# BlockMesh Drill down challenge

Objective:-

The objective of this study is to make a backward facing step block by editing the blockMeshDict file and simulate it in openFoam and post process the results like velocity profile near the time step zone and to check the effect of grading scheme on velocity profile.

First the backward facing block was devided into 5 blocks and each block is build one by one of given dimension and number of cells in x direction is taken as 200 while in y direction it is taken as 10 and particular faces are assigned to particular boundary condition. The blockMeshDict file for grading scheme 0.2 is given below.

/*--------------------------------*- 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 1;

vertices
(

(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)
(0 0 0.005)
(0.08 0 0.005)
(0.08 0.005 0.005)
(0 0.005 0.005)
(0.08 0.01 0.005)
(0 0.01 0.005)
(0.2 0.01 0.005)
(0.2 0.005 0.005)
(0.2 0 0.005)
(0.2 -0.01 0.005)
(0.08 -0.01 0.005)
);

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)
);
}
Front
{
type empty;
faces
(
(11 12 13 14)
(14 13 15 16)
(13 18 17 15)
(12 19 18 13)
(21 20 19 12)
);
}
Back
{
type empty;
faces
(
(3 2 1 0)
(5 4 2 3)
(4 6 7 2)
(2 7 8 1)
(1 8 9 10)
);
}
NoSlipWalls
{
type wall;
faces
(
(4 5 16 15)
(4 15 17 6)
(21 12 1 10)
(0 1 12 11)
(10 9 20 21)
);
}
);
mergePatchPairs
(
);

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

After editing the blockMeshDict file blocks can be seen in the paraFoam using the command paraFoam -block and it will give you the results which are shown below:-

After this the next step is to give the boundary conditions, The boundary condition for pressure and velocity in 0 folder is given as:-

Pressure boundary conditions:-

/*--------------------------------*- 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
{
}

outlet
{
type            fixedValue;
value           uniform 0;
}

Front
{
type            empty;
}
Back
{
type            empty;
}
NoSlipWalls
{
}
}

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

Velocity boundary conditions:-

/*--------------------------------*- 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       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
{
}

Front
{
type            empty;
}
Back
{
type            empty;
}
NoSlipWalls
{
type            noSlip;
}
}

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

The veocity and pressure file in the 0 folder will remain same for every grading scheme. We just have to change the simple grading points in blockMeshDict.

Simulation Results:-

The memshing and the velocity profile for grading scheme of 0.8 is shown below. In the given condition the grading near all wall is 0.8

    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)

velocity profile near the stepat 0.085 metre is plotted and is showen below:-

The grading scheme is then changed from 0.8 to 0.5 to see the change in meshing and grid size. the grading scheme is used for this is given below:-

    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)

The velocity profile near the step at 0.085 metres is plotted and is showen below:-

As we are reducing the grading scheme from 0.8 to 0.5 to 0.2 the mesh becomes more finer and you will get more accurate results compare to 1 grading scheme. so in case of any simulation grading scheme should be choosen accordingly in required areas.

Now after that the grading scheme of 0.2 is used.

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)

Velocity contour:-

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