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  1. Home/
  2. Yogessvaran T/
  3. Week 8 - Simulation of a backward facing step in OpenFOAM

Week 8 - Simulation of a backward facing step in OpenFOAM

        To create the geometry as shown in the figure, we need edit the blockMeshDict file that is available in the system folder of the case setup. Geometry Creation: Let us consider creating the geometry using 5 blocks as shown in the figure below, NOTE: We have demonstrated the geometry creation…

  • CFD

  • Yogessvaran T

    updated on 10 Sep 2022

 

 

   

To create the geometry as shown in the figure, we need edit the blockMeshDict file that is available in the system folder

of the case setup.

Geometry Creation:

Let us consider creating the geometry using 5 blocks as shown in the figure below,

NOTE: We have demonstrated the geometry creation using 5 blocks and have indexed the vertices this way. This can be

varied, according to your comfort.

The Isometric View, 

Now let’s, start indexing the Vertices on the Back side of the Blocks, Since openFoam uses C++ programming, we start

the index from “0”. (The Numbering of the vertices can be varied according to your comfort)

Similarly, the indexing for the frontside of the Blocks are,



To View the Bigger Picture,



Finally, to define the blocks, we need to specify the right order of vertex indices

For example:

to create the 1st Block for this case,

 

                                                               

hex (0 1 2 3 11 12 13 14) (1 1 1) simpleGrading (1 1 1)

Similarly proceed by defining the vertices of the other blocks in the Geometry.

To View geometry that you have created, clear out all the entries inside the boundary section of the blockMeshDict file

and execute the blockMesh command in the terminal.

Now open paraview and view the geometry that you have created.

 

Meshing:

Now to mesh the geometry fill in the number of cells in x, y and z direction inside the second parenthesis of the Hex

command,

i.e., hex(0 1 2 3 11 12 13 14) (10 10 1) simplegrading(1 1 1).

since this is a 2D problem, increasing the number of cells in the z direction, doesn’t make sense. To view the mesh in

paraFoam, select the surface with edges option and view the mesh.

Similarly to refine the mesh near the walls of the geometry, you need to play around with the numbers in the third

parenthesis. We leave that for you to figure out, how It can be done.

 

Boundary Conditions:

Before we run the simulations, we need to specify the initial and boundary conditions.

The given boundary conditions are,

 

1.Inlet: (Type patch)

         

 

The faces that contribute to inlet boundary are shown above. If we are looking at these faces from the outside, the

definition of inlet boundary will be in counter-clockwise direction,

(0 11 14 3)

(3 14 16 5)



2.Outlet: (Type patch)

                                               

Similarly, we have 3 faces contributing to the outlet boundary condition. Since we are viewing the faces from outside,

hence the definition of the outlet boundary will be in counter-clockwise direction,

(9 8 19 20)

(8 7 18 19)

(7 6 17 18)

Now for the other boundary look at the images below and repeat the steps to define the boundaries.

 

3.Front & Back: (Type empty)

 

4.No Slip walls: (Type wall)

After defining the boundaries correctly, to check if you have given defined them correctly, execute the blockMesh

command in the terminal.Now open paraFoam, select all the patches and click apply. You will now be able to see the

boundary conditions that have been applied to the geometry.

As part of the Challenge, figure out the right initial conditions.

Case 1 - Simulate the flow without using any grading factor (i.e., GF = 1).

BlockMeshDict:

/*--------------------------------*- 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 0.1;

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) (80 5 1) simpleGrading (1 1 1)
    hex (3 2 4 5 14 13 15 16) (80 5 1) simpleGrading (1 1 1)
    hex (10 9 8 1 21 20 19 12) (120 10 1) simpleGrading (1 1 1)
    hex (1 8 7 2 12 19 18 13) (120 5 1) simpleGrading (1 1 1)
    hex (2 7 6 4 13 18 17 15) (120 5 1) simpleGrading (1 1 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)
            (10 21 12 1)
            (0 1 12 11)
            (10 9 20 21)
        );
    }
);

mergePatchPairs
(
);

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

ControlDict:

/*--------------------------------*- 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         0.5;

deltaT          0.001;

writeControl    timeStep;

writeInterval   20;

purgeWrite      0;

writeFormat     ascii;

writePrecision  6;

writeCompression off;

timeFormat      general;

timePrecision   6;

runTimeModifiable true;


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

Here i have given step time =0.001 to satsify courant number condition.

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 0 0);
    }

    outlet
    {
        type            zeroGradient;
    }
    noslipwalls
    {
        type           noSlip;
    }
    frontAndBack
    {
        type            empty;
    }
}

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

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;
    }
    
}

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

 

Plots that we get from the above simulation:

 

Surfacewithedges:

 

 

Velocity profile:

Velocity plot:

 

Case 2 - Simulate the flow with grading factor of 0.2.

Here we have to alter the blockmeshdict block:

BlockMeshDict:

/*--------------------------------*- 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 0.1;

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) (80 5 1) simpleGrading (1 5 1)
    hex (3 2 4 5 14 13 15 16) (80 5 1) simpleGrading (1 0.2 1)
    hex (2 7 6 4 13 18 17 15) (120 5 1) simpleGrading (5 0.2 1)
    hex (10 9 8 1 21 20 19 12) (120 10 1) simpleGrading (5 5 1)
    hex (1 8 7 2 12 19 18 13) (120 5 1) simpleGrading (5 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)
            (10 21 12 1)
            (0 1 12 11)
            (10 9 20 21)
        );
    }
);

mergePatchPairs
(
);

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

Controldict:

/*--------------------------------*- 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         0.5;

deltaT          0.001;

writeControl    timeStep;

writeInterval   20;

purgeWrite      0;

writeFormat     ascii;

writePrecision  6;

writeCompression off;

timeFormat      general;

timePrecision   6;

runTimeModifiable true;


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

 

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 0 0);
    }

    outlet
    {
        type            zeroGradient;
    }
    noslipwalls
    {
        type           noSlip;
    }
    frontAndBack
    {
        type            empty;
    }
}

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

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;
    }
    
}

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

Plots that we get from the above simulation:

surfacewithedges:

 

 Velocity profile:

Velocity plot:

comparing

two different meshes indicates the significance of grading/growth factor as the mesh without any grading factor (GF = 1) was

not refined enough to capture the near wall gradients of the flow properties, especially velocity, as compared to the mesh

using a grading factor of 0.2 in the relevant blocks of the decomposed domain.

Conclusion:

By changing the simple grading factor from the value of 1 to 0.2 the mesh becomes finer at walls and the solution becomes

very accurate.

From the graph it can be seen that the velocity in two cases are same that is 1.2 m/s at distance of 0.085 from the inlet in

x-axis.

Overall the change in the size of simple grading factor (mesh size) the results remains the same as approximately.

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