BlogFor BusinessTalk to a counsellor
CoursesIIT ProgramsPlacementsLog in
BlogFor Business
Talk to a counsellor
Challenge

Week 8 - Simulation of a backward facing step in OpenFOAM

AAkarsh Choudhary13 Feb 2023
CFDOPENFOAM

AIM

We have to simulate the incompressible-laminar-viscous flow in a backward facing step with help of OPENFOAM with ParaView interface. As a result if the simulation, velocity, pressure profile for different mesh grading factor can be achieved. 

The geometry of backward facing step:-

I used the IcoFoam solver to solve the equations mentioned in the application of system file. IcoFoam is the transient solver for incompressible, laminar flow for Newtonian flow as it is in our case.

I will be solving the system for 2 cases:-

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

Case 2 - Simulate the flow with grading factor of 0.2. The cells should be finer near the walls (includig the step wall).

You can use the following mesh parameters but you can feel free to modify these values.

Mesh specification

  1. Number of cells along the x direction (longer dimension) = 200
  2. Number of cells along the longer y direction = 20;

THEORY

Grading Factor

Meshing is one of the most crucial step for solving this problem. Higher the number of mesh elements , higher would be the result accuracy and same trend for the time taken to reach the solution. Grading could not change the number of elements but could increase it's size at one place and decrease at another thus the Courant Number is not affected and further no discrepencies in our solutionj, this could increase the result accuracy at places where the mesh size decreses as expected near the the step walls(in our case) 

PROCEDURE

CASE 1) Grading Factor(GF) = 0

We will find the cavity folder under tutorial

I coppied this folder in the run folder and named it 'Case_1'

akarsh@DESKTOP-0FNFUIA:~$ tut
akarsh@DESKTOP-0FNFUIA:/opt/openfoam10/tutorials$ incompressible/
-bash: incompressible/: Is a directory
akarsh@DESKTOP-0FNFUIA:/opt/openfoam10/tutorials$ cd incompressible/
akarsh@DESKTOP-0FNFUIA:/opt/openfoam10/tutorials/incompressible$ ls
SRFPimpleFoam  adjointShapeOptimisationFoam  icoFoam     pisoFoam          shallowWaterFoam
SRFSimpleFoam  boundaryFoam                  pimpleFoam  porousSimpleFoam  simpleFoam
akarsh@DESKTOP-0FNFUIA:/opt/openfoam10/tutorials/incompressible$ cd icoFoam/
akarsh@DESKTOP-0FNFUIA:/opt/openfoam10/tutorials/incompressible/icoFoam$ ls
cavity  elbow
akarsh@DESKTOP-0FNFUIA:/opt/openfoam10/tutorials/incompressible/icoFoam$ cd cavity/
akarsh@DESKTOP-0FNFUIA:/opt/openfoam10/tutorials/incompressible/icoFoam/cavity$ ls
Allclean  Allrun  cavity  cavityClipped  cavityGrade
akarsh@DESKTOP-0FNFUIA:/opt/openfoam10/tutorials/incompressible/icoFoam/cavity$ cd cavity
akarsh@DESKTOP-0FNFUIA:/opt/openfoam10/tutorials/incompressible/icoFoam/cavity/cavity$ ls
0  constant  system
akarsh@DESKTOP-0FNFUIA:/opt/openfoam10/tutorials/incompressible/icoFoam/cavity/cavity$ cd ..
akarsh@DESKTOP-0FNFUIA:/opt/openfoam10/tutorials/incompressible/icoFoam/cavity$ cp -r cavity/ $FOAM_RUN/case_1
akarsh@DESKTOP-0FNFUIA:/opt/openfoam10/tutorials/incompressible/icoFoam/cavity$ run
akarsh@DESKTOP-0FNFUIA:~/OpenFOAM/akarsh-10/run$ ls
case_1  case_2  cavity  flange_te  flange_test  pitzDaily
akarsh@DESKTOP-0FNFUIA:~/OpenFOAM/akarsh-10/run$ cd case_1/
akarsh@DESKTOP-0FNFUIA:~/OpenFOAM/akarsh-10/run/case_1$ ls
0  constant  system
akarsh@DESKTOP-0FNFUIA:~/OpenFOAM/akarsh-10/run/case_1$

