How to calculate solution at every cell center point?

[JiaShumei92]

Dear every dealii user,
Wish you good day!
I am a beginner to dealii. The version that I am using is 9.3.0.
I have caught with a problem when I am solving my FEM problems. Could anyone please help me with it?
I am intended to get all active cells` center point values, so I followed the tutorials and use the following coding.

size_t cellCount = 0; //keep the cell`s count；
for (auto& cell : triangulation.active_cell_iterators())
{
//### get the cell`s center point solution：
double adata = dealii::VectorTools::point_value(dof_handler, solution, cell->center());

//### put the solution to a "VectorXd"：
phantVValues(vindex) = adata;

//### next cell：
++cellCount;
}

The problem is that when my active cell`s amount is large, this coding is in vastly slow calculation speed. I find another dealii method to solve the problem,
which is as follows. However, it is also very 'expensive' for very large amount of cells.

//additional.
//### define a FEFieldFunction object 'solution_function',
dealii::Functions::FEFieldFunction<3, dealii::DoFHandler<3>, dealii::Vector<double>>
solution_function(dof_handler, solution);

size_t cellCount = 0; //keep the cell`s count；
for (const auto& cell : dof_handler.active_cell_iterators())
{
//### cell`s center axis：
dealii::Point<3> cCell = cell->center();

//### set this cell as active cell：
solution_function.set_active_cell(cell);

//### get the cell`s center point solution：
double adata = solution_function.value(cCell);

//### put the solution to a "VectorXd"：
phantVValues(vindex) = adata;

//### next cell：
++cell_Count;
}

Are there any convenient and immediate methods in dealii version9.3.0 to get every cell center value?
I would be very appreciated if you can give me some suggestions.
Looking forward to your reply. Best wishes to you!

--

Yours，
sincerely.

[peterrum]

Take a look at step-19 and how FEPointEvaluation is used:

dealii/examples/step-19/step-19.cc

Line 655 in 9e55702

FEPointEvaluation<1, dim> evaluator(mapping, fe, update_gradients);

You can simply fill particle_positions with your (0.5)^d.

Hope this helps! A general remark: normally we ask such type of questions on the google mailing list.

PM

[JiaShumei92]

Take a look at step-19 and how FEPointEvaluation is used:

dealii/examples/step-19/step-19.cc

Line 655 in 9e55702

FEPointEvaluation<1, dim> evaluator(mapping, fe, update_gradients);

You can simply fill particle_positions with your (0.5)^d.

Hope this helps! A general remark: normally we ask such type of questions on the google mailing list.

PM

Thank you very much for your suggestion. I am learning step-19 now. I am sorry that I cannot open "https://groups.google.com/g/dealii" net address. So I put my question as an issue.

[drwells]

If you always need values at the cell center, another option is to use QMidpoint and FEValues::get_function_values().

[JiaShumei92]

Take a look at step-19 and how FEPointEvaluation is used:

dealii/examples/step-19/step-19.cc

Line 655 in 9e55702

FEPointEvaluation<1, dim> evaluator(mapping, fe, update_gradients);

You can simply fill particle_positions with your (0.5)^d.

Hope this helps! A general remark: normally we ask such type of questions on the google mailing list.

PM

Thank you again! I have gotten my cell centers' solutions following @drwells suggestions and step-9 tutorial 'GradientEstimation' class.
The kernal lines are as follwing:

//
scratch_data.fe_midpoint_value.reinit(cell); //initialize FEValues object at an active cell;

scratch_data.fe_midpoint_value.get_function_values(scratch_data.solution, //my whole solutions at nodes;
scratch_data.cmVal); //vector cmVal(1), to get this cell center solution.

scratch_data.cell_midpoint_value(cell->active_cell_index()) = scratch_data.cmVal[0]; //push this cell center value to a full vector.
//

[JiaShumei92]

If you always need values at the cell center, another option is to use QMidpoint and FEValues::get_function_values().

Thank you very much for your helpful suggestions! I have solved my problem successfully with QMidpoint and FEValues::get_function_values() methods. I put some of my kernal lines at the reply to @peterrum. (It is maybe useful to other users.)
Wish you good day!