[petsc-users] Question about DMPLEX/P4EST with different Sections

[Berend van Wachem]

Dear All,

I have started to use DMPLEX with P4EST for a computational fluid 
dynamics application. I am solving a coupled system of 4 discretised 
equations (for 3 velocity components and one pressure) on a mesh. 
However, next to these 4 variables, I also have a few single field 
variables (such as density and viscosity) defined over the mesh, which I 
don't solve for (they should not be part of the matrix with unknowns). 
Most of these variables are at the cell centers, but in a few cases, it 
want to define them at cell faces.

With just DMPLEX, I solve this by:

DMPlexCreateMesh, so I get an initial DM
DMPlexCreateSection, indicating the need for 4 variables
DMSetLocalSection
DMCreateGlobalVector (and Matrix), so I get an Unknown vector, a RHS 
vector, and a matrix for the 4 variables.

To get a vector for a single variable at the cell center or the cell 
face, I clone the original DM, I define a new Section on it, and then 
create the vector from that which I need (e.g. for density, viscosity or 
a velocity at the cell face).

Then I loop over the mesh, and with MatSetValuesLocal, I set the 
coefficients. After that, I solve the system for multiple timesteps 
(sequential solves) and get the solution vector with the 4 variables 
after each solve.

So-far, this works fine with DMPLEX. However, now I want to use P4EST, 
and I have difficulty defining a variable vector other than the original 4.

I have changed the code structure:

DMPlexCreateMesh, so I get an initial DM
DMPlexCreateSection, indicating the need for 4 variables
DMSetLocalSection
DMForestSetBaseDM(DM, DMForest) to create a DMForest
DMCreateGlobalVector (and Matrix), so I get a Unknown vector, a RHS 
vector, and a matrix for the 4 variables

then I perform multiple time-steps,
   DMForestTemplate(DMForest ->  DMForestPost)
   Adapt DMForestPost
   DMCreateGlovalVector(DMForestPost, RefinedUnknownVector)
   DMForestTransferVec(UnknownVector , RefinedUnknownVector)
   DMForestPost -> DMForest
and then DMConvert(DMForest,DMPLEX,DM)
and I can solve the system as usual. That also seems to work.

But my conceptual question: how can I convert the other variable vectors 
(obtained with a different section on the same DM) such as density and 
viscosity and faceVelocity within this framework?
The DMForest has the same Section as the original DM and will thus have 
the space for exactly 4 variables per cell. I tried pushing another 
section on the DMForest and DMForestPost, but that does not seem to 
work. Please find attached a working example with code to do this, but I 
get the error:

PETSC ERROR: PetscSectionGetChart() line 513 in 
/usr/local/petsc-3.12.4/src/vec/is/section/interface/section.c Wrong 
type of object: Parameter # 1

So, I is there a way to DMForestTransferVec my other vectors from one 
DMForest to DMForestPost. How can I do this?

Many thanks for your help!

Best wishes, Berend.
-------------- next part --------------
A non-text attachment was scrubbed...
Name: example-dmplexp4est.c
Type: text/x-csrc
Size: 8137 bytes
Desc: not available
URL: <http://lists.mcs.anl.gov/pipermail/petsc-users/attachments/20200312/9a175b23/attachment.bin>