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Block Mesh drill down challenge

PPrakash Kotian9 Jan 2020
CFD

Objective: Using the icoFoam solver to simulate the flow through a backward facing step. Creating multiple meshes and comparing results.

Theory:

A backward facing step is generated using OpenFOAM as per the given dimensional specifications.

Mesh specification:

  • Number of cells in x-direction = 200
  • Number of cells in y-direction = 10
  • Grading near the walls = 0.2

Solutions steps:

  1. Open the tutorials folder by navigating through command prompt. Open folder named incompressible inside it and then navigate to icoFoam folder. Copy the folder named inside icoFoam folder.
  2. Paste a copy of cavity folder inside Run folder.
  3. Open the blockmesh file in system folder and edit it.
  4. Open the ‘p’ and ‘u’ file and apply boundary conditions for pressure and velocity.
  5. Open the controlDict file and edit time steps according to requirement.
  6. Visualize the results using Paraview post processing tools
For Grading factor of 0.2

.blockMesh file

/*--------------------------------*- C++ -*----------------------------------*\\
  =========                 |
  \\\\      /  F ield         | OpenFOAM: The Open Source CFD Toolbox
   \\\\    /   O peration     | Website:  https://openfoam.org
    \\\\  /    A nd           | Version:  7
     \\\\/     M anipulation  |
\\*---------------------------------------------------------------------------*/
FoamFile
{
    version     2.0;
    format      ascii;
    class       dictionary;
    object      blockMeshDict;
}
// * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //

convertToMeters 0.01;

vertices
(
    (0 0 0) // point 0 
    (8 0 0) // point 1 
    (8 0.5 0) // point 2 
    (0 0.5 0) // point 3 
    (8 1 0) // point 4 
    (0 1 0) // point 5 
    (20 1 0) // point 6 
    (20 0.5 0) // point 7 
    (20 0 0) // point 8 
    (20 -1 0) // point 9 
    (8 -1 0) // point 10 
    (0 0 0.1) // point 11 
    (8 0 0.1) // point 12 
    (8 0.5 0.1) // point 13
    (0 0.5 0.1) // point 14
    (8 1 0.1) // point 15
    (0 1 0.1) // point 16
    (20 1 0.1) // point 17 
    (20 0.5 0.1) // point 18
    (20 0 0.1) // point 19
    (20 -1 0.1) // point 20
    (8 -1 0.1) // point 21
);

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

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



ControDil file

/*--------------------------------*- C++ -*----------------------------------*\\
  =========                 |
  \\\\      /  F ield         | OpenFOAM: The Open Source CFD Toolbox
   \\\\    /   O peration     | Website:  https://openfoam.org
    \\\\  /    A nd           | Version:  7
     \\\\/     M anipulation  |
\\*---------------------------------------------------------------------------*/
FoamFile
{
    version     2.0;
    format      ascii;
    class       dictionary;
    location    \"system\";
    object      controlDict;
}
// * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //

application     icoFoam;

startFrom       startTime;

startTime       0;

stopAt          endTime;

endTime         0.5;

deltaT          0.00001;

writeControl    timeStep;

writeInterval   20;

purgeWrite      0;

writeFormat     ascii;

writePrecision  6;

writeCompression off;

timeFormat      general;

timePrecision   6;

runTimeModifiable true;


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

pressure Boundary condition file 


/*--------------------------------*- C++ -*----------------------------------*\\
  =========                 |
  \\\\      /  F ield         | OpenFOAM: The Open Source CFD Toolbox
   \\\\    /   O peration     | Website:  https://openfoam.org
    \\\\  /    A nd           | Version:  7
     \\\\/     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 Boundary conditions file 
 
/*--------------------------------*- C++ -*----------------------------------*\\
  =========                 |
  \\\\      /  F ield         | OpenFOAM: The Open Source CFD Toolbox
   \\\\    /   O peration     | Website:  https://openfoam.org
    \\\\  /    A nd           | Version:  7
     \\\\/     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;
    }  
}

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










For Grading factor of 0.5

only blockMesh is updated while remaining other conditions remains same .

/*--------------------------------*- C++ -*----------------------------------*\\
  =========                 |
  \\\\      /  F ield         | OpenFOAM: The Open Source CFD Toolbox
   \\\\    /   O peration     | Website:  https://openfoam.org
    \\\\  /    A nd           | Version:  7
     \\\\/     M anipulation  |
\\*---------------------------------------------------------------------------*/
FoamFile
{
    version     2.0;
    format      ascii;
    class       dictionary;
    object      blockMeshDict;
}
// * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //

convertToMeters 0.01;

vertices
(
    (0 0 0) // point 0 
    (8 0 0) // point 1 
    (8 0.5 0) // point 2 
    (0 0.5 0) // point 3 
    (8 1 0) // point 4 
    (0 1 0) // point 5 
    (20 1 0) // point 6 
    (20 0.5 0) // point 7 
    (20 0 0) // point 8 
    (20 -1 0) // point 9 
    (8 -1 0) // point 10 
    (0 0 0.1) // point 11 
    (8 0 0.1) // point 12 
    (8 0.5 0.1) // point 13
    (0 0.5 0.1) // point 14
    (8 1 0.1) // point 15
    (0 1 0.1) // point 16
    (20 1 0.1) // point 17 
    (20 0.5 0.1) // point 18
    (20 0 0.1) // point 19
    (20 -1 0.1) // point 20
    (8 -1 0.1) // point 21
);

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)
);

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
(
);

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




For Grading factor of 0.8


/*--------------------------------*- C++ -*----------------------------------*\\
  =========                 |
  \\\\      /  F ield         | OpenFOAM: The Open Source CFD Toolbox
   \\\\    /   O peration     | Website:  https://openfoam.org
    \\\\  /    A nd           | Version:  7
     \\\\/     M anipulation  |
\\*---------------------------------------------------------------------------*/
FoamFile
{
    version     2.0;
    format      ascii;
    class       dictionary;
    object      blockMeshDict;
}
// * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //

convertToMeters 0.01;

vertices
(
    (0 0 0) // point 0 
    (8 0 0) // point 1 
    (8 0.5 0) // point 2 
    (0 0.5 0) // point 3 
    (8 1 0) // point 4 
    (0 1 0) // point 5 
    (20 1 0) // point 6 
    (20 0.5 0) // point 7 
    (20 0 0) // point 8 
    (20 -1 0) // point 9 
    (8 -1 0) // point 10 
    (0 0 0.1) // point 11 
    (8 0 0.1) // point 12 
    (8 0.5 0.1) // point 13
    (0 0.5 0.1) // point 14
    (8 1 0.1) // point 15
    (0 1 0.1) // point 16
    (20 1 0.1) // point 17 
    (20 0.5 0.1) // point 18
    (20 0 0.1) // point 19
    (20 -1 0.1) // point 20
    (8 -1 0.1) // point 21
);

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 1.8A 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
(
);

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

Conclusion:

  1. Changing the grading factordoesnt have a significant impact on the results
  2. Maximum velocity in all cases falls in a certain range
  3. Maximum velocity for grading factor 0.2 is 2.59 for 0.8 is 2.62m/s
  4. Velocity contour has less noticable variations
  5. Looks like solutions is near convergence

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