Bad parallelization. Has this ever happened to you??? :O
 

Hi,

I was trying to solve the problem of a magnetically confined conductive fluid in a periodic cylinder. The results for a serial simulation gives resonable results... But when I run the simulation for 2 processors one can distinguish 2 zones in the cylinder (one per processor). If I am using 3 one can distinguish 3 zones. Same for 4.

I suppose that there is a problem in the comunication between the cells at the boundaries of each processors zone. I am using scotch method.

I tried (it did not work):

1. Change the method to simple.
2. Add a correctBoundaryConditions() in the equation for B in mhdFoam.

I suppose that the problem is in the solver that I am using (a mhdFoam slightly modified). mhdFoam works well with parallelization. But when one wants to add a external magnetic field one has to modify the equation for B (with , is the same for all t).

• Induction equation in mhdFoam looks like:
  
  Code:

  ---

          while (bpiso.correct())
        {
            fvVectorMatrix BEqn // the magnetic induction equation, calculates the magnetic field taking into account the influence of the velocity.
            (
                fvm::ddt(B)
              + fvm::div(phi, B)
              - fvm::laplacian(DB, B)
              - fvc::div(phiB, U)
            );

            BEqn.solve();
            phiB = fvc::flux(B);//phiB is calculated at faces.
           
            while (bpiso.correctNonOrthogonal())
            {
                fvScalarMatrix pBEqn //A Poisson equation is derived for the fictitious magnetic pressure pB and solved
                (
                    fvm::laplacian(pB) == fvc::div(phiB)
                );

                pBEqn.solve();

                if (bpiso.finalNonOrthogonalIter()) //The variable distribution is then employed to correct the phiB values
                {
                    phiB -= pBEqn.flux();
                }
               
            }// The loop exits after the prescribed iterations stated in the fvSolution file.
            #include "magneticFieldErr.H" //print magnetic field divergence deviation from null value
        }

  ---

• My induction equation in modified mhdFoam looks like
  
  Code:

  ---

          #include "readBFixed.H"
        while (bpiso.correct())
        {
            fvVectorMatrix BVbleEqn
            (
                fvm::ddt(BVble)
              + fvm::div(phi,BVble)
              + fvc::div(phi,BFixed)
              - fvc::div(phiBVble,U)
              - fvc::div(phiBFixed,U)
              - fvm::laplacian(lambda,BVble)
              - fvc::laplacian(lambda,BFixed)
            );
           
            BVbleEqn.solve();
            phiBVble = fvc::flux(BVble);

            while (bpiso.correctNonOrthogonal())
            {
                fvScalarMatrix pBVbleEqn
                (
                    fvm::laplacian(pBVble) == fvc::div(phiBVble)
                );

                pBVbleEqn.solve();

                if (bpiso.finalNonOrthogonalIter())
                {
                    phiBVble -= pBVbleEqn.flux();
                }
            }
            #include "magneticFieldErr.H"

            BVble.correctBoundaryConditions();         
        }
        B=BVble+BFixed;
        phiB = fvc::flux(B);

  ---

Maybe the problem is to sum B=BVble+BFixed in this way??

Attachment 73469

Attachment 73470

I tried the same case with the OF series solver for MHD. I used the same configuration and the same magnetic field profile but without an external magnetic field source at t> 0, that is, it is an MHD decay. The result has been the same as with the modified solver.

I have tried the same thing in a box with periodic conditions on the sides and I have not seen anything unusual.

It seems that the quality of the parallelization in OF depends on how simple the mesh is. If it is a bit complicated it seems that parallelization does not work so well. If this is true is an OF internal issue and I cannot do anything.

Am I right?

yes you can. submit an issue ticket in gitlab repo.

Hi HPE,

Thanks for answering. I do not understand what you mean :(

My apologies. I wrote that while I was in bed. :)

>> If this is true is an OF internal issue and I cannot do anything.

I meant that if you think that there is an internal issue in OF, you can submit a bug ticket in gitlab repository, so that the developers can have a look at the issue, and evaluate it for you.