I believe the formula is correct. the cubic term comes from p=z^3 being the pressure manufactured solution. so in (pI-2e) you get a z^3 term and indeed a linear term in the 2e portion.

I see. I missed that there is of course also a pressure.

This is how I have come to conclude that it is how i am implementing the Neumann conditions that are causing the problem, since with Dirichlet conditions the solver is good. All i can figure out is that perhaps we have calculated the vector g_N (Neumann condition) incorrectly, but that has been checked multiple times - if anyone notices something stupid we have done here then do let us know too.

Well, then that is the point to debug the problem. You've established that the solution is wrong. Now you need to figure out how exactly is it wrong. Is the solution off by a constant? Is the profile wrong? If you look at the error as a cellwise constant function, is there a pattern? What happens if you multiply the prescribed boundary values by two -- is the error also multiplied by two? Does the pressure stay correct in that case, but the velocity change in some way? Is the sign of the velocity correct? If you collect enough clues, you can typically guess where the problem must lie.

I will for now use vector extractors like you suggested and see if that helps at all?

As you already found out, that's not helping with the bug. It just makes the code simpler.


Wolfgang Bangerth          email:                 bange...@colostate.edu
                           www: http://www.math.colostate.edu/~bangerth/

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