# Navier Stokes demo

I have a math question about the Navier-Stokes demo. It uses the Chorin projection method and in the second step, the new pressure is computed from the tentative velocity. The problem to be solved at this step is laplace p=(grad u*)/k_n. So, in the variational formulation I would expect something like

-<grad p, grad q>+integral over boundary from (q times normal derivative of p) = rhs

However, in the implementation, this "boundary integral" is missing. If the variational problem is satisfied for all q in the demo, than it implies a boundary condition \partial_n p=0 everywhere on the boundary. However, in addition to this, the line

[bc.apply(A2, b2) for bc in bcp] ...

seems to require also a dirichlet boundary condition at the inflow and outflow. Aren't here too many boundary conditions on p?

Thanks for an explanation and sorry if I misunderstood something. Peter

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- Kent-Andre Mardal

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