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While re-implementing the staggered grid scheme, I thought it might be good to discuss which types of BCs we want to offer. So far, I could implement most of the tests using only dirichlet and neumann BCs and some helper classes.

We could have additionally (as in the old implementation)

• symmetry (technically a combination of Dirichlet with v*n = 0 and zero shear stress)
• beaversJoseph (technically a solution dependent Dirichlet for the tangential velocity)

Symmetry can already be easily achieved by combining dirichlet and neumann.

This is not so easy for beaversJoseph because we do not support solution dependent Dirichlet BCs. The old implementation circumvents that by not calling dirichlet but beaversJoseph with sol-dep arguments in the local residual.

It could be possible to implement beaversJoseph in terms of a Neumann BC, but that might become cumbersome, especially if inertia terms are considered.