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Symmetry

In SimScale, we have a symmetry boundary condition for fluid dynamics and also a symmetry plane boundary condition for finite element analysis. On this page, we will go through both of them.

Symmetry Boundary Condition (CFD)

The symmetry boundary condition defines a mirror face/surface. It should only be used if the physical object or geometry and the expected flow field pattern of the developed solution are mirrored along that surface. By using this boundary condition, the domain can essentially be halved, reducing the time to achieve a solution.

In detail, the symmetry condition applies the following constraints on the flow variables:

  • The fluxes across the symmetry are zero.
  • The normal components of all variables are set to zero.

It can be applied to both planar or non-planar faces/surfaces on the domain boundaries.

Important

This boundary condition should not be used for axisymmetric or periodic flow cases (please refer to the wedge or periodic boundary conditions for details on how to approach these.

Symmetry Plane Boundary Condition (FEA)

This boundary condition is used to apply mirror-symmetry conditions on a structure.

It can be applied to faces of a structure and no other user input is needed. If a symmetry plane condition is applied to a face, the displacement of this face is locked in a normal direction but free to slide in tangential directions.

symmetry plane boundary condition for fea
Figure 1: Symmetry plane boundary condition for a connecting rod cut in half

This boundary condition is particularly useful when the symmetry plane is not aligned with any global coordinate axis.

Important

If nodes in the symmetry plane are also constrained by other boundary conditions (for example, if the adjacent faces have fixed support, fixed value, or undeformable forces assigned to them), the system will be overconstrained.

For this reason, often it is preferred to replace the symmetry plane for a fixed value displacement, blocking the motion normal to the plane and leaving the other directions unconstrained. Using the same example from Figure 1, we would have:

fixed value to replace symmetry plane boundary condition
Figure 2: Fixed value boundary condition, alternative to a symmetry plane condition

With this approach, potential issues with overconstrained nodes are avoided.

Last updated: July 7th, 2020

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