[petsc-users] DOF in DMDACreate3d and staggered grid

[Jed Brown]

On Sun, Jun 17, 2012 at 7:26 AM, TAY wee-beng <zonexo at gmail.com> wrote:

>
> On 16/6/2012 9:24 AM, Jed Brown wrote:
>
> It depends how you want to solve the problem. I usually group all dofs
> together. There is a 2D Stokes+Thermodynamics example in SNES ex30 (or 31?).
>
>
> I tried to understand ex30. I have some questions since it's in C and I'm
> using to Fortran programming.
>

This looks about right, see src/dm/examples/tutorials/ex11f90.F.


>
> Supposed I have a field - u,v,w,p, so in order to use them, I do the
> following:
> *
> type field
>
> real u,v,w,p
>
> end type field
>
> type(field), pointer :: field1(:,:,:)    -> make a derived variable*
>
> Also:
> *
> Vec field_local,field_global
>
> call DMDACreate3d with dof = 4
>
> call DMCreateGlobalVector(da, field_local,ierr)
>
> call DMCreateLocalVector(da,field_global,ierr)
>
> call DMGetLocalVector(da, field_local,ierr)    ->    To insert values
>
> call DMDAVecGetArrayF90(da, field_local,field1,ierr)
>
> do k = zs, zs + zm - 1
>
> do j = ys, ys + ym -1
>
>     do i = xs, xs + xm - 1
>
>         field1(i,j,k)%u = ...     ->    evaluate u,v,w,p etc
>
>     end do
>
> end do
>
> call DMDAVecRestoreArrayF90(da,field_local,field1,ierr)
>
> call DMLocalToGlobalBegin(da,field_local,INSERT_VALUES,field_global,ierr)
>
> call DMLocalToGlobalEnd(da,field_local,INSERT_VALUES,field_global,ierr)
>
> call DMRestoreLocalVector(da,field_local,ierr)*
>
> Is this the correct way?
>
> Also, supposed I now want to solve my u,v,w momentum eqns. Although
> they're not coupled together, I believe it's faster if I assemble them into
> 1 big matrix.
>
> So for Ax = b, x =
> (field(1,1,1)%u,field(1,1,1)%v,field(1,1,1)%w,field(2,1,1)%u.... )
>
> For b, do I duplicate a Vec similar to field_local?
>
> What about matrix A? Do I use the MatSetValuesStencil?
>

Yes


>
> Lastly, the type field contains u,v,w and p. However, I'm only solving
> u,v,w. Do I have to skip some values or use identity matrix to solve it?
>

Why not make a field that contains only u,v,w. I don't see what you're
trying to do.
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