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Commit dc20951e authored by Christophe Geuzaine's avatar Christophe Geuzaine
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update for version 2
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$Id: FAQ,v 1.77 2006-12-01 15:07:12 geuzaine Exp $ $Id: FAQ,v 1.78 2006-12-03 21:13:24 geuzaine Exp $
This is the Gmsh FAQ This is the Gmsh FAQ
...@@ -110,7 +110,7 @@ Section 4: Geometry module ...@@ -110,7 +110,7 @@ Section 4: Geometry module
* 4.1 Does Gmsh support NURBS curves/surfaces? * 4.1 Does Gmsh support NURBS curves/surfaces?
Only via STEP, IGES or BREP models (not in GEO files). Yes, but only via STEP, IGES or BREP model import (not in .geo files).
* 4.2 Gmsh is very slow when I use many transformations (Translate, * 4.2 Gmsh is very slow when I use many transformations (Translate,
Rotate, Symmetry, Extrude, etc. ). What's wrong? Rotate, Symmetry, Extrude, etc. ). What's wrong?
...@@ -120,47 +120,35 @@ entities (points, lines and surfaces) each time a transformation ...@@ -120,47 +120,35 @@ entities (points, lines and surfaces) each time a transformation
command is issued. This can slow down things a lot if many command is issued. This can slow down things a lot if many
transformations are performed. There are two solutions to this transformations are performed. There are two solutions to this
problem: problem:
- you may save the unrolled geometry in another file (e.g. with - you may save the unrolled geometry in another file (e.g. with gmsh
gmsh file.geo -0 > flat.geo), and use this new file for subsequent file.geo -0), and use this new file for subsequent computations;
computations; - or you may set the 'Geometry.AutoCoherence' option to 0. This will
- you may set the 'Geometry.AutoCoherence' option to 0. This will prevent any automatic duplicate check/replacement. If you still need
prevent any automatic duplicate check/replacement. If you still to remove the duplicates entities, simply add 'Coherence;' at
need to remove the duplicates entities, simply add 'Coherence;' at
strategic locations in your geo files (e.g. before the creation of strategic locations in your geo files (e.g. before the creation of
line loops, etc.). line loops, etc.).
* 4.3 How can I display only selected parts of my model? * 4.3 How can I display only selected parts of my model?
By using 'Tool->Visibility': Use 'Tool->Visibility'.
- In the 'Browser' tab, you can select which group of entities to show There are three main modes: 'Elementary entities', in which the
(several entities can be selected at once by dragging the mouse or by selections will apply to elementary geometrical entities; 'Physical
holding the Ctrl and Shift keys while clicking). There are three main groups', in which the selections will apply to physical entities; and
modes: 'Elementary', in which the numbers in the list correspond to 'Mesh partitions', in which the selections will apply to mesh
elementary geometrical entities; 'Physical', in which the numbers in partitions.
the list correspond to physical entities; and 'Partitions', in which
the numbers in the list correspond to indices of mesh partitions (for If the 'Recursive' option is set, selecting an entity also selects all
multi-domain meshes read from "new" .msh files). If 'Geometry+Mesh' is its boundaries, recursively. For example, if 'Recursive' is set,
selected, the selection applies to both the geometrical entities and selecting a surface will automatically select its boundary curves, as
their associated meshes; if 'Mesh' is selected, the selection applies well as the boundaries of these curves (i.e., points).
only to the mesh; if 'Geometry' is selected, the selection applies
only to the geometry. If the 'Recursive' option is set, selecting an In the 'Browser' tab, you can select which entities to show or hide in
entity implies that all the entities that are used to construct it a list (several entities can be selected at once by dragging the mouse
(all the "boundaries") are selected too, recursively. For example, if or by holding the Ctrl or Shift keys while clicking). In the
'Recursive' is set, selecting a surface will automatically select its 'Numerical input' tab, you can choose which entities to show or hide
boundary curves, as well as the boundaries of these curves (i.e., by explicitly specifying their identification tags. In the
points). If 'Recursive' is not set, only the surface (the dashed 'Interactive' tab, you can hide/show entities using the mouse.
cross) would be shown.
- In the 'Selector' tab, you can choose to show or hide any entity or
mesh element by giving its number. You can also use the "*" character
to select all the entities/elements at once. For example, to show only
the mesh element 13245 in the mesh, you would enter "*" in the
'Element' input field and click on 'Hide' (this will hide all the mesh
elements); and then enter "13245" in the 'Element' input field and
click on 'Show' (this will show the element 13245). To show other
element, you would just enter another number in the input field, click
on 'Show', etc.
