[petsc-users] Regarding P4est

[Matthew Knepley]

On Wed, Jun 17, 2020 at 3:21 PM MUKKUND SUNJII <mukkundsunjii at gmail.com>
wrote:

> Yes, precisely! I am not sure how I can replicate using the original
> version of ex11.c because it does not support bathymetry.
>
> Regardless, to demonstrate the discrepancy, I have uploaded three plots.
> The scenario is a lake at rest. Essentially, you have a varying bathymetry
> but a level water surface. If the model is well balanced, then the water
> surface height must not change. The description of the files are below
>
> 1) Bathymetry.png : It shows you the bathymetry profile (z(x)) and the
> water surface height (H = h+z(x)) at t = 0.
>

Hmm, that does look like a bug. The purpose of TransferVec is to preserve
moments when transferring from one grid to another. Here it looks like it
is artificially diffusive, but I do not
see a reason for this pattern if you did the refinement initially, and not
with p4est, since then the base grid is the same as the Plex. Is that
what your code does?

  Thanks,

    Matt


>
> 2) Plex.png : This is the water surface height after 1 time step (0.007055
> sec)  and the dm type is Plex. As you can see, the water surface height
> is undisturbed as expected.
>
> 3) P4est.png : This is the result after 1 time step (same final time) if I
> set the dm type as p4est. The noise is in the order of 1e-3 to be a little
> more specific. Since its not specifically at the boundaries and more or
> less spread throughout, it could indeed be noise introduced. But of course
> I could be wrong.
>
> Maybe this paints a better picture.
>
> Regards,
>
> Mukkund
>
> For your reference, the Riemann Solver is a modified version of the HLL
> solver: *A simple well-balanced and positive numerical scheme for the
> shallow-water system by **Emmanuel Audusse, Christophe Chalons, Philippe
> Ung. *
> (
> https://www.intlpress.com/site/pub/files/_fulltext/journals/cms/2015/0013/0005/CMS-2015-0013-0005-a011.pdf
> )
>
> On 17 Jun 2020, at 20:47, Mark Adams <mfadams at lbl.gov> wrote:
>
> So you get this noise with a regular grid in p4est. So the same grid as
> will Plex, and you are not getting the same results.
>
> I don't know of any difference from p4est on a non-adapted grid. Can you
> reproduce this with ex11?
>
> Matt and Toby could answer this better.
>
>
> On Wed, Jun 17, 2020 at 1:33 PM MUKKUND SUNJII <mukkundsunjii at gmail.com>
> wrote:
> Greetings,
>
> I am a master’s student working on the shallow water model of the TS
> example 'ex11.c' as part of my thesis. Therefore, I am working with
> DMForest for the implementation of adaptive grids. I have a question and an
> observation.
>
> I am trying to find relevant information about interpolation that takes
> place through the routine DMForestTransferVec. Perhaps it could be my
> inability to find it, but I am unable to locate the implementation of the
> routine
>
> (forest->transfervec)(dmIn,vecIn,dmOut,vecOut,useBCs,time).
>
> Any information on this particular routine is highly appreciated.
>
> Furthermore, I have developed a well balanced Riemann Solver that includes
> topography in the model. In the process of testing both the non-adaptive
> and adaptive version, I found that my results differed when I changed the
> type of DM. For instance, when I run a scenario in a fixed, non-adaptive
> grid  with a DM of type 'P4est', I find that the well balanced nature is
> lost due to small perturbations all across the domain. However, this does
> not occur when I use a DM of type ‘plex’. Is there a radical change in the
> routines between the two DM’s? This is not as much of a question as it is
> an observation.
>
> Thank you for all of your suggestions!
>
> Regards,
>
> Mukkund
>
>
>
>

-- 
What most experimenters take for granted before they begin their
experiments is infinitely more interesting than any results to which their
experiments lead.
-- Norbert Wiener

https://www.cse.buffalo.edu/~knepley/ <http://www.cse.buffalo.edu/~knepley/>
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