Week 8 - Simulation of a backward facing step in OpenFOAM
Objective :
1.Transient simulation of an incompressible-laminar-viscous flow through the backward facing step geometry
2. Have to perform 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).
3. Have to compare velocity profil at 0.085 m from the inlet of the geomentry for the both cases.
4.Explain why Grading/Growth factor is essential for meshing.
Geomentry :
The backward facing step geometry with specifications is given. This geometry is to be simulated 2 dimensionally,
recreating the geometry as blocks, we get to see the following arrangement with all the vertices given.
Geometry Creation:
Let us consider creating the geometry using 5 blocks as shown in the figure below,
Case 1 - Simulate the flow without using any grading factor:
As per the geometry and the blocks we edit the blockMeshDict file so that it has all the vertices from 0-21, 5 blocks without grading factor and specifying the type of each boundary.
- 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 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
(
// 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
(
);
// ************************************************************************* //
- ControlDict
/*--------------------------------*- 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 5;
deltaT 0.001;
writeControl timeStep;
writeInterval 20;
purgeWrite 0;
writeFormat ascii;
writePrecision 6;
writeCompression off;
timeFormat general;
timePrecision 6;
runTimeModifiable true;
// ************************************************************************* //
- Pressure p code
/*--------------------------------*- 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;
}
frontAndBack
{
type empty;
}
otherwall
{
type zeroGradient;
}
}
}
// ************************************************************************* //
- Velocity U code
/*--------------------------------*- 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 (0.2 0 0);
}
outlet
{
type zeroGradient;
}
frontAndBack
{
type empty;
}
otherwall
{
type noSlip; // zero at walls
}
}
// ************************************************************************* //
- Transporet Property
/*--------------------------------*- 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 "constant";
object physicalProperties;
}
// * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //
nu [0 2 -1 0 0 0 0] 0.00001;
// ************************************************************************* //
Post-Processing Results :
- Surface with edges
- Velocity Profile U
- Velocity Plot
velocity profile at x = 0.085 m without grading factor
Case 2: Simulating the flow with grading factor 0.2
This simulation is similar the previous one but has a change in the grading factors for the blocks and that needs
to be edited.
All other files [ controlDict, initial pressure and velocity conditions and transport properties ] are same.
- 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 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
(
// block 1
hex (0 1 2 3 11 12 13 14) (80 5 1) simpleGrading (1 5 1)
// block 2
hex (3 2 4 5 14 13 15 16) (80 5 1) simpleGrading (1 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 5 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
(
);
Post-Processing Results :
- Surface with edges:
- Velocity Profile:
- Velocity Plot:
Conclusion :
There is a small change in velocity profiles when grading size decreases we get fine mesh.
The Grading factor of 0.2 shows fine mesh.