Flow over a Backward facing step in OpenFOAM
The blockMeshDict file has been edited to obtain the geometry as follows:
For ungraded mesh, the grading factor is kept as 1. Also to ensure proper capture of flow properties at the boundaries, grading factor of 0.2 is kept at walls so that mesh is fine in that section.
Mesh grading factor= Size of last cell/Size of the first cell
- When mesh grading factor<<1, the mesh is finer at the end as compared to start of blockMesh.
- When mesh grading factor>>1, the mesh is finer at the start as compared to end of blockMesh.
BLOCKMESHDICT FILE:(no grading)
/*--------------------------------*- C++ -*----------------------------------*\\
| ========= | |
| \\\\ / F ield | OpenFOAM: The Open Source CFD Toolbox |
| \\\\ / O peration | Version: 4.1 |
| \\\\ / A nd | Web: www.OpenFOAM.org |
| \\\\/ M anipulation | |
\\*---------------------------------------------------------------------------*/
FoamFile
{
version 2.0;
format ascii;
class dictionary;
object blockMeshDict;
}
// * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //
convertToMeters 0.01;
vertices
(
(0 0 0) //0
(8 0 0) //1
(8 0.5 0) //2
(0 0.5 0) //3
(0 0 0.1) //4
(8 0 0.1) //5
(8 0.5 0.1) //6
(0 0.5 0.1) //7
(0 1 0) //8
(8 1 0) //9
(0 1 0.1) //10
(8 1 0.1) //11
(8 -1 0) //12
(20 -1 0) //13
(20 -0.5 0) //14
(8 -0.5 0) //15
(8 -1 0.1) //16
(20 -1 0.1) //17
(20 -0.5 0.1) //18
(8 -0.5 0.1) //19
(20 0 0) //20
(20 0.5 0) //21
(20 0 0.1) //22
(20 0.5 0.1) //23
(20 1 0) //24
(20 1 0.1) //25
);
blocks
(
hex (0 1 2 3 4 5 6 7) (80 5 1) simpleGrading (1 1 1)
hex (3 2 9 8 7 6 11 10 ) (80 5 1) simpleGrading (1 1 1)
hex (2 21 24 9 6 23 25 11) (120 5 1) simpleGrading (1 1 1)
hex (1 20 21 2 5 22 23 6) (120 5 1) simpleGrading (1 1 1)
hex (15 14 20 1 19 18 22 5) (120 5 1) simpleGrading (1 1 1)
hex (12 13 14 15 16 17 18 19) (120 5 1) simpleGrading (1 1 1)
);
edges
(
);
boundary
(
inlet
{
type patch;
faces
(
(0 3 7 4)
(3 8 10 7)
);
}
outlet
{
type patch;
faces
(
(23 25 24 21)
(22 23 21 20)
(18 22 20 14)
(17 18 14 13)
);
}
noslipwall
{
type wall;
faces
(
(8 9 11 10)
(13 12 16 17)
(15 1 5 19)
(9 24 25 11)
(12 15 19 16)
(0 4 5 1)
);
}
frontAndBack
{
type empty;
faces
(
(0 1 2 3)
(3 2 9 8)
(2 21 24 9)
(1 20 21 2)
(15 14 20 1)
(12 13 14 15)
(4 7 6 5)
(7 10 11 6)
(6 11 25 23)
(5 6 23 22)
(19 5 22 18)
(16 19 18 17)
);
}
);
mergePatchPairs
(
);
// ************************************************************************* //Initial condition for Pressure :
/*--------------------------------*- C++ -*----------------------------------*\\
| ========= | |
| \\\\ / F ield | OpenFOAM: The Open Source CFD Toolbox |
| \\\\ / O peration | Version: 4.1 |
| \\\\ / A nd | Web: www.OpenFOAM.org |
| \\\\/ 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;
}
noslipwall
{
type zeroGradient;
}
frontAndBack
{
type empty;
}
}
// ************************************************************************* //Initial condition for Velocity:
Here 0.5 m/s is taken as the inlet velocity. The initial velocity is selected to satisfy the Courant number criteria which should be less than 1. Only then the solution will converge.
/*--------------------------------*- C++ -*----------------------------------*\\
| ========= | |
| \\\\ / F ield | OpenFOAM: The Open Source CFD Toolbox |
| \\\\ / O peration | Version: 4.1 |
| \\\\ / A nd | Web: www.OpenFOAM.org |
| \\\\/ 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 (0.5 0 0);
}
outlet
{
type zeroGradient;
}
noslipwall
{
type noSlip;
}
frontAndBack
{
type empty;
}
}
// ************************************************************************* //Ungraded Mesh:
Velocity Profile
When Grading Factor = 0.2
blocks
(
hex (0 1 2 3 4 5 6 7) (80 5 1) simpleGrading (0.2 5 1)
hex (3 2 9 8 7 6 11 10) (80 5 1) simpleGrading (0.2 0.2 1)
hex (2 21 24 9 6 23 25 11) (120 5 1) simpleGrading (5 0.2 1)
hex (1 20 21 2 5 22 23 6) (120 5 1) simpleGrading (5 5 1)
hex (15 14 20 1 19 18 22 5) (120 5 1) simpleGrading (5 0.2 1)
hex (12 13 14 15 16 17 18 19) (120 5 1) simpleGrading (5 5 1)
);Velocity Profile
When Grading Factor = 0.5
blocks
(
hex (0 1 2 3 4 5 6 7) (80 5 1) simpleGrading (0.5 2 1)
hex (3 2 9 8 7 6 11 10) (80 5 1) simpleGrading (0.5 0.2 1)
hex (2 21 24 9 6 23 25 11) (120 5 1) simpleGrading (2 0.2 1)
hex (1 20 21 2 5 22 23 6) (120 5 1) simpleGrading (2 2 1)
hex (15 14 20 1 19 18 22 5) (120 5 1) simpleGrading (2 0.5 1)
hex (12 13 14 15 16 17 18 19) (120 5 1) simpleGrading (2 5 1)
);Velocity Profile
When Grading Factor = 0.8
blocks
(
hex (0 1 2 3 4 5 6 7) (80 5 1) simpleGrading (0.8 1.25 1)
hex (3 2 9 8 7 6 11 10) (80 5 1) simpleGrading (0.8 0.2 1)
hex (2 21 24 9 6 23 25 11) (120 5 1) simpleGrading (1.25 0.2 1)
hex (1 20 21 2 5 22 23 6) (120 5 1) simpleGrading (1.25 1.25 1)
hex (15 14 20 1 19 18 22 5) (120 5 1) simpleGrading (1.25 0.8 1)
hex (12 13 14 15 16 17 18 19) (120 5 1) simpleGrading (1.25 5 1)
);Velocity Profile
RESULT:
We can see that as we increase the grading factor from 0.2 to 0.8, there is not much difference,but this will have major effect for much larger change in grading factor.
As we decrease the grading factor, the mesh becomes finer and finer. This makes the solution becomes more accurate.
In all the plots, velocity at point 0.085m from the inle,t is 0.55 m/s . From this we can conclude that ,at the mesh near the step ,the velocity is maximum.