On 22/04/16 18:49, Colin Cotter wrote:
OK, are you trying to match an analytical solution?
I think it depends on whether you integrated by parts in the grad term, or the divergence. If you integrated by parts in the grad term, then you can get the 0 pressure boundary condition naturally i.e. without setting anything.
all the best --cjc
To add to that, the natural ("do nothing") boundary condition leads to a zero stress condition. This is I think also where the difference between including and excluding the grad-div term comes from. The tangential component of stress is different (dv/dx vs. dv/dx+du/dy) and is not sufficiently constrained by the boundary conditions. If you include the grad-div term the tangential stress is non-zero and so you either have to explicitly add that as a boundary term to weakly enforce the correct value of dv/dx+du/dy, or you apply a strong dirichlet on the tangential component of velocity. Without the grad-div term the "do-nothing" bc for the tangential stress automatically enforces dv/dx=0. Cheers Stephan