CFD Online Discussion Forums - checkerboard pressure and velocity field

Hi foamers,

I'm computing the periodic flow through a ribbed duct. To achieve the desired flow rate, I coupled simpleFoam and channelFoam as follows:

Code:

   Info<< "\nStarting time loop\n" << endl;



    while (simple.loop())

    {

        Info<< "Time = " << runTime.timeName() << nl << endl;



        // --- Pressure-velocity SIMPLE corrector



        // U equation--------------------------------------------------------------------------------------------



       // Momentum predictor



       tmp<fvVectorMatrix> UEqn

       (

           fvm::div(phi, U)

         + turbulence->divDevReff(U)

         ==

           flowDirection*gradP

       );



       UEqn().relax();



       sources.constrain(UEqn());



       solve(UEqn() == -fvc::grad(p));



      // p equation ---------------------------------------------------------------------------------------------

    



          p.boundaryField().updateCoeffs();



          volScalarField rAU(1.0/UEqn().A());

          U = rAU*UEqn().H();

          UEqn.clear();



          phi = fvc::interpolate(U, "interpolate(HbyA)") & mesh.Sf();

          adjustPhi(phi, U, p);



          // Non-orthogonal pressure corrector loop

          while (simple.correctNonOrthogonal())

          {

              fvScalarMatrix pEqn

              (

                  fvm::laplacian(rAU, p) == fvc::div(phi)

              );



              pEqn.setReference(pRefCell, pRefValu             pEqn.solve();



              if (simple.finalNonOrthogonalIter())

              {

                  phi -= pEqn.flux();

              }

           }



          #include "continuityErrs.H"



          // Explicitly relax pressure for momentum corrector

          p.relax();



         // Momentum corrector

          U -= rAU*fvc::grad(p);

          U.correctBoundaryConditions();

          sources.correct(U);



          turbulence->correct();



      // Correct driving force for a constant mass flow rate



      // Extract the velocity in the flow direction

      dimensionedScalar magUbarStar =

            (flowDirection & U)().weightedAverage(mesh.V());



      // Calculate the pressure gradient increment needed to

      // adjust the average flow-rate to the correct value

      dimensionedScalar gragPplus =

          (magUbar - magUbarStar)/rAU.weightedAverage(mesh.V());

       U += flowDirection*rAU*gragPplus;



       gradP += gragPplus;



       Info<< "Uncorrected Ubar = " << magUbarStar.value() << tab

           << "pressure gradient = " << gradP.value() << endl;



       runTime.write();



       #include "writeGradP.H"



       Info<< "ExecutionTime = " << runTime.elapsedCpuTime() << " s"

           << "  ClockTime = " << runTime.elapsedClockTime() << " s"

           << nl << endl;

    }



    Info<< "End\n" << endl;



    return 0;

}

The code works, but I get a strange checkerboard pattern.

https://www.dropbox.com/s/43r2go9ohz...eckerBoard.png

Has anybody an idea where it may come from and how I could remedy this issue? I already tried different meshes.

Any help appreciated!!!
Best,
Pascal