On 04/15/2018 07:30 PM, Jaekwang Kim wrote:

and I think I should add this cell_matrix(i,j) and cell_rhs(i) to my system matrix.

Yes.


but not overlapping the rows that describe weakform of original poisson equation.. so I think i should not use them...



for (unsigned int i=0; i<dofs_per_cell; ++i)

         {

for (unsigned int j=0; j<dofs_per_cell; ++j)

                 system_matrix.add (local_dof_indices[i],

                                    local_dof_indices[j],

                                    cell_matrix(i,j));

                 system_rhs(local_dof_indices[i]) += cell_rhs(i);

         }



In this situation, how should I expand (to add rows) my system matrix and connect to it boundary dof ?

No. These are just regular DoFs like all others that you have enumerated. There is definitely no need to expand the size of the system.

I don't think that your boundary conditions are any different than the Neumann conditions that we treat in one of the earlier tutorial programs (step-7?). Your just happen to be Robin boundary conditions. Take a look at how this is done there, and maybe also consider the video lectures on boundary conditions.

I'll point out that in your code snippet where you assemble the boundary contributions, you need to make sure that the `fe_values` object you use there is really an FE*Face*Values.

Best
 W.

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

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