4.4 When compiled with OpenCascade support, Gmsh crashes at startup 4.4 When compiled with OpenCascade support, Gmsh crashes at startup
...@@ -176,80 +164,42 @@ Section 5: Mesh module ...@@ -176,80 +164,42 @@ Section 5: Mesh module
* 5.1 What should I do when the 2D unstructured algorithm fails? * 5.1 What should I do when the 2D unstructured algorithm fails?
Try one of the other 2D algorithms, e.g.: Send us a bug report (including your geometry).
- on the command line: 'gmsh -algo tri'
- in the interface: 'Tools->Options->Mesh->2D->Isotropic algorithm (Triangle)'
- in input files: 'Mesh.Algorithm = 3'
The old 2D algorithm may disappear once all its features are
integrated in the new ones, so please don't send bug reports on the
old algorithm anymore.
* 5.2 The new 2D unstructured algorithms also fail! Then what?
Send us your geometry, and we will investigate. Please keep the
following in mind though: 2D (surface) meshes are generated by
projecting the 2D mesh in a "mean plane" of the surface. This gives
nice results only if the surface curvature is small enough. Otherwise
you must cut the surface into several pieces (patches). For example,
using half circles to define a cylinder will fail with the
unstructured algorithm: you should define arcs with angles smaller
than Pi, and thus define the cylinder with at least three patches.
* 5.3 What should I do when the 3D unstructured algorithm fails? * 5.2 What should I do when the 3D unstructured algorithm fails?
The 3D algorithm is still very experimental. Try to change some Try to change some characteristic lengths in your input file to
characteristic lengths in your input file to generate meshes that generate meshes that better suit the geometrical details of your
better suit the geometrical details of your structure. structure.
* 5.4 I changed the characteristic lengths, but the 3D algorithm still * 5.3 I changed the characteristic lengths, but the 3D algorithm still
does not work. What should I do? does not work. What should I do?
Buy a professional mesh generator ;-) You can also try to use Netgen Try one of the other algorithms (Tool->Options->Mesh->General->3D
instead of the default algorithm for the 3D mesh. Note that all algorithm). If everything fails, buy a professional mesh generator ;-)
surface meshes have to be oriented with exterior normals in this case.
* 5.5 The 3D algorithm is reaaaaally slow. Can you improve it? * 5.4 The quality of the elements generated by the 3D algorithm is
We are working on it. But since we have a (very) limited amount of
time to spend on the development of Gmsh, this may take a while. For
very big meshes, see the answer to the previous question...
* 5.6 The quality of the elements generated by the 3D algorithm is
very bad. very bad.
Upgrade to Gmsh >= 1.54 and use 'Optimize quality'. If badly shaped Use 'Optimize quality' in the mesh menu.
elements still exist due to the surface recovery step, you can try to
use Netgen instead of the default algorithm for the 3D mesh. Note that
all surface meshes have to be oriented with exterior normals in this
case.
* 5.7 Non-recombined 3D extruded meshes sometimes fail. * 5.5 Non-recombined 3D extruded meshes sometimes fail.
The swapping algorithm is not very clever at the moment. Try to change 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 the surface mesh a bit, or recombine your mesh to generate prisms or
hexahedra instead of tetrahedra. hexahedra instead of tetrahedra.
* 5.8 Tools->Visibility does not seem to work with extruded meshes. * 5.6 Can I explicitly assign region numbers to extruded layers?
This is fixed in Gmsh >= 1.54. Note that in Gmsh < 2.0, when region
numbers are explicitly assigned to mesh entities in the extrude
commands (which partially destroys the geometry/mesh relationship),
the 'Visibility' tool will only work as expected when displaying
elementary entities. In this particular case, the only solution to
visualize extruded physical entities is to save the mesh, and to read
it again.
* 5.9 Can I explicitly assign region numbers to extruded layers? No, this feature has been removed in Gmsh 2.0. You must use the
No, this feature has been removed in Gmsh 2.0. You must now use the
standard entity number instead. standard entity number instead.
* 5.10 Did you remove the elliptic mesh generator in Gmsh 2.0? * 5.7 Did you remove the elliptic mesh generator in Gmsh 2.0?
Yes. You can achieve the same result by using the transfinite Yes. You can achieve the same result by using the transfinite
algorithm with smoothing (e.g., with "Mesh.Smoothing = 10"). algorithm with smoothing (e.g., with "Mesh.Smoothing = 10").
