Dear Daniel,
Thank you for a very good answer, adding the fe_values_scalar and
cell_scalar fixed the problem.
I have a follow-up question about the solution I get out, how is the data
organized in the solution vector? Say for example I want to loop over all
the x components, how would I do that?
Best,
Joel
On Friday, August 12, 2016 at 5:35:07 PM UTC+2, Daniel Arndt wrote:
>
> Joel,
>
> Yes the matrix you are assembling is a vector-valued mass matrix now.
> For me your code is failing with
>
> void dealii::FEValuesBase<dim, spacedim>::get_function_gradients(const
> InputVector&, std::vector<dealii::Tensor<1, spacedim, typename
> InputVector::value_type> >&) const [with InputVector =
> dealii::Vector<double>; int dim = 3; int spacedim = 3; typename
> InputVector::value_type = double]
> The violated condition was:
> (fe->n_components()) == (1)
> The name and call sequence of the exception was:
> dealii::ExcDimensionMismatch((fe->n_components()),(1))
> Additional Information:
> Dimension 3 not equal to 1
>
> and this is not surprising. What you are doing is to extract the gradients
> of a vector-valued finite element solution. The object that should store
> this should therefore be a std::vector<std::vector<Tensor<1,dim>> as you
> want to store for each quadrature point and each component a Tensor<1,dim>.
>
> What you want to do is really that in "Do not work". As you have two
> DoFHandlers, you should also have two FEValues objects and two
> cell_iterator corresponding to the correct DoFHandler. Then you can extract
> the gradient of your scalar field and project it onto the ansatz space for
> the vector-valued field. This should look like:
>
> typename DoFHandler<dim>::active_cell_iterator cell_scalar =
> dof_handler_scalar.begin_active();
> typename DoFHandler<dim>::active_cell_iterator cell =
> dof_handler.begin_active();
> ...
>
> for (; cell!=endc; ++cell, ++cell_vector)
> {
> fe_values.reinit (cell);
> fe_values_scalar.reinit (cell_scalar);
> fe_values_scalar.get_function_gradients(test,fe_function_grad);
> ...
> }
>
> Best,
> Daniel
>
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