Extracting element solution in step-40

[Wasim Niyaz Munshi ce21d400]

Hello everyone.

I want to extract the element solution vector from the global solution once the problem is solved in step-40. For a serial code, I would do something like this:

int i=0;

for (const auto vertex : cell->vertex_indices())
{
int a = (cell->vertex_dof_index(vertex, 0));
element_sol[i] =  solution_vector[a];

              i=i+1;
}

I don't have a solution_vector for a parallel code, but a locally_relevant_solution. I want to know that, given this locally_relevant_solution and the cell, how do I get the element_sol?

The global_dof will not be helpful here, as the solution_vector is distributed across a number of processors.

Thanks and regards

Wasim

[Daniel Arndt]

Wasim,

The answer depends very much on what you actually want to do with that solution vector.

Do you want a representation of the solution (assuming you are using Q1? nodal elements) on a single process/all processes

or are you just interested in the partial solution on every process separately?

What you are doing looks quite similar to what DataOut would give you for visualizing solutions.

Best,

Daniel

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[Wolfgang Bangerth]

On 4/6/23 06:02, Wasim Niyaz Munshi ce21d400 wrote:
>
> I don't have a solution_vector for a parallel code, but a
> locally_relevant_solution. I want to know that, given this
> locally_relevant_solution and the cell, how do I get the element_sol?
> The global_dof will not be helpful here, as the solution_vector is
> distributed across a number of processors.

Daniel's question is correct, but to this specific point: A distributed
vector (and it's locally relevant incarnation) is still a global vector,
indexed by global degree of freedom numbers, and so the code remains
correct.

Best
W.

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------------------------------------------------------------------------
Wolfgang Bangerth email: bang...@colostate.edu
www: http://www.math.colostate.edu/~bangerth/

[Wasim Niyaz Munshi ce21d400]

I appreciate the clarification. I thought that global indexing was no longer present as the solution vector is distributed.

I have one more doubt. I want to create a vector (H_vector) that stores some value for each Gauss point in the domain.

For a serial problem, I was doing something like this: H_vector = Vector<double> (8*(triangulation.n_active_cells()));

8 because the problem is in 3d, so I have 8 Gauss points per cell.

Now, for a MPI code, this H_vector would also be  LA::MPI::Vector, and its size should be 8*no. of cells owned by the processor.

How do I get the no.of cells owned by the processor?

Thanks and regards

Wasim

[Wolfgang Bangerth]

On 4/6/23 10:18, Wasim Niyaz Munshi ce21d400 wrote:
> How do I get the no.of cells owned by the processor?

Triangulation::n_locally_owned_active_cells().

[Wasim Niyaz Munshi ce21d400]

Thank You, Prof. Bangerth.

I have one more doubt about the output of step-40.

For cycle 0, there are 1024 cells (32*32 mesh). So, we have 33 nodes each, along x and y. So, shouldn't the number of DOFs be 1089 instead of 4225?

Regards

Wasim

[Daniel Arndt]

Wasim,

step-40 uses Q2 elements which means that you have 9 dofs/cell in 2D and for 32 cells in each direction (33+32)^2=65^2=4225 dofs.

Best,

Daniel

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