Symmetry Condition with Translation
A standard symmetry condition prohibits translation in the symmetry direction.
The normal to the symmetry plane, ns, is either computed from the geometry, or given as a user input.
When allowing translation of the symmetry plane, the constraint is modified to
Here, un is a single scalar displacement offset which is the same for all locations where the symmetry constraint is applied.
If the selected normal direction condition is Prescribed displacement, then un is the given value.
If the selected normal direction condition is Free displacement or Prescribed force, then un is added as a degree of freedom to be determined during the solution.
The weak contribution for this equation is
where Fn is the prescribed force. Note that in the Free displacement case, the force is zero, so no weak contribution is added. This corresponds to a solution with zero reaction force.