer cannot identify two (close) curves as actually being identical.

   The best solution is to not use IGES and use STEP instead. If you really have to use IGES, check that you don’t have duplicate curves (e.g. by displaying their numbers in the GUI with ‘Tools->Options->Geometry->Visibility->Line numbers’). If there are duplicates, try to change the geometrical tolerance and sew the faces (see options in ‘Tools->Options->Geometry->General’).

4. The quality of the elements generated by the 3D algorithm is very bad.

   Use ‘Optimize quality’ in the mesh menu.

5. Non-recombined 3D extruded meshes sometimes fail.

   The swapping algorithm is not very clever at the moment. Try to change the surface mesh a bit, or recombine your mesh to generate prisms or hexahedra instead of tetrahedra.

6. Does Gmsh automatically couple unstructured tetrahedral meshes and structured hexahedral meshed using pyramids?

   Only in simple geometrical cases. We need your help to improve this.

7. Can I explicitly assign region numbers to extruded layers?

   No, this feature has been removed in Gmsh 2.0. You must use the standard entity number instead.

8. Did you remove the elliptic mesh generator in Gmsh 2.0?

   Yes. You can achieve the same result by using the transfinite algorithm with smoothing (e.g., with Mesh.Smoothing = 10).

9. Does Gmsh support curved elements?

   Yes, Gmsh can generate both 1st order and 2nd order elements. To generate second order elements, click on ‘High order’ in the mesh menu after the mesh is completed. To always generate 2nd order elements, select ‘Generate second order elements’ in the mesh option panel. From the command line, you can also use -order 2.

10. Can I import an existing surface mesh in Gmsh and use it to build a 3D mesh?

    Yes, you can import a surface mesh in any one of the supported mesh file formats, define a volume, and mesh it. For an example see ‘demos/sphere-discrete.geo’.

11. How do I define boundary conditions or material properties in Gmsh?

    By design, Gmsh does not try to incorporate every possible definition of boundary conditions or material properties—this is a job best left to the solver. Instead, Gmsh provides a simple mechanism to tag groups of elements, and it is up to the solver to interpret these tags as boundary conditions, materials, etc. Associating tags with elements in Gmsh is done by defining Physical entities (Physical Points, Physical Lines, Physical Surfaces and Physical Volumes). See the reference manual as well as the tutorials (in particular ‘tutorial/t1.geo’) for a detailed description and some examples.

12. How can I display only the mesh associated with selected geometrical entities?

    See “How can I display only selected parts of my model?”.

13. How can I “explore” a mesh (for example, to see inside a complex structure)?

    You can use ‘Tools->Clipping Planes’ to clip the region of interest. You can define up to 6 clipping planes in Gmsh (i.e., enough to define a “cube” inside your model) and each plane can clip either the geometry, the mesh, the post-processing views, or any combination of the above. The clipping planes are defined using the four coefficients A,B,C,D of the equation A*x+B*y+C*y+D=0, which can be adjusted interactively by dragging the mouse in the input fields.

14. What is the signification of Rho, Eta and Gamma in Tools->Statistics?

    They measure the quality of the tetrahedra in a mesh:

    Gamma ~ inscribed_radius / circumscribed_radius Eta ~ volume^(2/3) / sum_edge_length^2 Rho ~ min_edge_length / max_edge_length

    For the exact definitions, see Geo/MElement.cpp. The graphs plot the the number of elements vs. the quality measure.

15. Why don’t the vertex and/or elememt numbers on the screen match the numbers in the mesh file?

    Gmsh reindexes the mesh vertices and elements so that they are numbered in a continuous sequence in the output files. The numbers displayed on screen after mesh generation thus usually differ from the ones saved in the mesh files. To check the actual numbers saved in the output file just load the mesh file back using ‘File->Open’.

This document was generated on February 9, 2014 using texi2html 5.0.