Under the 'case_1' we can find 3 folders:-

0

system

constant

From directory i changed the source code for 

1)blockMeshDict

/*--------------------------------*- C++ -*----------------------------------*
  =========                 |
  \      /  F ield         | OpenFOAM: The Open Source CFD Toolbox
   \    /   O peration     | Website:  https://openfoam.org
    \  /    A nd           | Version:  9
     \/     M anipulation  |
*---------------------------------------------------------------------------*/
FoamFile
{
    format      ascii;
    class       dictionary;
    object      blockMeshDict;
}
// * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //

convertToMeters 1;

vertices
(
    // vertices from point 0 to 10
    (0 0 0)
    (0.08 0 0)
    (0.08 0.005 0)
    (0 0.005 0)
    (0.08 0.01 0)
    (0 0.01 0)
    (0.2 0.01 0)
    (0.2 0.005 0)
    (0.2 0 0)
    (0.2 -0.01 0)
    (0.08 -0.01 0)
    
    // vertices from point 11 to 21
    (0 0 0.1)
    (0.08 0 0.1)
    (0.08 0.005 0.1)
    (0 0.005 0.1)
    (0.08 0.01 0.1)
    (0 0.01 0.1)
    (0.2 0.01 0.1)
    (0.2 0.005 0.1)
    (0.2 0 0.1)
    (0.2 -0.01 0.1)
    (0.08 -0.01 0.1)
);

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

edges
(
);

boundary
(
    inlet
    {
        type patch;
        faces
        (
            (11 14 3 0)
            (14 16 5 3)
        );
    }
    outlet
    {
        type patch;
        faces
        (
            (6 17 18 7)
            (8 7 18 19)
            (9 8 19 20)
        );
    }
    
    otherwall  //no slip walls
    {	type wall;
        faces
        (
        	//top
            (15 4 5 16)
            (15 17 6 4)
            //step
            (21 12 1 10)
            //bottom left
            (11 0 1 12)
            //bottom right
            (21 10 9 20) 
        );
    }
    frontAndBack
    {
        type empty;
        faces
        (	//back
            (0 3 2 1)
            (3 5 4 2)
            (2 4 6 7)
            (1 2 7 8)
            (10 1 8 9)
            //front
            (13 15 16 14)
            (12 13 14 11)
            (20 19 12 21)
            (19 18 13 12)
            (18 17 15 13)
        );
    }
);
mergePatchPairs
(
);
// ************************************************************************* //

2) controlDict

/*--------------------------------*- C++ -*----------------------------------*
  =========                 |
  \      /  F ield         | OpenFOAM: The Open Source CFD Toolbox
   \    /   O peration     | Website:  https://openfoam.org
    \  /    A nd           | Version:  9
     \/     M anipulation  |
*---------------------------------------------------------------------------*/
FoamFile
{
    format      ascii;
    class       dictionary;
    location    "system";
    object      controlDict;
}
// * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //

application     icoFoam;

startFrom       startTime;

startTime       0;

stopAt          endTime;

endTime         5;

deltaT          0.001;

writeControl    timeStep;

writeInterval   20;

purgeWrite      0;

writeFormat     ascii;

writePrecision  6;

writeCompression off;

timeFormat      general;

timePrecision   6;

runTimeModifiable true;

//adaptive timestep
/*
adjustTimeStep true;

maxCo 1;*/


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

3) U (Velocity) under '0'

/*--------------------------------*- C++ -*----------------------------------*
  =========                 |
  \      /  F ield         | OpenFOAM: The Open Source CFD Toolbox
   \    /   O peration     | Website:  https://openfoam.org
    \  /    A nd           | Version:  9
     \/     M anipulation  |
*---------------------------------------------------------------------------*/
FoamFile
{
    format      ascii;
    class       volVectorField;
    object      U;
}
// * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //

