point_value for a vector-valued Problem / FESystem

[Jost Arndt]

Dear everyone,

I have trouble understanding in how to evaluate a solution at a given point correctly.

Described in Step-3 for example the function point_value is used to evaluate the solution at (0.3, 0.3), and I want to do something quite similar.

However, my solution is vector valued (i.e. I use FESystem ), and I get the following error message

>>An error occurred in line <181> of file <./include/deal.II/numerics/vector_tools_point_value.templates.h> in function                                                                                                    
    typename VectorType::value_type dealii::VectorTools::point_value(const dealii::Mapping<dim, spacedim>&, const dealii::DoFHandler<dim, spacedim>&, const VectorType&, const dealii::Point<spacedim>&) [with int dim =
2; VectorType = dealii::Vector<double>; int spacedim = 2; typename VectorType::value_type = double]                                                                                                                      
The violated condition was:                                                                                                                                                                                              
    dof.get_fe(0).n_components() == 1                                                                                                                                                                                    
Additional information:                                                                                                                                                                                                  
    Finite element is not scalar as is necessary for this function

However in the documentation it says "Evaluate a possibly vector-valued finite element function defined by the given DoFHandler and nodal vector fe_function at the given point point". So I am curious how to evaluate my solution at a few fixed points? Did I miss something?

Performance-wise it does not have to be perfect at all, as I want to evaluate only ~400 points on a few hundered solutions (time-steps).

Best,

Jost

[Simon Sticko]

Hi.

There are several point_value functions. Right now, you are calling one that only works for scalar elements:

https://www.dealii.org/current/doxygen/deal.II/namespaceVectorTools.html#a7be5c7eed52308898dfaad91c4cff204

You need to call the more general function:

https://www.dealii.org/current/doxygen/deal.II/namespaceVectorTools.html#acd358e9b110ccbf4a7f76796d206b9c7

by passing a vector with each components value as the last argument to the function:

Vector<double> value(n_solution_components);
VectorTools::point_value(dof_handler,
solution,
point,
value);

Best,
Simon

On 20/07/2023 12:54, 'Jost Arndt' via deal.II User Group wrote:
> Dear everyone,
>
> I have trouble understanding in how to evaluate a solution at a given point correctly.
> Described in Step-3 for example the function point_value is used to evaluate the solution at (0.3, 0.3), and I want to do something quite similar.
> However, my solution is vector valued (i.e. I use FESystem ), and I get the following error message
>
> >>An error occurred in line <181> of file <./include/deal.II/numerics/vector_tools_point_value.templates.h> in function
>     typename VectorType::value_type dealii::VectorTools::point_value(const dealii::Mapping<dim, spacedim>&, const dealii::DoFHandler<dim, spacedim>&, const VectorType&, const dealii::Point<spacedim>&) [with int dim =
> 2; VectorType = dealii::Vector<double>; int spacedim = 2; typename VectorType::value_type = double]
> The violated condition was:
>     dof.get_fe(0).n_components() == 1
> Additional information:
>     Finite element is not scalar as is necessary for this function
>
>

> However in the documentation it says "Evaluate a possibly vector-valued finite element function defined by the given DoFHandler <https://dealii.org/developer/doxygen/deal.II/classDoFHandler.html> and nodal vector fe_function at the given point point". So I am curious how to evaluate my solution at a few fixed points? Did I miss something?

>
> Performance-wise it does not have to be perfect at all, as I want to evaluate only ~400 points on a few hundered solutions (time-steps).
>
> Best,
>
> Jost
>

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