* 5.11 Does Gmsh support curved elements? * 5.8 Does Gmsh support curved elements?
Yes, Gmsh can generate both 1st order and 2nd order elements. To Yes, Gmsh can generate both 1st order and 2nd order elements. To
generate second order elements, click on 'Second order' in the mesh generate second order elements, click on 'Second order' in the mesh
...@@ -257,12 +207,14 @@ menu after the mesh is completed. To always generate 2nd order ...@@ -257,12 +207,14 @@ menu after the mesh is completed. To always generate 2nd order
elements, select 'Generate second order elements' in the mesh option elements, select 'Generate second order elements' in the mesh option
panel. From the command line, you can also use '-order 2'. panel. From the command line, you can also use '-order 2'.
* 5.12 Can I import an existing surface mesh in Gmsh and use it to * 5.9 Can I import an existing surface mesh in Gmsh and use it to
build a 3D mesh? build a 3D mesh?
Yes, see for example 'demos/sphere-discrete.geo'. 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'.
* 5.13 How do I define boundary conditions or material properties in * 5.10 How do I define boundary conditions or material properties in
Gmsh? Gmsh?
By design, Gmsh does not try to incorporate every possible definition By design, Gmsh does not try to incorporate every possible definition
...@@ -271,19 +223,19 @@ to the solver. Instead, Gmsh provides a simple mechanism to tag groups ...@@ -271,19 +223,19 @@ 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 of elements, and it is up to the solver to interpret these tags as
boundary conditions, materials, etc. Associating tags with elements in boundary conditions, materials, etc. Associating tags with elements in
Gmsh is done by defining Physical entities (Physical Points, Physical Gmsh is done by defining Physical entities (Physical Points, Physical
Lines, Physical Surfaces and Physical Volumes): see the reference Lines, Physical Surfaces and Physical Volumes). See the reference
manual as well as the tutorials (in particular 'tutorial/t1.geo') for manual as well as the tutorials (in particular 'tutorial/t1.geo') for
a detailed description and some examples. a detailed description and some examples.
* 5.14 How can I display only the mesh associated with selected * 5.11 How can I display only the mesh associated with selected
geometrical entities? geometrical entities?
See question 4.3. See question 4.3.
* 5.15 How can I "explore" a mesh (for example, to see inside a * 5.12 How can I "explore" a mesh (for example, to see inside a
complex structure)? complex structure)?
You can use 'Tools->Clipping Planes' to extract the region of 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 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 to define a "cube" inside your model) and each plane can clip either
the geometry, the mesh, the post-processing views, or any combination the geometry, the mesh, the post-processing views, or any combination
...@@ -294,7 +246,7 @@ fields. There is also one additional clipping plane available for ...@@ -294,7 +246,7 @@ fields. There is also one additional clipping plane available for
"cutting" only the mesh (by keeping entire elements), in "cutting" only the mesh (by keeping entire elements), in
'Tools->Options->Mesh->Cut Plane'. 'Tools->Options->Mesh->Cut Plane'.
* 5.16 What is the signification of Rho, Eta and Gamma in * 5.13 What is the signification of Rho, Eta and Gamma in
Tools->Statistics? Tools->Statistics?
They measure the quality of the tetrahedra in a mesh: They measure the quality of the tetrahedra in a mesh:
...@@ -303,10 +255,8 @@ Rho ~ min_edge_length / max_edge_length ...@@ -303,10 +255,8 @@ Rho ~ min_edge_length / max_edge_length
Eta ~ volume^(2/3) / sum_edge_length^2 Eta ~ volume^(2/3) / sum_edge_length^2
Gamma ~ volume / sum_face_area / max_edge_length Gamma ~ volume / sum_face_area / max_edge_length
For the exact definitions, look at Mesh/Element.cpp and For the exact definitions, see Geo/MElement.cpp. The graphs plot the
Mesh/Simplex.cpp. the number of elements vs the quality measure.
The graphs plot the the number of elements vs the quality measure.
******************************************************************** ********************************************************************
...@@ -486,7 +436,7 @@ line option. ...@@ -486,7 +436,7 @@ line option.
* 7.14 How can I see "inside" a complicated post-processing view? * 7.14 How can I see "inside" a complicated post-processing view?
See question 5.13. See question 5.12.
When viewing 3D scalar fields, you can also modify the colormap When viewing 3D scalar fields, you can also modify the colormap
('Tools->Options->View->Map') to make the iso-surfaces "transparent": ('Tools->Options->View->Map') to make the iso-surfaces "transparent":
......
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