[dnl_xr]

Hello,

i am doing research on the pressure drop due to rapid diameter expansion of pipes for small Reynoldsnumbers (laminar flow). My geometry/mesh can be seen in the attachments. First i tried a structured Mesh with cyclic boundary conditions. The segment itself is 45°. I did this to save computation time bc its an Axisymmetric flow type. But something strange happens. The Line which is also the symmetry axis seems to do strange things. I looked up a few tutorials and there they defined the centerline curve as physical curve in Blockmesh-then its set to empty in the constant/polyMesh/boundary file. I tried to do the same in GMSH but after gsmhToFoam import there is no Axis entry.

Nevertheless i came across another Model- Axisymmetric flow wiht wegde boundary condition for the sides. This works perfectly fine for my case. I did follow these instructions : https://openfoamwiki.net/index.php/M...s/AxiSymmetric

BUT - i wanted to check if the results match up with a real 3D simulation with a unstructured mesh. The velocity for in and outlet are nearly equal.

But the inletPressure is in the 3D case a lot higher then in the axisymmetric case with the wedge bc.

I am wondering what am i doing wrong.

1.: why is the case with the cyclic bc not working properly

2.: why is there a huge difference between the inletPressure (3D compared to axisymmetric with wedge bc)

3.: i implemented the fully developed inlet condition using codedFixedValue bc i need a parabolic inlet velocityprofile. I think its working fine but there is a little difference in the gradient near the wall- i will include a picture. So is the velocityprofile implemented correct.

here are my used boundary conditions p/U for the cyclic case:

/*--------------------------------*- C++ -*----------------------------------*\
| =========                 |                                                 |
| \\      /  F ield         | OpenFOAM: The Open Source CFD Toolbox           |
|  \\    /   O peration     | Version:  v2312                                 |
|   \\  /    A nd           | Website:  www.openfoam.com                      |
|    \\/     M anipulation  |                                                 |
\*---------------------------------------------------------------------------*/
FoamFile
{
    version     2.0;
    format      ascii;
    class       volScalarField;
    object      p;
}
// * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //

dimensions      [0 2 -2 0 0 0 0];

internalField   uniform 0;

boundaryField
{
    inlet
    {
    type            zeroGradient;
    value           uniform 0;

    }

    outlet
    {
        type            fixedMean;
    meanValue    0;
        value           uniform 0;
    }

    wall
    {
        type            zeroGradient;
    }
    
    "(cyclic_0|cyclic_1)"
    {
        type            cyclic;
    }

}


// ************************************************************************* //

/*--------------------------------*- C++ -*----------------------------------*\
| =========                 |                                                 |
| \\      /  F ield         | OpenFOAM: The Open Source CFD Toolbox           |
|  \\    /   O peration     | Version:  v2312                                 |
|   \\  /    A nd           | Website:  www.openfoam.com                      |
|    \\/     M anipulation  |                                                 |
\*---------------------------------------------------------------------------*/
FoamFile
{
    version     2.0;
    format      ascii;
    class       volVectorField;
    object      U;
}
// * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //

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

internalField   uniform (0 0 0);

boundaryField
{
    inlet
    {
        type            codedFixedValue;
        value           uniform (0 0 0);
        
        name inletLaminarProfil;
        code
        #{
             const fvPatch& boundaryPatch = patch(); //generic
             const vectorField& Cf = boundaryPatch.Cf(); //generic
             vectorField& field = *this; //generic

             const scalar yc      =     0; // definition of the center y coordinate
             const scalar rpipe    =     0.00025; // definition of the pipe radius
             const scalar Umax  =     0.596; // definition of the maximum velocity Re=10

             forAll(Cf, faceI) // loop over all the patch faces
             {
//                const scalar x = Cf[faceI].x(); // x coordinate of the faces i
                const scalar y = Cf[faceI].y(); // y coordinate of the faces i
                const scalar z = Cf[faceI].z(); // z coordinate of the faces i
                const scalar radius = pow((y-yc)*(y-yc)+z*z,0.5); // compute radius from center patch

        field[faceI] = vector( Umax *(1-pow(radius/rpipe,2)), 0, 0); // define velocity value on the face i
             }

        #};
    }


    outlet
    {
        type            zeroGradient;
    }

    wall
    {
        type            noSlip;
    }
    
    "(cyclic_0|cyclic_1)"
    {
        type            cyclic;
    }

}

createPatchDict:

FoamFile
{
    version     2.0;
    format      ascii;
    class       dictionary;
    object      createPatchDict;
}

// This application/dictionary controls:
// - optional: create new patches from boundary faces (either given as
//   a set of patches or as a faceSet)
// - always: order faces on coupled patches such that they are opposite. This
//   is done for all coupled faces, not just for any patches created.
// - optional: synchronise points on coupled patches.