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

internalField   uniform (0.1 0 0);

boundaryField
{
    inlet
    {
        type            fixedValue;
        value           uniform (0.2 0 0);
    }

    outlet
    {
        type            zeroGradient;
    }

    frontAndBack
    {
        type            empty;
    }
    otherwall
    {
    	type 		noSlip; // zero at walls
    }
}

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

4) p (pressure) under '0'

/*--------------------------------*- C++ -*----------------------------------*
  =========                 |
  \      /  F ield         | OpenFOAM: The Open Source CFD Toolbox
   \    /   O peration     | Website:  https://openfoam.org
    \  /    A nd           | Version:  9
     \/     M anipulation  |
*---------------------------------------------------------------------------*/
FoamFile
{
    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;
    }

    frontAndBack
    {
        type            empty;
    }
    otherwall
    {
    	type 		zeroGradient;
    }
}

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

5) Constant code

transportProperties
/*--------------------------------*- C++ -*----------------------------------*
  =========                 |
  \      /  F ield         | OpenFOAM: The Open Source CFD Toolbox
   \    /   O peration     | Website:  https://openfoam.org
    \  /    A nd           | Version:  9
     \/     M anipulation  |
*---------------------------------------------------------------------------*/
FoamFile
{
    format      ascii;
    class       dictionary;
    location    "constant";
    object      transportProperties;
}
// * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //

nu              [0 2 -1 0 0 0 0] 1e-5;


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

Then I ran the following commands:-

1) blockMesh

2) checkMesh

3) icoFoam

4) touch test.foam

5) explorer.exe .

I opened the test file in ParaView to display our mesh, grid and results.

Case 2) Grading Factor = 0.2

I followewd the same steps and name the new cavity file 'Case_2' and editted the source code as given below to specify G.F =0.2

1) blockMeshDict

/*--------------------------------*- C++ -*----------------------------------*
  =========                 |
  \      /  F ield         | OpenFOAM: The Open Source CFD Toolbox
   \    /   O peration     | Website:  https://openfoam.org
    \  /    A nd           | Version:  9
     \/     M anipulation  |
*---------------------------------------------------------------------------*/
FoamFile
{
    format      ascii;
    class       dictionary;
    object      blockMeshDict;
}
// * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //

convertToMeters 1;

vertices
(
    // vertices from point 0 to 10
    (0 0 0)
    (0.08 0 0)
    (0.08 0.005 0)
    (0 0.005 0)
    (0.08 0.01 0)
    (0 0.01 0)
    (0.2 0.01 0)
    (0.2 0.005 0)
    (0.2 0 0)
    (0.2 -0.01 0)
    (0.08 -0.01 0)
   
    // vertices from point 11 to 21
    (0 0 0.1)
    (0.08 0 0.1)
    (0.08 0.005 0.1)
    (0 0.005 0.1)
    (0.08 0.01 0.1)
    (0 0.01 0.1)
    (0.2 0.01 0.1)
    (0.2 0.005 0.1)
    (0.2 0 0.1)
    (0.2 -0.01 0.1)
    (0.08 -0.01 0.1)
);

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

edges
(
);

boundary
(
    inlet
    {
        type patch;
        faces
        (
            (11 14 3 0)
            (14 16 5 3)
        );
    }
    outlet
    {
        type patch;
        faces
        (
            (6 17 18 7)
            (8 7 18 19)
            (9 8 19 20)
        );
    }
   
    otherwall  //no slip walls
    { type wall;
        faces
        (
        //top
            (15 4 5 16)
            (15 17 6 4)
            //step
            (21 12 1 10)
            //bottom left
            (11 0 1 12)
            //bottom right
            (21 10 9 20)
        );
    }
    frontAndBack
    {
        type empty;
        faces
        ( //back
            (0 3 2 1)
            (3 5 4 2)
            (2 4 6 7)
            (1 2 7 8)
            (10 1 8 9)
            //front
            (13 15 16 14)
            (12 13 14 11)
            (20 19 12 21)
            (19 18 13 12)
            (18 17 15 13)
        );
    }
);
mergePatchPairs
(
);
// ************************************************************************* //

