Symmetry Constraints
In many cases symmetry of the geometry and loads can be used to your advantage in modeling. Symmetries can often greatly reduce the size of a model and hence reduce the memory requirements and solution time. When a structure exhibits axial symmetry, use the axisymmetric physics interfaces. A solid that is generated by rotating a planar shape about an axis is said to have axial symmetry. In order to make use of the axisymmetric physics interfaces, all loads and constraints must also be the same around the circumference.
For other types of symmetry, use the predefined symmetry and antisymmetry constraints. This means that no expressions need to be entered—instead just add the type of constraint to apply to the model.
Physics Interface Axial Symmetry Node in the COMSOL Multiphysics Reference Manual
If the geometry exhibits two symmetry planes (Figure 2-6), model a quarter of the geometry by using the Symmetry node for the two selected surfaces.
Figure 2-6: If the geometry exhibits two symmetry planes, model a quarter of the geometry by using the Symmetry feature for the two selected surfaces.
Both geometric symmetry and loads are important when selecting the correct constraints for a model.
In an eigenfrequency or buckling analysis, the eigenmodes might be nonsymmetric even if the structure is symmetric.
Figure 2-7 shows symmetric and antisymmetric loading of a symmetric geometry. When modeling half of the geometry, the correct constraint for the face at the middle of the object would be Antisymmetry in the case of antisymmetric loading and Symmetry in the case of symmetric loading of the object.
Figure 2-7: Symmetry plane (left) and antisymmetry plane (right).
Symmetry in 2D Axisymmetry
In an axisymmetric model, the only possible symmetry is when the symmetry plane is normal to the Z-axis. For models in 2d axisymmetry, the Symmetry Plane node is used for prescribing this type of symmetry.
Antisymmetry cannot exist in this case.
Translation of The Symmetry Plane
In some situations, you may want to use a symmetry condition, in which the symmetry boundary actually can move along its normal. This may for example the case when you use symmetry conditions to terminate your modeled region even though the situation is not truly symmetric. The best approximation may then be that the boundary remains planar, but that there is no resultant reaction force from the boundary condition.
You can modify the symmetry condition, so that it can translate in various ways by using the controls in the Normal Direction Condition section of the settings for the Symmetry constraint. You can model the following cases:
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Reaction force free translation.
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Prescribed total force acting on the constrained part.
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The displacement in the normal direction is prescribed.
Note that allowing translation in the symmetry constraint is only meaningful if the geometry selection corresponds to a single symmetry plane.
For an example showing how to force a boundary to remain plane, but still allow it to translate in its normal direction using this special version of Symmetry, see Thermo-Mechanical Analysis of a Surface-Mounted Resistor: Application Library path Structural_Mechanics_Module/Thermal-Structure_Interaction/surface_resistor.
Symmetry Condition with Translation in the Structural Mechanics Theory chapter.