Week 8 - Simulation of a backward facing step in OpenFOAM
- Considering a 2D laminar, incompressible and Newtonian fluid flow over the Backward-facing step with given kinamatic viscosity as `10^-5 m/s`.
- Considering the 'No-slip boundary condition' on every wall under the domain.
- For the results, we need to perform simulations for given grading factors.
- We need to change the time step for different meshes such that satisfies our courant-Freidrichs-lewy condition.
- the problem satement should be carried out with the aid of tututorial named cavity under the icoFoam solver with appropriate Geometry, Mesh , Boundary and initial conditions are created by modifying the script files from the respective solver.
MESH SPECIFICATION:
- Number of cell in X direction = 200(for whole)
- number of cell in Y direction =20 (for first region)
- number of cell in Y direction = 40(for second region)
FOR GRADING FACTOR 1:
- Copy the cavity folder to run folder
- then open and edit the following files
1.GEOMETRY and MESH GENERATION
BlockMeshDict File:
System --> blockMeshDict
/*--------------------------------*- C++ -*----------------------------------*\
========= |
\\ / F ield | OpenFOAM: The Open Source CFD Toolbox
\\ / O peration | Website: https://openfoam.org
\\ / A nd | Version: 8
\\/ 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
(8 1 0) //4
(0 1 0) //5
(20 1 0) //6
(20 0.5 0) //7
(20 0 0) //8
(20 -1 0) //9
(8 -1 0) //10
(0 0 0.1) //11
(8 0 0.1) //12
(8 0.5 0.1) //13
(0 0.5 0.1) //14
(8 1 0.1) //15
(0 1 0.1) //16
(20 1 0.1) //17
(20 0.5 0.1) //18
(20 0 0.1) //19
(20 -1 0.1) //20
(8 -1 0.1) //21
);
blocks
(
hex (0 1 2 3 11 12 13 14) (80 10 1) simpleGrading (1 1 1)
hex (3 2 4 5 14 13 15 16) (80 10 1) simpleGrading (1 1 1)
hex (1 8 7 2 12 19 18 13) (120 10 1) simpleGrading (1 1 1)
hex (2 7 6 4 13 18 17 15) (120 10 1) simpleGrading (1 1 1)
hex (10 9 8 1 21 20 19 12) (120 20 1) simpleGrading (1 1 1)
);
edges
(
);
boundary
(
inlet
{
type patch;
faces
(
(0 11 14 3)
(3 14 16 5)
);
}
outlet
{
type patch;
faces
(
(8 7 18 19)
(7 6 17 18)
(9 8 19 20)
);
}
frontAndBack
{
type empty;
faces
(
(0 3 2 1)
(3 5 4 2)
(10 1 8 9)
(1 2 7 8)
(2 4 6 7)
(11 12 13 14)
(14 13 15 16)
(21 20 19 12)
(12 19 18 13)
(13 18 17 15)
);
}
noslipwalls
{
type wall;
faces
(
(5 16 15 4)
(4 15 17 6)
(0 1 12 11)
(10 9 20 21)
(21 12 1 10)
);
}
);
mergePatchPairs
(
);
// ************************************************************************* //ControlDict File:
System --> controlDict
/*--------------------------------*- C++ -*----------------------------------*\
========= |
\\ / F ield | OpenFOAM: The Open Source CFD Toolbox
\\ / O peration | Website: https://openfoam.org
\\ / A nd | Version: 8
\\/ M anipulation |
\*---------------------------------------------------------------------------*/
FoamFile
{
version 2.0;
format ascii;
class dictionary;
location "system";
object controlDict;
}
// * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //
application icoFoam;
startFrom startTime;
startTime 0;
stopAt endTime;
endTime 0.5;
deltaT 0.0005;
writeControl timeStep;
writeInterval 20;
purgeWrite 0;
writeFormat ascii;
writePrecision 6;
writeCompression off;
timeFormat general;
timePrecision 6;
runTimeModifiable true;
// ************************************************************************* //2.SPECIFYING PHYSICAL PROPERTIES:
Pressure and velocity profiles:
0 --> P&V
P:
/*--------------------------------*- C++ -*----------------------------------*\
========= |
\\ / F ield | OpenFOAM: The Open Source CFD Toolbox
\\ / O peration | Website: https://openfoam.org
\\ / A nd | Version: 8
\\/ 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;
}
frontAndBack
{
type empty;
}
noslipwalls
{
type zeroGradient;
}
}
// ************************************************************************* //V:
/*--------------------------------*- C++ -*----------------------------------*\
========= |
\\ / F ield | OpenFOAM: The Open Source CFD Toolbox
\\ / O peration | Website: https://openfoam.org
\\ / A nd | Version: 8
\\/ 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
{
type zeroGradient;
}
frontAndBack
{
type empty;
}
noslipwalls
{
type noSlip;
}
}
// ************************************************************************* //TransportProperties:
constant --> TransportProperties
/*--------------------------------*- C++ -*----------------------------------*\
========= |
\\ / F ield | OpenFOAM: The Open Source CFD Toolbox
\\ / O peration | Website: https://openfoam.org
\\ / A nd | Version: 8
\\/ M anipulation |
\*---------------------------------------------------------------------------*/
FoamFile
{
version 2.0;
format ascii;
class dictionary;
location "constant";
object transportProperties;
}
// * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //
transportModel Newtonian;
nu [0 2 -1 0 0 0 0] 1e-5;
// ************************************************************************* //FOR GRADING FACTOR 0.2:
blocks
(
hex (0 1 2 3 11 12 13 14) (80 10 1) simpleGrading (1 5 1)
hex (3 2 4 5 14 13 15 16) (80 10 1) simpleGrading (1 0.2 1)
hex (1 8 7 2 12 19 18 13) (120 10 1) simpleGrading (5 5 1)
hex (2 7 6 4 13 18 17 15) (120 10 1) simpleGrading (5 0.2 1)
hex (10 9 8 1 21 20 19 12) (120 20 1) simpleGrading (5 1 1)
);- After setup all the files we need to create a BlockMesh
- for post processing we need to solve the problem using icoFoam solver.
- we can use paraFoam to view mesh and run simulation.
RESULTS:
For grading factor 1.
refined mesh.
For Grading factor 0.2
refined mesh
EFFECTS OF GRADED MESH:
- with Grading , the wall boundary layer effects are captured well.
- with Grading , the mesh is coarse in the middle region, which makes the velocity curve more detailed to see.
- for better results we should have a finer mesh near the walls and the mesh with0.2 grading provides us the finest mesh generated .
CONCLUSIONS:
- For finer mesh , we can visualize the accurate output.
- After ploting the graphs at x = 0.085, it can be seen that the velocity at the walls is zero , whereas the velocity at the center is maximum. This is due to the No-Slip condition at the walls.