Dear Joel,
In general, no you cannot. Here you assume a specific ordering of the DoFs
in the global solution vector, and this changes for each finite element.
For example, using FE_Q's might result the ordering [x0 y0 z0 x1 y1 z1 ...]
and FE_DGQ's might lead to [x0 x1 x2 ... y0 y1 y2 ... z0 z1 z2...]. To do
this safely you should definitely do this using the built in tools, such as
FEValueExtractors
<https://www.dealii.org/developer/doxygen/deal.II/namespaceFEValuesExtractors.html>
or fe.system_to_component_index
<https://www.dealii.org/developer/doxygen/deal.II/classFiniteElement.html#a27220a135402b96c7e6eecbb04acda56>.
See the tutorials for examples on how to use them.
Regards,
J-P
On Tuesday, September 13, 2016 at 2:05:56 PM UTC+2, Joel Davidsson wrote:
>
> Dear all,
>
> If I have a Vector-valued solution in 3D and I would like to get the size
> of each vector in another smaller Vector, can I do it like this:
>
> for (unsigned int i=0;i<solution.size();i=i+3)
> {
> unsigned int x=i;
> unsigned int y=i+1;
> unsigned int z=i+2;
> unsigned int j = i / 3.0;
>
> size(j)=pow(solution(x),2.0)+pow(solution(y),2.0)+pow(solution(z),2.0);
> }
>
> Where solution is the vector valued Vector that is 3 times bigger that the
> scalar Vector called size.
>
> Best,
> Joel
>
> On Tuesday, August 30, 2016 at 6:48:46 PM UTC+2, Jean-Paul Pelteret wrote:
>>
>> Dear Joel,
>>
>> No, this functionality doesn't exist. However, you can create a
>> quadrature rule with the quadrature points located at the support points
>> (see the FAQ
>> <https://github.com/dealii/dealii/wiki/Frequently-Asked-Questions#how-to-get-the-mapped-position-of-support-points-of-my-element>),
>>
>> and could then extract the solution based on your custom quadrature rule.
>>
>> Regards,
>> J-P
>>
>> On Tuesday, August 30, 2016 at 5:32:13 PM UTC+2, Joel Davidsson wrote:
>>>
>>> Dear Daniel,
>>>
>>> I was just wondering if their exist a similar function
>>> to DoFTools::map_dofs_to_support_points, but that maps a vector-valued
>>> solution Vector into to a std::vector<Point<dim>> instead?
>>>
>>> Best,
>>> Joel
>>>
>>> On Monday, August 22, 2016 at 9:03:22 PM UTC+2, Daniel Arndt wrote:
>>>>
>>>> Joel,
>>>>
>>>> You should not rely on any particular sorting of the dofs in the
>>>> solution vector. Instead you can ask the FiniteElement on each cell for
>>>> the
>>>> component of a local dof by
>>>> const unsigned int component = fe.system_to_component_index(i).first;
>>>>
>>>> Apart from that the DataOut object knows about components and you can
>>>> do some postprocessing on the solution vector using DataPostprocessor [1]
>>>>
>>>> Best,
>>>> Daniel
>>>>
>>>> [1]
>>>> https://dealii.org/8.4.1/doxygen/deal.II/classDataPostprocessor.html
>>>>
>>>> Am Montag, 22. August 2016 17:05:08 UTC+2 schrieb Joel Davidsson:
>>>>>
>>>>> 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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