# FEM MeshGmshFromShape

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FEM MeshGmshFromShape

Системное название
FEM MeshGmshFromShape
Расположение в меню
Mesh → Сетка МКЭ из фигуры генерируемая построителем Gmsh
Верстаки
FEM
Быстрые клавиши
нет
Представлено в версии
-
См. также
FEM tutorial

## Описание

Для анализа методом конечных элементов геометрия должна быть дискретизирована в сетке МКЭ. Эта команда использует для рассчета сетки gmsh, который должен быть установлен в системе.

Gmsh is bundled with the FreeCAD installation binaries. Alternatively you can install it separately from FreeCAD and then use the menu Edit → Preferences → FEM → Gmsh to set the path to the gmsh.exe.

## Использование

1. Выберите форму, которую хотите проанализировать. Для объёмных МКЭ это должно быть твердое тело или композит. Композит необходим, если ваша деталь сделана из нескольких материалов. (Композит может быть создан с помощью команды BooleanFragments.) -- Для МКЭ оболочки и балки кто-то должен указать здесь детали.
2. Нажмите кнопку
3. При желании отредактируйте минимальный и максимальный размер элемента. (Автоопределение отлично работает, если вы не применяете сложные граничные условия.)
4. Нажмите кнопку Apply и дождитесь завершения вычисления сетки
5. Закройте задачу. Теперь вы должны увидеть новый объект FEMMeshGMSH в вашем активном контейнере анализа.

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

## Properties

• ДанныеAlgorithm2D: The algorithm to create 2D meshes. The different algorithms are explained here. For Delaunay, see Delaunay triangulation.
• ДанныеAlgorithm3D: The algorithm to create 3D meshes. The different algorithms are explained here.
• ДанныеCharacteristic Length Max: 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.
• ДанныеCharacteristic Length Min: 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.
• ДанныеCoherence Mesh:
• true (default); duplicate mesh nodes will be removed
• false
• ДанныеElement Dimension: 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
• ДанныеElement Order: The mesh element order. This property can also be changed in the Gmsh dialog in the field Mesh order. introduced in version 0.20
• 1st
• 2nd (default)
• ДанныеGeometrical Tolerance: 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.
• ДанныеGroups Of Nodes: 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.
• ДанныеHigh Order Optimize: If and how meshes with ДанныеElement Order = 2nd are optimized. The optimization is done by a deformation of the element borders.
introduced in version 0.20 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.
• ДанныеMesh Size From Curvature introduced in version 0.20: The number of mesh elements per ${\displaystyle 2\pi }$ times the radius of the curvature. To get a finer mesh at small corners or holes, this value can be increased for better results

Effect of Mesh Size From Curvature'; left: set to 12, right: deactivated

• ДанныеOptimize Netgen: 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 ДанныеElement Dimension is not 3D.
• ДанныеRecombination Algorithm introduced in version 0.20: The algorithm used for ДанныеRecombine 3D All and also for ДанныеRecombine All. For more info, see section Element Recombination and for technical details see the Gmsh documentation.
• ДанныеRecombine 3D All introduced in version 0.20: Applies a recombination 3D-algorithm to all volumes. Tetrahedra will be recombined into prisms, hexahedra or pyramids if possible.
• ДанныеRecombine All: Applies a recombination algorithm to all surfaces. Triangles will be recombined into quadrangles when possible.
• ДанныеOptimize Std Optimizes the mesh to improve the quality of tetrahedral elements.
• ДанныеSecond Order Linear: Option if second order nodes (if ДанныеElement Order set to 2nd) and/or mesh refinement points are created by linear interpolation.
• true; linear interpolation is used
• false (default); curvilinear interpolation is used

## Notes

### Nonpositive Jacobians

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

• Set ДанныеSecond Order Linear to true but keep ДанныеElement Order at 2nd.
• Set ДанныеElement Order to 1st.
• Use a smaller element size by reducing the ДанныеCharacteristic Length Max.
• If solver ccxtools is used and the run button is used (not the task panel) the nodes of non positive 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 edges. The growing strategy of Gmsh is to grow between edges with different sizes. So the growing fails when an area has the same sized edges like for example this tube:

Failing mesh growing because the cylindrical area is surrounded be the same edges

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.

Sensible mesh growing due to the additional edge in the middle of the cylindrical aread

### 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 ДанныеRecombine All alone usually only makes sense for 2D meshes.
When we use now also ДанныеRecombine 3D All, 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

Effect of mesh element recombination. Upper left: standard mesh, Upper right: recombination only at the surface using the Simple algorithm, Lower left: recombination at the surface and in the volume using the Simple algorithm, Lower right: recombination at the surface and in the volume using the Simple full-quad algorithm