// 1. Create cyclic:
// - specify where the faces should come from
// - specify the type of cyclic. If a rotational specify the rotationAxis
//   and centre to make matching easier
// - always create both halves in one invocation with correct 'neighbourPatch'
//   setting.
// - optionally pointSync true to guarantee points to line up.

// 2. Correct incorrect cyclic:
// This will usually fail upon loading:
//  "face 0 area does not match neighbour 2 by 0.0100005%"
//  " -- possible face ordering problem."
// - in polyMesh/boundary file:
//      - loosen matchTolerance of all cyclics to get case to load
//      - or change patch type from 'cyclic' to 'patch'
//        and regenerate cyclic as above

// Do a synchronisation of coupled points after creation of any patches.
// Note: this does not work with points that are on multiple coupled patches
//       with transformations (i.e. cyclics).
pointSync false;

// Patches to create.
patches
(
    {   
        // Name of new patch
        name cyclic_0;

        // Dictionary to construct new patch from
        patchInfo
        {   
            type cyclic;
            neighbourPatch cyclic_1;

            // Optional: explicitly set transformation tensor.
            // Used when matching and synchronising points.
            transform rotational;
            rotationAxis (1 0 0);
            rotationCentre (0 0 0);
            rotationAngle 22.5;
            // transform translational;
            // separationVector (1 0 0);

            // Optional non-default tolerance to be able to define cyclics
            // on bad meshes
            matchTolerance 1E-4;
        }

        // How to construct: either from 'patches' or 'set'
        constructFrom patches;

        // If constructFrom = patches : names of patches. Wildcards allowed.
        patches (front);
    }
    {   
        // Name of new patch
        name cyclic_1;

        // Dictionary to construct new patch from
        patchInfo
        {   
            type cyclic;
            neighbourPatch cyclic_0;

            // Optional: explicitly set transformation tensor.
            // Used when matching and synchronising points.
            transform rotational;
            rotationAxis (1 0 0);
            rotationCentre (0 0 0);
            rotationAngle -22.5;
            // transform translational;
            // separationVector (1 0 0);

            // Optional non-default tolerance to be able to define cyclics
            // on bad meshes
            matchTolerance 1E-4;
        }

        // How to construct: either from 'patches' or 'set'
        constructFrom patches;

        // If constructFrom = patches : names of patches. Wildcards allowed.
        patches (back);
    }
);

wedge type case:

/*--------------------------------*- C++ -*----------------------------------*\
| =========                 |                                                 |
| \\      /  F ield         | OpenFOAM: The Open Source CFD Toolbox           |
|  \\    /   O peration     | Version:  v2312                                 |
|   \\  /    A nd           | Website:  www.openfoam.com                      |
|    \\/     M anipulation  |                                                 |
\*---------------------------------------------------------------------------*/
FoamFile
{
    version     2.0;
    format      ascii;
    class       volVectorField;
    object      U;
}
// * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //

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

internalField   uniform (0 0 0);

boundaryField
{
    
    /*  
    inlet
    {
        type            fixedValue;
        value             uniform (1 0 0);
        
    }
    */
    
    inlet
    {
        type            codedFixedValue;
        value           uniform (0 0 0);
        
        name inletLaminarProfil;
        code
        #{
             const fvPatch& boundaryPatch = patch(); //generic
             const vectorField& Cf = boundaryPatch.Cf(); //generic
             vectorField& field = *this; //generic

             const scalar yc      =     0; // definition of the center y coordinate
             const scalar rpipe    =     0.00025; // definition of the pipe radius
             const scalar Umax  =     0.596; // definition of the maximum velocity Re=1

             forAll(Cf, faceI) // loop over all the patch faces
             {
//                const scalar x = Cf[faceI].x(); // x coordinate of the faces i
                const scalar y = Cf[faceI].y(); // y coordinate of the faces i
                const scalar z = Cf[faceI].z(); // z coordinate of the faces i
                const scalar radius = pow((y-yc)*(y-yc)+z*z,0.5); // compute radius from center patch

        field[faceI] = vector( Umax *(1-pow(radius/rpipe,2)), 0, 0); // define velocity value on the face i
             }

