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I calculated the flow around an object using OpenFOAM's simpleFoam and wanted to evaluate it's drag coefficent. Actually the drag coefficient is in a reasonable range for the object so I thought the simulation went well. However when I looked at the flow fields they appear to have some strange diagonal artefacts in them.

The mesh I used looks like this where the flow is coming from the left. It is refined near the object

Mesh

The resulting pressure field looks for some reason like this:

Resulting pressure field

And the velocity field like this:

Resulting velocity field

The flow is coming from the left with a uniform velocity of ~0.03. This corresponds roughly to a Reynolds number of 800 for the object. The calculation was done in laminar mode and the following interpolating schemes where used:

ddtSchemes
{
    default         steadyState;
}

gradSchemes
{
    default         Gauss linear;
    grad(U)         cellLimited Gauss linear 1;
}

divSchemes
{
    default         none;
    div(phi,U)      bounded Gauss linearUpwindV grad(U);
    div(phi,k)      bounded Gauss upwind;
    div(phi,epsilon) bounded Gauss upwind;
    div(phi,omega)  bounded Gauss upwind;
    div((nuEff*dev2(T(grad(U))))) Gauss linear;
}

laplacianSchemes
{
    default         Gauss linear corrected;
}

interpolationSchemes
{
    default         linear;
}

snGradSchemes
{
    // Korrigiert für Non-Orthogonalität
    default         corrected;
}

wallDist
{
    method meshWave;
}

For the solvers the following set-up was used:

solvers
{
    p
    {
        solver          GAMG;
        tolerance       1e-8;
        relTol          0.01;
        smoother        GaussSeidel;
    }

    Phi
    {
        $p;
    }

    U
    {
        solver          smoothSolver;
        smoother        GaussSeidel;
        tolerance       1e-8;
        relTol          0.1;
        nSweeps         1;
    }

    k
    {
        solver          smoothSolver;
        smoother        GaussSeidel;
        tolerance       1e-8;
        relTol          0.1;
        nSweeps         1;
    }

    epsilon
    {
          solver          smoothSolver;
          smoother        GaussSeidel;
          tolerance       1e-08;
          relTol          0.1;
            nSweeps         1;
    }

    omega
    {
        solver          smoothSolver;
        smoother        GaussSeidel;
        tolerance       1e-8;
        relTol          0.1;
        nSweeps         1;
    }
}

SIMPLE
{
    nNonOrthogonalCorrectors 0;
    consistent      yes;
}

potentialFlow
{
    nNonOrthogonalCorrectors 10;
}

relaxationFactors
{
    equations
    {
        U               0.9;
        k               0.7;
        epsilon         0.7;
        omega           0.7;
    }
    fields
    {
        p               0.8;
    }
}

cache
{
    grad(U);
}

What could cause these strange diagonal patterns?

EDIT: As suggested by the comments, I increased the distance between the object and the surrounding walls. However I still got the same issue with the diagonal fragments.

Velocity field with distant walls

Pressure field with distant walls

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  • $\begingroup$ Just at a glance, it looks like the wind tunnel should be made taller, so there's more room for flow to develop around the model - the mesh also looks quite coarse in the wake - I'd refine this a little. How long did it take to run 'as is'? Would it be fairly easy to run again with these changes? $\endgroup$ Feb 13 '20 at 18:10
  • $\begingroup$ What are the boundary conditions at the walls? It looks like flow could be reflecting off them? If they are non-penetrable boundaries, then they need to be much further from the object... $\endgroup$ Feb 13 '20 at 18:12
  • $\begingroup$ Hi, @JonathanRSwift thanks for your comment. Right now I have slip boundary condition for the velocity at the four walls of the channel and zeroGradient boundary condition for pressure. I can try a calculation where the walls are further away from the object, that should not be too much work. I will post the results of that as soon as I'm done. $\endgroup$
    – Axel
    Feb 13 '20 at 19:39
  • $\begingroup$ To add something that has not been said yet; Perhaps the numerical method is oscillating on its solution and what you are seeing is noise from that, there are methods to circumvent that which are ENO and WENO (essentially non oscillatory) which are already used in grid methods with good results. I do not know if openfoam has an ENO method already implemented. $\endgroup$
    – Media
    Feb 14 '20 at 16:29
  • $\begingroup$ @JonathanRSwift I increased the distant between the object and the wall, but unfortunately I still got the issue with the diagonal fragments. You also suggested to refine the mesh in the wake of the object. I will do this next. $\endgroup$
    – Axel
    Feb 16 '20 at 0:33

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