Week 8 - BlockMesh Drill down challenge

AIM : To create blockMesh and to simulate the flow through backward facing step by using openFoam. And to measure the velocity profile at 0.085 m from the inlet of the geometry. And compare velocity magnitude contours near the step region for the 0.2, 0.5, & 0.8 grading factors.

Mesh specification:

1. Number of cells along the x direction (longer dimension) = 200
2. Number of cells along the y direction = 10
3. Use a graded mesh. Grading factor near all walls should be 0.2, 0.5, & 0.8.
   

Boundary Conditions:

The following are the conditions which need to be followed.

Mesh:

The mesh is generated from a dictionary file named blockMeshDict located in the constant/polyMesh directory of a case. blockMesh reads this dictionary, generates the mesh and writes out the mesh data to points and faces, cells and boundary files in the same directory.

Blocks:

The blocks are defined as follows:

     blocks
    (
        hex (0 1 2 3 4 5 6 7)    // vertex numbers
        (10 10 10)               // numbers of cells in each direction
        simpleGrading (1 2 3)    // cell expansion ratios
    );

blockMeshDict file:

/*--------------------------------*- C++ -*----------------------------------*\
  =========                 |
  \\      /  F ield         | OpenFOAM: The Open Source CFD Toolbox
   \\    /   O peration     | Website:  https://openfoam.org
    \\  /    A nd           | Version:  8
     \\/     M anipulation  |
\*---------------------------------------------------------------------------*/
FoamFile
{
    version     2.0;
    format      ascii;
    class       dictionary;
    object      blockMeshDict;
}
// * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //

convertToMeters 0.01;

vertices
(
    (0 0 0)
    (8 0 0)
    (8 0.5 0)
    (0 0.5 0)
    (8 1 0)
    (0 1 0)
    (20 1 0)
    (20 0.5 0)
    (20 0 0)
    (20 -1 0)
    (8 -1 0)
    (0 0 0.1)
    (8 0 0.1)
    (8 0.5 0.1)
    (0 0.5 0.1)
    (8 1 0.1)
    (0 1 0.1)
    (20 1 0.1)
    (20 0.5 0.1)
    (20 0 0.1)
    (20 -1 0.1)
    (8 -1 0.1)
);

blocks
(
    hex (0 1 2 3 11 12 13 14) (200 10 1) simpleGrading (0.2 1.2 1)
    hex (3 2 4 5 14 13 15 16) (200 10 1) simpleGrading (0.2 0.2 1)
    hex (2 7 6 4 13 18 17 15) (200 10 1) simpleGrading (1.2 0.2 1)
    hex (1 8 7 2 12 19 18 13) (200 10 1) simpleGrading (1.2 1.2 1)
    hex (10 9 8 1 21 20 19 12) (200 10 1) simpleGrading (1.2 1.2 1)
);

edges
(
);

boundary
(
    inlet
    {
        type patch;
        faces
        (
            (0 11 14 3)
            (3 14 16 5)
        );
    }
    outlet
    {
        type patch;
        faces
        (
            (9 8 19 20)
            (8 7 18 19)
            (7 6 17 18)
        );
    }
    frontAndBack
    {
        type empty;
        faces
        (
            (0 3 2 1)
            (3 5 4 2)
            (10 1 8 9)
            (1 2 7 8)
            (2 4 6 7)
            (11 12 13 14)
            (14 13 15 16)
            (21 20 19 12)
            (12 19 18 13)
            (13 18 17 15)
        );
    }
    noslipwalls
    {
        type wall;
        faces
        (
            (5 16 15 4)
            (4 15 17 6)
            (0 1 12 11)
            (10 21 12 1)
            (10 9 20 21)
        );
     }      
);

mergePatchPairs
(
);

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

controlDict file:

/*--------------------------------*- C++ -*----------------------------------*\
  =========                 |
  \\      /  F ield         | OpenFOAM: The Open Source CFD Toolbox
   \\    /   O peration     | Website:  https://openfoam.org
    \\  /    A nd           | Version:  8
     \\/     M anipulation  |
\*---------------------------------------------------------------------------*/
FoamFile
{
    version     2.0;
    format      ascii;
    class       dictionary;
    location    "system";
    object      controlDict;
}
// * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //

application     icoFoam;

startFrom       startTime;

startTime       0;

stopAt          endTime;

endTime         0.05;

deltaT          1e-5;

writeControl    timeStep;

writeInterval   20;

purgeWrite      0;

writeFormat     ascii;

writePrecision  6;

writeCompression off;

timeFormat      general;

timePrecision   6;

runTimeModifiable true;


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

Pressure file:

/*--------------------------------*- C++ -*----------------------------------*\
  =========                 |
  \\      /  F ield         | OpenFOAM: The Open Source CFD Toolbox
   \\    /   O peration     | Website:  https://openfoam.org
    \\  /    A nd           | Version:  8
     \\/     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;
    }
    outlet
    {
        type            fixedValue;
        value           uniform 0;
    }

    noslipwalls
    {
        type            zeroGradient;
    }

    frontAndBack
    {
        type            empty;
    }
}

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

velocity file:

/*--------------------------------*- C++ -*----------------------------------*\
  =========                 |
  \\      /  F ield         | OpenFOAM: The Open Source CFD Toolbox
   \\    /   O peration     | Website:  https://openfoam.org
    \\  /    A nd           | Version:  8
     \\/     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 (1.5 0 0);
    }
    outlet
    {
        type            zeroGradient;
    }

    noslipwalls
    {
        type            noSlip;
    }

    frontAndBack
    {
        type            empty;
    }
}

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

Results:

1. For 0.2  grading factor:

Mesh block:

Pressure variation:

Velocity variation:

Velocity plot:

2. For 0.5  grading factor:

The code is same except for the change of values in simpleGrading in blocks.

blocks
(
    hex (0 1 2 3 11 12 13 14) (200 10 1) simpleGrading (0.5 1.5 1)
    hex (3 2 4 5 14 13 15 16) (200 10 1) simpleGrading (0.5 0.5 1)
    hex (2 7 6 4 13 18 17 15) (200 10 1) simpleGrading (1.5 0.5 1)
    hex (1 8 7 2 12 19 18 13) (200 10 1) simpleGrading (1.5 1.5 1)
    hex (10 9 8 1 21 20 19 12) (200 10 1) simpleGrading (1.5 1.5 1)
);

Mesh block:

Pressure variation:

Velocity variation:

Velocity Plot:

2. For 0.8  grading factor:

The code is same except for the change in simpleGeading values.

blocks
(
    hex (0 1 2 3 11 12 13 14) (200 10 1) simpleGrading (0.8 1.8 1)
    hex (3 2 4 5 14 13 15 16) (200 10 1) simpleGrading (0.8 0.8 1)
    hex (2 7 6 4 13 18 17 15) (200 10 1) simpleGrading (1.8 0.8 1)
    hex (1 8 7 2 12 19 18 13) (200 10 1) simpleGrading (1.8 1.8 1)
    hex (10 9 8 1 21 20 19 12) (200 10 1) simpleGrading (1.8 1.8 1)
);

Mesh block:

Pressure variation:

Velocity variation:

Velocity plot:

Conclusion:

The Velocity obtained from all the Mesh Grading Factors are all same except with the minor difference. The high accuracy can be obtained by lowering the mesh grading size.