[petsc-users] DMDAGetElements and global/local element number

[Dave May]

On Thu, 12 Sep 2019 at 20:21, Emmanuel Ayala via petsc-users <
petsc-users at mcs.anl.gov> wrote:

> Hi everyone, it would be great if someone can give me a hint for this
> issue, i have been trying to figure out how to solve it, but i did not
> succeed
>
> I'm using DMDA to generate a 3D mesh (DMDA_ELEMENT_Q1). I'm trying to fill
> a MPI matrix with some values wich are related to the dofs of each element
> node, moreover i need to set this values based on the element number.
> Something like:
>
> mpi_A(total_elements X total_dofs)
>
>                                                  total_dofs
> row_0 (element_0)                      a_0 a_1 a_2 ... a_23
> row_1 (element_1)                                     a_0 a_1 a_2 ... a_23
> row_2 (element_2)
>   a_0 a_1 a_2 ... a_23
> .
> .
> .
> row_n (element_n)                            a_0 a_1 a_2 ... a_23
>
> The element number is related to the row index. And the matrix values are
> set depending of the DOFs related to the element.
>
> With DMDAGetElements i can read the LOCAL nodes connected to the element
> and then the DOFs associated to the element. I can handle the local and
> global relations with DMGetLocalToGlobalMapping, MatSetLocalToGlobalMapping
> and MatSetValuesLocal. BUT i CAN NOT understand how to know the element
> number in LOCAL or GLOBAL contex. DMDAGetElements gives the NUMBER OF
> ELEMENTS owned in the local process, but there is not any information about
> the local or global ELEMENT NUMBER.
>
> How to know the local or global element number related to the data
> provided by DMDAGetElements?
>

The DMDA defines cells of the same type (quads (2D) or hex (3D), hence
every cell defines the same number of vertices.

DMDAGetElements(DM dm,PetscInt *nel,PetscInt *nen,const PetscInt *e[])
nel - number of local elements
nen - number of element nodes
e - the local indices of the elements' vertices

e[] defines the ordering of the elements. e[] is an array containing all of
the element-vertex maps. Since each element in the DMDA has the same number
of vertices, the first nen values in e[] correspond to the vertices (local
index) associated with the first element. The next nen values in e[]
correspond to the vertices of the second element.  The vertices for any
(local) element with the index "cid" can be sought via e[nen*cid + i] where
i would range from 0 to nen-1.

Why would you ever want, or need, the global element number? What is the
use case?

Thanks,
Dave




>
> Thank you.
>
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