The step-22 code uses the stress form of the NS equations. If you use the form
-mu*Laplace(u) + grad(p) = 0
and use the same dirichlet+neumann bc, then it seems to give the correct
solution. Please check it.
These are the lines to be changed
std::vector<Tensor<2, dim>> symgrad_phi_u(dofs_per_cell);
symgrad_phi_u[k] =
fe_values[velocities].gradient(k, q);
local_matrix(i, j) +=
(scalar_product(symgrad_phi_u[i], symgrad_phi_u[j]) // (1)
- div_phi_u[i] * phi_p[j] // (2)
- phi_p[i] * div_phi_u[j]) // (3)
* fe_values.JxW(q); // * dx
Best
praveen
> On 18-Apr-2020, at 10:55 PM, Patrick Eastham <[email protected]>
> wrote:
>
> Praveen,
>
> Thanks for the suggestion on adding tolerance to assigning boundary
> conditions -- that could come back to bite me in the future.
>
> I also appreciate your other suggestions, although I'd like to get this
> working without those conditions. I know that the FEM problem of assigning
> dirichlet velocity BC on 3 inflow and walls, and neumann conditions on the
> outflow is well-posed and leads to the exact solution (as the exact velocity
> is quadratic and exact pressure is linear, just like my element family) as
> I've done this same problem in a different FEM code.
>
> It seems like the cell at the intersection of the dirichlet and neumann
> conditions is not being handled correctly based on how I have assigned
> constraints/boundary conditions. Has anyone else had any experience with this?
>
> Best,
> Patrick
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