Thanks Wolfgang,
indeed, as we have incompressibility, the last component of the rhs will
always be 0. I wrote a possible patch, I'll open the pr tomorrow so that I
can take another look at it!
Best,
Marco
Il giorno venerdì 19 novembre 2021 alle 23:15:50 UTC+1 Wolfgang Bangerth ha
scritto:
>
> Marco,
>
> > I was going through step-22 and I saw that since you're using primitive
> > elements, then you compute the local_rhs contribution using
> > /fe.system_to_component_index(i).first/
> >
> > However, as written in the program, we could as well multiply the dim+1
> tensor
> > having the values of the i-th shape function with the whole rhs vector
> (f,0).
> >
> > So, I have two questions:
> > 1) Are you using /fe.system_to_component_index(i).first /just to make
> the
> > program run faster? Why not using extractors?
>
> I think it is an oversight. I would accept a patch to correct that :-)
>
>
> > 2) Taking the scalar product between a rank-1 Tensor and a Vector with
> the
> > same size is not "conceptually" allowed. Did you have in mind something
> like
> > the following?
> >
> > //inside assemble_system()
> > std::vector<Tensor<1, dim>> phi_u(dofs_per_cell);
> > for (unsigned int q = 0; q < n_q_points; ++q)
> > {
> > for (unsigned int k = 0; k < dofs_per_cell; ++k)
> > {
> > //store symgrad_phi_u, div_phi_u, etc. using extractors
> > phi_u[k] = fe_values[velocities].value(k, q);
> > }
> > // compute local_matrix and local_preconditioner
> >
> > double sum = 0.0; //store scalar product between first dim components of
> i-th
> > shape function (corresponding to velocity) and first dim components of
> the rhs
> > for (unsigned int s = 0; s < dim; ++s)
> > {
> > sum += phi_u[i][s] * rhs_values[q][s];
> > }
> > local_rhs(i) +=
> > (sum + rhs_values[q][dim] * phi_p[i]) * fe_values.JxW(q);
> > }
> > }
>
> That would work. I think a better solution would be to say outright that
> only
> the first equation can have force terms, and then make the RightHandSide
> class
> derived from TensorFunction<1,dim> and let it return its values as
> Tensor<1,dim> objects for which you can take the dot product
> fe_values[u].shape_value(q) * rhs_values[q]
>
> Want to give this a try and write a patch? We'd be happy to walk you
> through
> what is necessary for such a patch!
>
> Best
> Wolfgang
>
> --
> ------------------------------------------------------------------------
> Wolfgang Bangerth email: [email protected]
> www: http://www.math.colostate.edu/~bangerth/
>
>
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