[petsc-users] Regarding P4est-Petsc Mapping

[MUKKUND SUNJII]

Greetings, 

This is with regards to the exchange (see below) I had pertaining to P4est and DMPlex. The problem only persists with DM of type P4est. 

As suggested by Mark Adams, the possible source of the problem (in my case) could be in the mapping from P4est to PETSc. In an effort to understand more, I went digging and I found this file:
https://gitlab.com/petsc/petsc/-/blob/master/src/dm/impls/forest/p4est/dmp4est.c#L4 <https://gitlab.com/petsc/petsc/-/blob/master/src/dm/impls/forest/p4est/dmp4est.c#L4>

In line 4, the mapping for the faces from p4est to PETSc are defined. Any explanation of the contents of the array P4estFaceToPetscFace[] can help me out a lot. 

Thank you in advance. 

Regards, 

Mukkund   

For Context: I am trying to expand the shallow water equation Riemann Solver provided in ./ts/tutorials/ex11.c to support bathymetry. I have successfully implemented a well-balanced Riemann Solver that works fine with PLEX DM but the behaviour changes when I use P4est DM (for adaptive mesh refinement). 


> Begin forwarded message:
> 
> From: MUKKUND SUNJII <mukkundsunjii at gmail.com>
> Subject: Re: [petsc-users] Regarding P4est
> Date: 17 June 2020 at 21:20:29 CEST
> To: Mark Adams <mfadams at lbl.gov>
> Cc: petsc-users <petsc-users at mcs.anl.gov>, Domenico Lahaye <D.J.P.Lahaye at tudelft.nl>
> 
> 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.
> 
> 
> 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 <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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