# [petsc-users] Could DMComposite handle two vectors with different grid sizes.

(Rebecca) Xuefei YUAN xy2102 at columbia.edu
Tue Jul 27 22:11:09 CDT 2010

```Dear Barry,

I went over the mp1.c as the multi physics example. However, what I
concerned is not addressed in that example. My concern is that will it
be fine if the grid size for the different das are different.

The different grid sizes of da_A and da_B are coming from the fact
that they share the same physical domain, but the computational domain
are slightly different due to that problem A has the Neumann boundary
condition (giving that the computational domain is half spacing
extended from the physical domain, each point is expressed at the
center of the cell) and problem B has the Dirichlet boundary condition
(giving that the computational domain is the same as the physical
domain, each point is expressed at the node of the cell.) Thus,
problem A ALWAYS needs 1 more grid in each direction than problem B at
each level.

If the coarsest grid has the difference 1(mA - mB = 1) in each
direction, according to the algorithm for multigrid, the next finer
level has the difference of ((2*mA-1) - (2*mB-1) = 2(mA-mB) = 2) in
each direction, the next level has the difference of ((2*(2*mA-1)-1) -
(2*(2*mB-1)-1) = 4*(mA-mB) = 4) in each direction. Because there is
only half spacing being extended from the physical domain as the
computational domain for problem A, the abandonNumber of grid is 1-1=0
for nl=1, 2-1=1 for nl=2 and 4-1=3 for nl=3.

To keep those extra points in the residual function and Jacobian, I
applied the Neumann boundary condition for those points (mirror with
the interior points).

Will this procedure be fine for such a multi physics problem?

Thanks very much!

Rebecca

Quoting Barry Smith <bsmith at mcs.anl.gov>:

>
>
>    What you are doing below looks ok. This builds the DM and DMMG.
> See tutorials/multiphysics/mp1.c for an example that uses SNES to
> solve a nonlinear equation on the grids.
>
>    Barry
>
>
>
> On Jul 27, 2010, at 7:36 PM, (Rebecca) Xuefei YUAN wrote:
>
>> Dear all,
>>
>> If I have two different vectors (with different dof, stencil width
>> and grid sizes), could I use DMComposite to handle this?
>>
>> The following is the piece of code for assemble this DMComposite in DMMG.
>>
>> 	tempparameters.mxfield				= 5;
>> 	tempparameters.myfield				= 5;
>>
>> 	tempparameters.mxgrid	= tempparameters.mxfield+1;
>> 	tempparameters.mygrid	= tempparameters.myfield+1;
>>
>> 	ierr	= DMMGCreate(comm, tempparameters.numberOfLevels, &appCtx,
>> &dmmg);CHKERRQ(ierr);
>>
>> 	ierr	= DMCompositeCreate(comm,&packer);CHKERRQ(ierr);
>>
>> 	ierr	= DACreate2d(comm,DA_NONPERIODIC,DA_STENCIL_BOX,
>> tempparameters.mxfield, tempparameters.myfield, PETSC_DECIDE,
>> PETSC_DECIDE, 4, 3, PETSC_NULL, PETSC_NULL, &da_fff);CHKERRQ(ierr);
>> 	ierr	= DASetFieldName(da_fff, 0, "phi");CHKERRQ(ierr);
>> 	ierr	= DASetFieldName(da_fff, 1, "vz");CHKERRQ(ierr);
>> 	ierr	= DASetFieldName(da_fff, 2, "psi");CHKERRQ(ierr);
>> 	ierr	= DASetFieldName(da_fff, 3, "bz");CHKERRQ(ierr);
>> 	ierr	= DACreate2d(comm,DA_NONPERIODIC,DA_STENCIL_BOX,
>> tempparameters.mxgrid, tempparameters.mygrid, PETSC_DECIDE,
>> PETSC_DECIDE, 1, tempparameters.gridStencilWidth, PETSC_NULL,
>> PETSC_NULL, &da_ggg);CHKERRQ(ierr);
>> 	ierr	= DASetFieldName(da_ggg, 0, "disp");CHKERRQ(ierr);
>>
>>
>> 	ierr	= DMMGSetDM(dmmg, (DM)packer);CHKERRQ(ierr);
>>
>>
>> .........................................................
>>
>> 	ierr	= DMCompositeDestroy(packer);CHKERRQ(ierr);
>> 	ierr	= DMMGDestroy(dmmg);CHKERRQ(ierr);
>>
>>
>> I understand that for numberOfLevels(nl)>1, the grid size
>> difference is no longer 1, but 3(nl=2), 5(nl=3), could I use
>> DMComposite to handle such a case successfully?
>>
>> I was not able to get examples for such a case, is there any?
>>
>> Thanks a lot!
>>
>>
>>
>> --
>> (Rebecca) Xuefei YUAN
>> Department of Applied Physics and Applied Mathematics
>> Columbia University
>> Tel:917-399-8032
>> www.columbia.edu/~xy2102
>>
>
>
>

--
(Rebecca) Xuefei YUAN
Department of Applied Physics and Applied Mathematics
Columbia University
Tel:917-399-8032
www.columbia.edu/~xy2102

```