Using PETSc in structured c-grid for CFD and multigrid

[Ben Tay]

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.
> >
>
>
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.mcs.anl.gov/pipermail/petsc-users/attachments/20070127/fd494295/attachment.htm>