The normal to the symmetry plane, ns, is either computed from the geometry, or given as a user input.
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 Free displacement or
Prescribed force, then
un is added as a degree of freedom to be determined during the solution.
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.