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Challenge

Week 8 - Simulation of a backward facing step in OpenFOAM

SShruti Jadhav8 Oct 2022
CFD
  • Simulate an incompressible-laminar-viscous flow through the backward facing step geometry. You should perform a transient simulation. The solver can be chosen based on the described physics of the flow.

    perform 2 case studies as described below and compare the results.

    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.

  • Objective- To simulate an incompressible-laminar-viscous flow through the backward facing step geometry and also perform the transient simulation.To perform 2 case studies as described below and compare the results.

    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).

  • The flow in which density is constant is known as incompressible flow.
  • The smooth and streamlined flows are called as laminar flows.
  • When the fluid has more resistance to flow then it is called as viscous flow.
  • Transient analysis by definition involves loads that are a function of time
  • Grading factor is defined as the ratio of last cell distance to the first cell distance.
  • Procedure ,1. Blockmesh

2. Checkmesh

3. Solve (icoFoam)

4. Paraview (visualize)

  • Steps- Open the openFoam tutorials and find solutions to solve the incompressible laminar viscous flow problem.
  • The next step is to copy the required file in the solver and paste it into the OpenFOAM run folder, leaving the official OpenFoam file intact.
  • Find out the block network script file to create the domain space and provide the network specification.
  • Find the "controlDict" file and edit the simulation start and end time.
  • In the initial folder "0", adjust the initial parameter for velocity and pressure according to the surfaces of the given domain.
  • Use the blockMesh command to generate the mesh and checkMesh to verify it.
  • Open the simulation file in paraView and plot the velocity profile at a point 0.085 m from the inlet.
  • Repeat the above procedure for a grading factor of 0.2 to check for differences in results.
    Write a detailed report explaining all the commands and codes used in the project.

  • Boundary condition specification

    The domain specifications are provided in the following figure.

  • OpenFoam has 3 basic folders,namely 0, constant , system.

  • The system folder consist of following categories,

    1. BlockMeshDict - Used to create a mesh information.

    2. ControlDict - Used to control solver time operations.

    3. fvSolutions- Used to assign instructions on how to solve discretized linear equations.

    4. fvSchemes- Used to assigned discretization schemes.

  • The constant folder consist of following categories,

    1. Polymesh- Polymesh contains mesh information generated by the BlockMeshDict

    2. TransportProperties- It contains information of transport coefficients.

    3. turbulenceProperties- It contains information about turbulence coefficients.

  • The 0 folder consist of following categories,

1. P - It contains BC and IC data for pressure at time t=0.

2. U- It contains BC and IC data for velocity at time t=0.

  • SimpleGrading- SimpleGrading defines the expansion ratio or mesh stretching in each local direction(x,y,z).
  • The boundary condition is specified in the blockmesh dict file.

  • BlockMeshDict 
/*--------------------------------*- C++ -*----------------------------------*\
  =========                 |
  \\      /  F ield         | OpenFOAM: The Open Source CFD Toolbox
   \\    /   O peration     | Website:  https://openfoam.org
    \\  /    A nd           | Version:  10
     \\/     M anipulation  |
\*---------------------------------------------------------------------------*/
FoamFile
{
    format      ascii;
    class       dictionary;
    object      blockMeshDict;
}
// * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //

convertToMeters 1;

vertices
(
   
    (0 0 0)//[0]
    (0.08 0 0)//[1]
    (0.08 0.005 0)//[2] 
    (0 0.005 0)//[3]
    (0.08 0.01 0)//[4]
    (0 0.01 0)//[5]
    (0.2 0.01 0)//[6]
    (0.2 0.005 0)//[7]
    (0.2 0 0)//[8]
    (0.2 -0.01 0)//[9]
    (0.08 -0.01 0)//[10]
    
    (0 0 0.1)//[11]
    (0.08 0 0.1)//[12]
    (0.08 0.005 0.1)//[13] 
    (0 0.005 0.1)//[14]
    (0.08 0.01 0.1)//[15]
    (0 0.01 0.1)//[16]
    (0.2 0.01 0.1)//[17]
    (0.2 0.005 0.1)//[18]
    (0.2 0 0.1)//[19]
    (0.2 -0.01 0.1)//[20]
    (0.08 -0.01 0.1)//[21]
);

blocks
(
    hex (0 1 2 3 11 12 13 14) (100 10 1) simpleGrading (0.2 1 1)
    hex (3 2 4 5 14 13 15 16) (100 10 1) simpleGrading (0.2 1 1)
    hex (10 9 8 1 21 20 19 12) (200 10 1) simpleGrading (2 1 1)
    hex (1 8 7 2 12 19 18 13) (200 10 1) simpleGrading (2 1 1)
    hex (2 7 6 4 13 18 17 15) (200 10 1) simpleGrading (2 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)
        );
    }
    Bottomwall 
    {
        type wall;
        faces
        (
            (11 0 1 12)
            (21 10 9 20)
            (10 21 12 1)
        );
    }
    Topwall 
    {
        type wall;
        faces
        (
           (16 15 4 5)
           (15 17 6 4)
        );
    }
    
  
    frontAndBack
    {
        type empty;
        faces
        (
            (14 13 15 16)
            (11 12 13 14)
            (21 20 19 12) //Front side 
            (12 19 18 13)
            (13 18 17 15)
          
