When you ask for free slip, then the term implies two things:
- slip: the normal component of the velocity is zero, i.e., no flux
across the boundary - free: the tangential force is zero.
The tangential force is (sigma.n).t where sigma is the stress, n is the
normal vector, and t is the tangent vector. sigma.n is the total force
acting, and it can of course have a nonzero normal component (sigma.n).n
to ensure that the velocity is in fact zero in normal component. “Free
slip” then implies that the tangential force is in fact zero.
We should probably fix this in the manual. Want to take a stab at it? Or
at least open a github issue?
There is more material on this whole topic here:
https://dealii.org/developer/doxygen/deal.II/step_22.html#Weakform
Cheers
W.