2) controlDict

/*--------------------------------*- C++ -*----------------------------------*
  =========                 |
  \      /  F ield         | OpenFOAM: The Open Source CFD Toolbox
   \    /   O peration     | Website:  https://openfoam.org
    \  /    A nd           | Version:  9
     \/     M anipulation  |
*---------------------------------------------------------------------------*/
FoamFile
{
    format      ascii;
    class       dictionary;
    location    "system";
    object      controlDict;
}
// * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //

application     icoFoam;

startFrom       startTime;

startTime       0;

stopAt          endTime;

endTime         5;

deltaT          0.001;

writeControl    timeStep;

writeInterval   20;

purgeWrite      0;

writeFormat     ascii;

writePrecision  6;

writeCompression off;

timeFormat      general;

timePrecision   6;

runTimeModifiable true;

//adaptive timestep
/*
adjustTimeStep true;

maxCo 1;*/


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

3) U (Velocity) from '0'

/*--------------------------------*- C++ -*----------------------------------*
  =========                 |
  \      /  F ield         | OpenFOAM: The Open Source CFD Toolbox
   \    /   O peration     | Website:  https://openfoam.org
    \  /    A nd           | Version:  9
     \/     M anipulation  |
*---------------------------------------------------------------------------*/
FoamFile
{
    format      ascii;
    class       volVectorField;
    object      U;
}
// * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //

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

internalField   uniform (0.1 0 0);

boundaryField
{
    inlet
    {
        type            fixedValue;
        value           uniform (0.2 0 0);
    }

    outlet
    {
        type            zeroGradient;
    }

    frontAndBack
    {
        type            empty;
    }
    otherwall
    {
    	type 		noSlip; // zero at walls
    }
}

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

4) p (pressure) from '0'

/*--------------------------------*- C++ -*----------------------------------*
  =========                 |
  \      /  F ield         | OpenFOAM: The Open Source CFD Toolbox
   \    /   O peration     | Website:  https://openfoam.org
    \  /    A nd           | Version:  9
     \/     M anipulation  |
*---------------------------------------------------------------------------*/
FoamFile
{
    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;
    }

    frontAndBack
    {
        type            empty;
    }
    otherwall
    {
    	type 		zeroGradient;
    }
}

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

5) Constant source code

/*--------------------------------*- C++ -*----------------------------------*
  =========                 |
  \      /  F ield         | OpenFOAM: The Open Source CFD Toolbox
   \    /   O peration     | Website:  https://openfoam.org
    \  /    A nd           | Version:  9
     \/     M anipulation  |
*---------------------------------------------------------------------------*/
FoamFile
{
    format      ascii;
    class       dictionary;
    location    "constant";
    object      transportProperties;
}
// * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //

nu              [0 2 -1 0 0 0 0] 1e-5;


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

Results

1) GF = 0

1.1) MESH

1,2) VELOCITY PROFILE

1.3) PRESSURE PROFILE

1.4) VELOCITY CHART AT LINE 0.85MM FROM INLET

1.6) Animation

https://drive.google.com/file/d/1rGUT9MTHIgj6vLhU790afyYrDGwg-8k1/view?usp=sharing

2) GF = 0.2

2.1) MESH

2.2) VELOCITY PROFILE

2.3) PRESSURE PROFILE

2.4) VELOCITY CHART AT LINE 0.85MM FROM INLET

2.6) Animation

https://drive.google.com/file/d/1dIVgYYcxCxOFmeZep3EqKP0X-G25AWu-/view?usp=sharing

CONCLUSION

From the results we can conclude the following points:-

1) The flow is laminar and fast at inlet and slows down after the step and near the wall.

2) Presence of vortice can be found right after the step due to the pressure difference.

3) Steady state is achieved near the 25th step.

4) Case 1 with no grading factor gives the smoother plot for velocity as compared to the 2nd plot with 0.2 grading factor but no much difference in the slopes.

Your portfolio, next

Ship work like this
in your own course.

Talk to a career counsellor

Free 20-minute call. We'll map the right course to where you want to work.

By submitting you agree to our Terms and Privacy Policy.