            (0 3 2 1)
            (3 5 4 2)
            (2 4 6 7)//Back side
            (1 2 7 8)
            (10 1 8 9)
            
        );
    }
);
mergePatchPairs
(
);
  • ControlDict
  • In the controlDict file timesteps can be controlled.
  • In this simulation start and end time is provided as 0 and 0.5. deltaT is assumed to be 0.002. 
/*--------------------------------*- C++ -*----------------------------------*\
  =========                 |
  \\      /  F ield         | OpenFOAM: The Open Source CFD Toolbox
   \\    /   O peration     | Website:  https://openfoam.org
    \\  /    A nd           | Version:  10
     \\/     M anipulation  |
\*---------------------------------------------------------------------------*/
FoamFile
{
    format      ascii;
    class       dictionary;
    location    "system";
    object      controlDict;
}
// * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //

application     icoFoam;

startFrom       startTime;

startTime       0;

stopAt          endTime;

endTime         0.5;

deltaT          0.002;

writeControl    timeStep;

writeInterval   20;

purgeWrite      0;

writeFormat     ascii;

writePrecision  6;

writeCompression off;

timeFormat      general;

timePrecision   6;

runTimeModifiable true;


// ************************************************************************* //
  • Initial Condition
  • The initial condition for velocity and pressure can be provided in the p and U file located in the 0 folder.
  • Pressure
/*--------------------------------*- C++ -*----------------------------------*\
  =========                 |
  \\      /  F ield         | OpenFOAM: The Open Source CFD Toolbox
   \\    /   O peration     | Website:  https://openfoam.org
    \\  /    A nd           | Version:  10
     \\/     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;
    }

    Bottomwall
    {
        type            zeroGradient;
    }
    
    Topwall
    {
        type            zeroGradient;
    }
   
    frontAndBack
    {
        type              empty;
    }

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

Velocity-

/*--------------------------------*- C++ -*----------------------------------*\
  =========                 |
  \\      /  F ield         | OpenFOAM: The Open Source CFD Toolbox
   \\    /   O peration     | Website:  https://openfoam.org
    \\  /    A nd           | Version:  10
     \\/     M anipulation  |
\*---------------------------------------------------------------------------*/
FoamFile
{
    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;
    }
    
    Bottomwall

    {
        type            noSlip;
    }

    Topwall

    {
        type            noSlip;
    }

    frontAndBack
    {
        type            empty;
    }
}

// ************************************************************************* //
  • Procedure
  • Open terminal by clicking clt+alt+T on Linux operating system.
  • In a terminal window, type $FOAM_TUTORIALS or TUT.
  • ls represents a directory with a list.
  • pwd represents the current working directory.
  • Type cd incompressible/icoFoam/cavity to enter in cavities folder.
  • Copy the cavity directory with cp and paste the directory into RUN
  • Enter the RUN window by typing cd $FOAM_RUN.
  • In the RUN window, type cd cavity/cavity/system to enter the system folder.
  • The notepad.exe command is used to open text files.
  • notepad.exe blockmeshdict is used to open a blockmesh file.
  • notepad.exe controldict is used to open the controldict file.
  • Cd .. is used to go to the previous folder.
  • Constants related to fluid flow, such as diffusivity, can be changed in the constants component.
  • The initial state related to the fluid flow can be changed under the 0 component.
  • Geometry:

    The block mesh dict is used to create the geometry, generate the mesh and define the boundary condition. The following procedures should be followed when creating geometry in Blockmesh.

  • The entire piece is created by combining 5 blocks.

  • First, vertices should be defined for each block.

  • After the blocks are created, meshing is done using hex(a,b,c,..) (x,y,z) simpleGrading(p,q,r)

  • Hex represents a hexahedral mesh.

  • a,b,c represent the vertices of the block.

  • x,y,z represents meshing in x,y and z direction.

  • Results - Mesh with grading and without grading

  • Plot -Velocity profile with grading factor 0.2

  • Velcontour

  • Precontour

  • Plot -Velocity profile with grading factor 1

velcontour

precontour

  • To calculate velocity at distance 0.085 from inlet modify following parameters:-

  • Results discussion-
  • Mesh are generated for grading factor 0.2 and grading factor 1.
  • Flow is simulated without using grading factor and with using grading factor 0.2
  • Graphs are plotted for the velocity profile.
  • When dx value is small ,accuracy increases also as the finer mesh , the more accurate will be the results.
  • Explain why Grading/Growth factor is essential for meshing.
  • Specifies the maximum ratio of adjacent mesh edges for the transition between coarse and fine regions.
    A smaller sorting factor creates a more even mesh.
    In the program, we can change the factor in the blockMeshDict code.
  • SimpleGrading defines the expansion ratio or mesh stretching in each local direction(x,y,z).
  • Conclusion:-
  • As we can observe from the graphs, there is not much difference between the two meshes.
  • But if the values are taken, we can see a slight variation among the two graphs.
  • By changing the simple grading factor from 0.2 to 1, the mesh becomes finer and the solution changes to precision.
  • Overall, changing the size of the simple grading factor,the result remains the same as approx.
  • Execution time is lower for a lower mesh sorting factor.
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