[petsc-users] DMSwarm losing particles with a non-uniform mesh

[Dave May]

On Mon, 4 Apr 2022 at 12:07, Joauma Marichal <joauma.marichal at uclouvain.be>
wrote:

> Hello,
>
> I have written before as I am trying use the DMSwarm library to track
> particles over a collocated non-uniform mesh with ghost cells.
> I have been able to deal with the collocated and ghost cell issues by
> creating an intermediate DMDA.
> However, I lose particles when my mesh is non-uniform. I have re-written a
> function similar to DMDASetUniformCoordinates but I still have issues when
> my cells have varying sizes.
>

Right.

As I wrote in my previous email, the native PIC support with DA will only
work with coordinates created using DMDASetUniformCoordinates.
The point location DMDA provides is very simple - you can find it here
  src/dm/impls/da/dageometry.c : DMLocatePoints_DA_Regular()

If your cell DM coordinates are not uniform, then you need to provide your
own point location routine to the DM.
In your code, you would do this as follows
  da_swarm->ops->locatepoints = CUSTOM_POINT_LOCATION_FUNCTION
with a signature matching

*PetscErrorCode DMLocatePoints(DM dm,Vec pos,DMPointLocationType
ltype,PetscSF cellSF)*




> I attach a small code to this e-mail that reads particles coordinates from
> a file and stores them in a DMSwarm structure. My code works well when I
> use uniform coordinates but whenever I change this, I lose several
> particles after calling the migration function.
>
> Can it be due to how I define my coordinates? If yes, why? Or is it due to
> how the migrate function is implmented?
>

The problem has nothing to do with Migrate. Here is the issue.
When you set the type to DMSWARM_PIC, a particular migration function was
selected.
The migration function selected does the following:
* It calls the point location routine (DMLocatePoints) for the cellDM you
provided for all swarm points
* Any swarm points which were not located in the sub-domain of the cellDM
are scattered to the neighbouring cellDM sub-domains.
* When the scatter has finished, the DM point location routine is called
again on the received swarm points.
* If any received swarm points are located in the sub-domain, they are
added to the swarm object.

Thanks,
Dave


> Best regards,
>
> Joauma
>
> PS. the code is run with: mpirun -np 3 ./cobpor.
>
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