BC prescribing only normal-components of solution

Hi!

What's the best practice for prescribing boundary conditions that only make a statement about the normal component of the solution on a Dirichlet boundary?

In case that this question was silly, let me ask it in a different way. How can I achieve boundary conditions like a non-penetration BC without restricting the tangential component for a fluid problem (slip-boundary) or having a support that only restricts the displacement in normal direction to the boundary for a structure problem (sliding support)?

Simply not specifying the respective component in a dirichlet BC led to errors in my case, so I guess there must be a better way.

Thank you!