Jane,

I believe the formula is correct. the cubic term comes from p=z^3 being thepressure manufactured solution. so in (pI-2e) you get a z^3 term and indeed alinear 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 theNeumann conditions that are causing the problem, since with Dirichletconditions the solver is good. All i can figure out is that perhaps we havecalculated the vector g_N (Neumann condition) incorrectly, but that has beenchecked multiple times - if anyone notices something stupid we have done herethen 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 helpsat all?

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

`code simpler.`

Best W. -- ------------------------------------------------------------------------ Wolfgang Bangerth email: bange...@colostate.edu www: http://www.math.colostate.edu/~bangerth/ -- The deal.II project is located at http://www.dealii.org/ For mailing list/forum options, see https://groups.google.com/d/forum/dealii?hl=en

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