[Matt_B]

Hi Foamers,
I'm experiencing a problem with my solver. I have a 3D cylinder mesh, filled with air, with a rotating top lid at a constant angular velocity. I set my solver starting from pisoFoam, which is an incompressible fluid solver, and then I modified it, introducing all needed for a compressible fluid. Before I run it on the whole cylinder mesh and everything worked well; but after I decided to decrease the calculation effort by means of using just 1/4th of the cylinder cutting it by the 2 available symmetry planes. So, in brief, now I'm running on a 90° part of a cylinder.
Since I changed the geometry, the calculation blows up very soon, with just a few time steps, either with bigger or smaller timesteps and independently on the mesh size. I think I defined in a proper way the two simmetry plane with a simmetryPlane patch and every paramether in "0" folder has this defined.
Going deeper into the calculation results, I understand that the U velocity calculation is responsible for this and it seems that the BC defined as symmetryPlane are considered as a wall instead; I see this because I have z components of U along the cylinder height which are not expected at all.
I'm wondering if there is something else I should define also for a symmetric calculation which I don't know, or if there is some known problem with this issue.
Ah, the turbulent model is k-epsilon.

Thanks in advance!

Matteo

[Matt_B]

Has somebody experienced my same troubles with a symmetric volume?
I attach here below a picture of Uz component rising after one timestep (deltat=0.05 s) and of U. I'm expecting not having such a Uz component.
I also attach U dictionary file with the starting boundaries values.

FoamFile
{
    version     2.0;
    format      ascii;
    class       volVectorField;
    object      U;
}
// * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //

dimensions      [0 1 -1 0 0 0 0];

internalField   uniform (0 0 0);

boundaryField
{
    top
    {
        type            rotatingWallVelocity;
        origin (0 0 10);  // è l'asse di rotazione rispetto all'origine degli assi
        axis (1 1 1);  // deve rimanere sempre uguale a (1 1 1)
        omega constant 1;
    }

    side
    {
        type            fixedValue;
        value           uniform (0 0 0);
    }

    bottom
    {
        type            fixedValue;
        value           uniform (0 0 0);
    }
    oneSym
    {
        type            symmetryPlane;
    }
    twoSym
    {
        type            symmetryPlane;
    }
    invisible
    {
        type            zeroGradient;
    }
}

[ngj]

Hi Matteo,

A symmetry plane does exactly what the name suggest, namely that it keeps the flow variables symmetric across the patch. For all scalar quantities this results in a zeroGradient condition, whereas the component of a vector normal to the patch most be zero, because that it the only possibility.

I believe that what you are after is to specify cyclic boundary conditions on the two patches. I have never used them for that type of layout, but the boundary condition do perform a transformation of vectors due to changes in direction of the normal vector for the two patches, thus it should work.

An alternative, again something I have never tried, is to consider whether you could benefit from the wedge-type boundary condition. Many post are available on this topic on the forum.

Good luck

Niels

[Matt_B]

Originally Posted by ngj

Hi Matteo,

A symmetry plane does exactly what the name suggest, namely that it keeps the flow variables symmetric across the patch. For all scalar quantities this results in a zeroGradient condition, whereas the component of a vector normal to the patch most be zero, because that it the only possibility.

I believe that what you are after is to specify cyclic boundary conditions on the two patches. I have never used them for that type of layout, but the boundary condition do perform a transformation of vectors due to changes in direction of the normal vector for the two patches, thus it should work.

An alternative, again something I have never tried, is to consider whether you could benefit from the wedge-type boundary condition. Many post are available on this topic on the forum.

Good luck

Niels

Hi Niels,
thank you for you reply. Now, everthing is clearer to me. I just don't understand how and when using symmetryPlane by the way....I thought it should be implemented for symmetric volume, diving it by means of symmetry axis and mirroring the result of a slice of the volume on the whole volume.
What you say about zeroGradient of scalar and vector quantities makes sense to me for my actual results.
As suggested by you, I will try to define cyclic or wedge patches, but I have to investigate more about their implementation.
Thank you again for now!

Matteo

[chaolian]

Originally Posted by Matt_B

Hi Niels,
thank you for you reply. Now, everthing is clearer to me. I just don't understand how and when using symmetryPlane by the way....I thought it should be implemented for symmetric volume, diving it by means of symmetry axis and mirroring the result of a slice of the volume on the whole volume.
What you say about zeroGradient of scalar and vector quantities makes sense to me for my actual results.
As suggested by you, I will try to define cyclic or wedge patches, but I have to investigate more about their implementation.
Thank you again for now!

Matteo

Hi, Matteo,

I have exactly the same issue. Do you have any idea how to do with symmetryPlane in OpenFOAM?
Thanks in advance.

Chao