Magda,
let me ask a few more specific questions about your post, even though I'm not
sure they help you with the bigger question:
> I'm a kind of new user of dealii and I'm trying to set up the boundary
> conditions for a domain [0, 1] X [0, 1] of incompressible fluid with fully
> periodic boundary conditions. I know that for an incompressible fluid with
> fully periodic boundary conditions (velocity both directions and pressure)
You can't prescribe periodic boundary conditions for the pressure. You can do
that for the velocity, but not the pressure because the pressure is only in L2
and consequently does not allow for Dirichlet boundary conditions.
More intuitively, though, think about a pipe section. You can make the
inflow=outflow periodicity work on the velocities, but along the pipe section
you will have a pressure drop that is necessary to drive the flow -- so the
pressure at the outflow must be lower than at the inflow, and they can't be
the same.
> I
> need to release one boundary DOF, otherwise the problem will be over
> constrained.
Why is this so? I don't think I follow why you would need to do that. On the
contrary, it seems to me like you need to *add* a constraint because otherwise
(if you had periodicity in both x and y direction), you could have arbitrarily
large rigid body translational velocity fields added to the flow you want to
compute -- they all satisfy the same periodic boundary conditions.
> Therefore, the question is:
> Is there any way, after I have set up my periodic boundary conditions, to
> release one DOF? I was thinking it should be something in the constraint
> matrix but I wasn't sure.
I don't think that's possible right now, but as mentioned above, it's also not
clear to me why one would need to do that.
Best
W.
--
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Wolfgang Bangerth email: bange...@colostate.edu
www: http://www.math.colostate.edu/~bangerth/
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