        #};
    }


    outlet
    {
        type            zeroGradient;
    }

    wall
    {
        type            noSlip;
    }
    
    
    "(front|back)"
    {
        type            wedge;
    }
    
}


// ************************************************************************* //

/*--------------------------------*- C++ -*----------------------------------*\
| =========                 |                                                 |
| \\      /  F ield         | OpenFOAM: The Open Source CFD Toolbox           |
|  \\    /   O peration     | Version:  v2312                                 |
|   \\  /    A nd           | Website:  www.openfoam.com                      |
|    \\/     M anipulation  |                                                 |
\*---------------------------------------------------------------------------*/
FoamFile
{
    version     2.0;
    format      ascii;
    class       volScalarField;
    object      p;
}
// * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //

dimensions      [0 2 -2 0 0 0 0];

internalField   uniform 0;

boundaryField
{
    inlet
    {
    type            zeroGradient;
    value           uniform 0;

    }

    outlet
    {
        type            fixedMean;
    meanValue    0;
        value           uniform 0;
    }

    wall
    {
        type            zeroGradient;
    }
    
    
    "(front|back)"
    {
        type            wedge;
    }
}


// ************************************************************************* //

and 3D :

/*--------------------------------*- C++ -*----------------------------------*\
| =========                 |                                                 |
| \\      /  F ield         | OpenFOAM: The Open Source CFD Toolbox           |
|  \\    /   O peration     | Version:  v2312                                 |
|   \\  /    A nd           | Website:  www.openfoam.com                      |
|    \\/     M anipulation  |                                                 |
\*---------------------------------------------------------------------------*/
FoamFile
{
    version     2.0;
    format      ascii;
    class       volVectorField;
    object      U;
}
// * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //

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

internalField   uniform (0 0 0);

boundaryField
{
    
    /*  
    inlet
    {
        type            fixedValue;
        value             uniform (1 0 0);
        
    }
    */
    
    inlet
    {
        type            codedFixedValue;
        value           uniform (0 0 0);
        
        name inletLaminarProfil;
        code
        #{
             const fvPatch& boundaryPatch = patch(); //generic
             const vectorField& Cf = boundaryPatch.Cf(); //generic
             vectorField& field = *this; //generic

             const scalar yc      =     0; // definition of the center y coordinate
             const scalar rpipe    =     0.00025; // definition of the pipe radius
             const scalar Umax  =     0.596; // definition of the maximum velocity Re=1

             forAll(Cf, faceI) // loop over all the patch faces
             {
//                const scalar x = Cf[faceI].x(); // x coordinate of the faces i
                const scalar y = Cf[faceI].y(); // y coordinate of the faces i
                const scalar z = Cf[faceI].z(); // z coordinate of the faces i
                const scalar radius = pow((y-yc)*(y-yc)+z*z,0.5); // compute radius from center patch

        field[faceI] = vector( Umax *(1-pow(radius/rpipe,2)), 0, 0); // define velocity value on the face i
             }

        #};
    }


    outlet
    {
        type            zeroGradient;
    }

    wall
    {
        type            noSlip;
    }
    
    
    "(front|back)"
    {
        type            wedge;
    }
    
}


// ************************************************************************* //

/*--------------------------------*- C++ -*----------------------------------*\
| =========                 |                                                 |
| \\      /  F ield         | OpenFOAM: The Open Source CFD Toolbox           |
|  \\    /   O peration     | Version:  v2312                                 |
|   \\  /    A nd           | Website:  www.openfoam.com                      |
|    \\/     M anipulation  |                                                 |
\*---------------------------------------------------------------------------*/
FoamFile
{
    version     2.0;
    format      ascii;
    class       volScalarField;
    object      p;
}
// * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //

dimensions      [0 2 -2 0 0 0 0];

internalField   uniform 0;

boundaryField
{
    inlet
    {
    type            zeroGradient;
    value           uniform 0;

    }

    outlet
    {
        type            fixedMean;
    meanValue    0;
        value           uniform 0;
    }

    wall
    {
        type            zeroGradient;
    }
    
    
    "(front|back)"
    {
        type            wedge;
    }
}


// ************************************************************************* //

Did i do something wrong with the implementation of U? or did i set my bc completely wrong?

Hope someone could help me out.

I cant explain myself why there is a huge difference in inlet pressure... even though the velocities match perfect for 3D and wedge case