Using PETSc in structured c-grid for CFD and multigrid

[Berend van Wachem]

Hi Ben,

The challenge in your problem is how you "glue" the C grid in the back; 
there you will need to do some additional scattering. I would set-up 
the IS for this, and then use that to scatter the values 
into "ghostcells" which will be present on the block(s).

Berend.

> Thank you Berend! I'll go through DA again. I'm also looking at
> HYPRE. Its way of creating grids and linking them seems intuitive.
> Btw, is there a mailing list for HYPRE similar to PETSc to discuss
> problems? I find that their explanation are quite brief.
>
> I tried to install HYPRE 2.0 on windows using cygwin but it failed.
> I then install it as an external software thru PETSc. I think it's
> installing HYPRE 1.0 or something. But similarly, there's illegal
> operation.
>
> Installing HYPRE 2.0 on my school's linux worked, though there's
> seems to be some minor error. So what's the best way to employ
> multigird? Is it to install as an external software thru PETSc or
> just use HYPRE on its own?
>
> Btw, it will be great if you can send me parts of your code
> regarding DA.
>
> Thank you very much!
>
> On 1/26/07, Berend van Wachem <berend at tfd.chalmers.se> wrote:
> > Hi,
> >
> > I am not an expert - but have used PETSc for both structured and
> > unstructured grids.
> >
> > When you use an unstructured code for a structured grid, there is
> > additional overhead (addressing, connectivity) which is redundant;
> > this information is not required for solving on a structured grid.
> > I would say this is maximum a 10% efficiency loss for bigger
> > problems - it does not affect solving the matrix, only in
> > gathering your coefficients. I would not rewrite my CFD code for
> > this.
> >
> > If you only deal with structured grids, using the PETSc DA
> > framework should work for you - you are not saving all
> > connectivity. The DA framework is not difficult at all, according
> > to my opinion. Look at a few examples that come with PETSc. I use
> > a block structured solver - using multiple DA's within one
> > problem. Let me know if you are interested in this, and I can send
> > you parts of code.
> >
> > Multigrid is certainly possible (I would reccomend through HYPRE,
> > discussed on the mailinglist, although I still have problems with
> > it), but the question is how efficient it will be for your CFD
> > problem. For an efficient multigrid in CFD, it is important to
> > consider the coefficient structure arising from the momentum
> > equations - the grouping of cells should occur following the
> > advection term. Only then will you achieve linear scaling with the
> > problem size. For instance, consider a rotating flow in a square
> > box. Most multigrid algorithms will group cells in "squares" which
> > will not lead to a significant improvement, as the flow
> > (advection, pressure grad) does not move in these squares. In
> > fact, to have an efficient multigrid algorithm, the cels should be
> > grouped along the circular flow. As this cannot be seen directly
> > from the pressure coefficients, I doubt any "automatic" multigrid
> > algorithm (in Hypre or Petsc) would be able to capture this, but
> > don't quote me on it - I am not 100% sure. So concluding, if you
> > want to do efficient multigridding for CFD, you will need to point
> > out which cells are grouped into which structure, based upon the
> > upwind advection coefficients.
> >
> > Good luck,
> >
> > Berend.
> >
> > > Hi,
> > >
> > > I was discussing with another user in another forum
> > > (cfd-online.com)
> >
> > about
> >
> > > using PETSc in my cfd code. I am now using KSP to solve my
> > > momentum and poisson eqn by inserting values into the matrix. I
> > > was told that using PETSc
> > > this way is only for unstructured grids. It is very inefficient
> > > and much slower if I'm using it for my structured grid because I
> > > am not
> >
> > exploiting
> >
> > > the regular structure of my grid.
> > >
> > > Is that true? I'm solving flow around airfoil using c-grid.
> > >
> > > So how can I improve? Is it by using DA? I took a glance and it
> > > seems quite
> > > complicated.
> > >
> > > Also, is multigrid available in PETSc? Chapter 7 discusses about
> > > it but
> >
> > it
> >
> > > seems very brief. Is there a more elaborate tutorial besides
> > > that c examples?
> > >
> > > Hope someone can give me some ideas.
> > >
> > > Thank you.