[petsc-users] Regarding P4est

[MUKKUND SUNJII]

I agree with the structured nature of the noise. I did play around with the PetscFV implementation a bit to allow for the computation of different fluxes left and right side of every interface. 

Nevertheless it is indeed strange that the problem disappears when I use a PLEX dm.

Regards, 

Mukkund  

> On 17 Jun 2020, at 22:53, Dave May <dave.mayhem23 at gmail.com> wrote:
> 
> 
> 
> On Wed 17. Jun 2020 at 21:21, MUKKUND SUNJII <mukkundsunjii at gmail.com <mailto: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.
> <Bathymetry.png>
> 
> 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.
> <Plex.png>
> 
> 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. 	
> <p4est.png>
> 
> 
> The (wrong) result has seemingly a lot of structure. Have you verified your code using p4est is valgrind clean? This looks too much like a weird indexing bug for me to not ask this question.
> 
> Thanks,
> Dave
> 
> 
> 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 <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 <mailto: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 <mailto: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 

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