FEM MeshGmshFromShape/de

Beschreibung
Für eine Finite-Elemente-Analyse muss die Geometrie in ein FEM-Netz diskretisiert werden. Dieser Befehl verwendet das Programm Gmsh (das auf dem System installiert sein muss) zur Berechnung des Netzes.

Abhängig vom Betriebssystem und dem Installationspaket kann Gmsh in FreeCAD enthalten sein oder auch nicht. Für weitere Informationen siehe FEM Installation.

Anwendung

 * 1) Wähle die Form, die du analysieren möchtest. Bei der Volumen FEM muss es sich um einen Festkörper oder Compsolid (zusammengesetzten Festkörper) handeln. Ein Compsolid ist erforderlich, wenn dein Teil aus mehreren Materialien besteht. (Ein Compsolid kann mit dem Befehl BoolescheFragmente erstellt werden).
 * 2) * Die Schaltfläche drücken.
 * 3) * Den Menüeintrag auswählen.
 * 4) Bei Bedarf minimale und maximale Elementgröße anpassen. (Automatische Erkennung funktioniert gut, es sei denn, du wendest komplizierte Randbedingungen an).
 * 5) Die Schaltfläche  anklicken und warten, bis die Berechnung des Netzes abgeschlossen ist.
 * 6) Die Aufgabe abschließen. Jetzt sollte sich ein neues FEMMeshGMSH-Objekt im aktiven Analysebehälter befinden.

After the mesh has been created, you can change its properties using the property editor. After you change a property, you must reopen the Gmsh dialog again and click the button (you can leave the dialog open while changing properties).

Properties

 * : The algorithm to create 2D meshes. The different algorithms are explained here. For Delaunay, see Delaunay triangulation.
 * : The algorithm to create 3D meshes. The different algorithms are explained here.
 * : The maximal size of the mesh elements. If set to 0.0, the size will be set automatically. This property can also be changed in the Gmsh dialog in the field Max element size.
 * : The minimal size of the mesh elements. If set to 0.0, the size will be set automatically. This property can also be changed in the Gmsh dialog in the field Min element size.
 * true (default); duplicate mesh nodes will be removed
 * false
 * : The dimension of the mesh elements. This property can also be changed in the Gmsh dialog in the field Mesh element dimension.
 * From Shape (default); the dimension will be determined from the dimension of the object that is meshed
 * 1D
 * 2D
 * 3D
 * : The mesh element order. This property can also be changed in the Gmsh dialog in the field Mesh order.
 * 1st
 * 2nd (default) Note: If you use the solver Elmer you may get this error: ERROR:: GetEdgeBasis: Can't handle but linear elements, sorry. This means the solver equation cannot handle 2nd order meshes. Use either 1st order meshes then, or check the FreeCAD Wiki page for the solver equation for possible options to handle 2nd order meshes.
 * : The geometrical tolerance for the mesh to match the object edges. The default 0.0 means that Gmsh's default of 1e-8 is used.
 * : All nodes and not only the elements will be saved for each physical mesh group. Physical groups are collections of mesh entities (points, curves, surfaces and volumes). They and are identified by their dimension and by a tag. For example a mesh of the same object region is internally tagged the same. So all surfaces of this region will form one physical group.
 * : If and how meshes with = 2nd are optimized. The optimization is done by a deformation of the element borders.  Gmsh supports different optimization algorithms. Elastic is an algorithm in which the mesh elements are treated as a collection of deformable viscoelastic solids. 1st order meshes cannot be optimized because their element borders are linear an cannot be deformed.
 * : The number of mesh elements per $$2\pi$$ times the radius of the curvature. To get a finer mesh at small corners or holes, this value can be increased for better results
 * : The number of mesh elements per $$2\pi$$ times the radius of the curvature. To get a finer mesh at small corners or holes, this value can be increased for better results


 * : Whether the mesh will be optimized using the 3D mesh generator Netgen to improve the quality of tetrahedral elements. Note: since Netgen can only create tetrahedral elements, this option is ignored for meshes whose  is not 3D.
 * : The algorithm used for and also for . For more info, see section Element Recombination and for technical details see the Gmsh documentation.
 * : Applies a recombination 3D-algorithm to all volumes. Tetrahedra will be recombined into prisms, hexahedra or pyramids if possible.
 * : Applies a recombination algorithm to all surfaces. Triangles will be recombined into quadrangles when possible.
 * Optimizes the mesh to improve the quality of tetrahedral elements.
 * : Option if second order nodes (if set to 2nd) and/or mesh refinement points are created by linear interpolation.
 * true; linear interpolation is used
 * false (default); curvilinear interpolation is used

Nonpositive Jacobians
When you get a meshing error about nonpositive Jacobians, you can try out the following strategies:


 * Set to true but keep  at 2nd.
 * Set to 1st.
 * Use a smaller element size by reducing the.
 * If solver ccxtools is used and the run button is used (not the task panel) the nodes of nonpositive jacobian elements will be green.

Mesh Growth
At edges and small geometric entities, the mesh has to be smaller than in areas without edges. So the mesh element size grows away from the edges. The growing strategy of Gmsh is to grow between edges of different sizes. So the growing fails when an area has the same sized edges like for example this tube:



To enable a sensible mesh growing, you must in this case add an edge to the area. In the example, this would be a circle in the middle of the cylinder. The circle is added as part of a BooleanFragments compound (to form a CompSolid), see the project file of the example.



Element Recombination
Elements can be recombined in two ways, on the surface of objects so that triangles will be recombined into quadrangles if possible and in the volume of objects so that tetrahedra will be recombined into prisms, hexahedra or pyramids if possible. Thinking about the geometry, it becomes clear that the recombination result depends strongly on the geometry of the body and that recombining a 3D body only at the surface will mostly lead to strange results.

To illustrate this, look at the image below. A cuboid body is meshed using the standard settings (tetrahedra, 2nd order mesh). This is the subimage at the upper left. The image at the upper right shows the result, when additionally the elements are recombined only at the surface of the body. The result is bad because the changed surface elements don't fit to the unchanged volume elements. So alone usually only makes sense for 2D meshes. When we use now also, the result is better, see the lower left subimage. However, the result doesn't show a great difference compared to the mesh without recombinations. Since our body is a cuboid, it is therefore sensible to use a recombination algorithm that tries to create cuboids as well. And this result is shown in the subimage at the lower right.

The Simple recombination algorithm will leave some triangles in the mesh in case the recombining leads to badly shaped quads. In such cases use a full-quad recombination algorithm, which will automatically perform a coarser mesh followed by the recombination, smoothing and subdividing. See forum topic