Week 8 - Simulation of a backward facing step in OpenFOAM
Aim:
- Tosimulate an incompressible -lamainar flow throgh the backward facing step geometry.
- To perform transient simulation and choosing solver based on the described physics of the flow in problem statement
To simulate the flow with out using any grading factor (GF=1) and with grading factor=0.2
Method
- Open openFoam and find the tutorials and solver to solve the incompressible -laminar viscous problem
- In the next step copy the required file in solver and paste it in openFoam run folder so that official file of openFoam untouched
- Find out blockMesh script file to create domainspace and provide mesh specification
- Find 'controlDict file to edit start and end time for simulation
- In initial folder 'O' edit initial parameternfor velocity and pressure
- Use blockmesh command to generate to mesh and checkMesh to varify the mesh quality
- Open simulation file in paraView and plot the velocity profile at a perticular point from inlet
- Repeat the above procedure for grading factor 0.2 to check the difference in the results
Problem Statement
Simulate an incompressible -laminar -viscous flow through the backward facing step geometry and perform two case studis
case1:Simulate the flow without using any grading factor GF=1
case2: simualte the folow with grading factor 0.2. The cells should be near the walla(including the step wall).
Mesh specification
1) Number of cells along the x direction(longer dimension)=200
2) Number of cells along the longer y direction=20;
Boundary condition specification
The domain specifications are given in the following picture
Grading Factor or Expansion ratio
Gading factor or expansion ratio improves the quality of mesh by refining the mesh in specified diirection
Expansion ratio =(size of end cell)/(size of starting cell)
Mesh Grid (GF=1)
velocity profile
Velocity Plot at distance 0.085
Blockmesh Dict. FOR GF=1
/*--------------------------------*- 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 5 1) simpleGrading (1 1 1) //block1
hex (3 2 4 5 14 13 15 16) (80 5 1) simpleGrading (1 1 1) //block2
hex (2 7 6 4 13 18 17 15) (120 5 1) simpleGrading (1 1 1) //block3
hex (1 8 7 2 12 19 18 13) (120 5 1) simpleGrading (1 1 1) //block4
hex (10 9 8 1 21 20 19 12) (120 10 1) simpleGrading (1 1 1) //block5
);
edges
(
);
boundary
(
inlet
{
type wall;
faces
(
(0 11 14 3)
(3 14 16 5)
);
}
outlet
{
type wall;
faces
(
(9 8 19 20)
(8 7 18 19)
(7 6 17 18)
);
}
frontAndBack
{
type empty;
faces
(
(11 12 13 14)
(14 13 15 16)
(13 18 17 15)
(12 19 18 13)
(21 20 19 12)
(0 3 2 1)
(3 5 4 2)
(2 4 6 7)
(1 2 7 8)
(10 1 8 9)
);
}
fixedWalls
{
type wall;
faces
(
(5 16 15 4)
(0 1 12 11)
(4 15 17 6)
(10 9 20 21)
(10 21 12 1)
);
}
);
mergePatchPairs
(
);
// ************************************************************************* //
Control Dict.
/*--------------------------------*- 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 0.2;
deltaT 0.00001;
writeControl timeStep;
writeInterval 20;
purgeWrite 0;
writeFormat ascii;
writePrecision 6;
writeCompression off;
timeFormat general;
timePrecision 6;
runTimeModifiable true;
// ************************************************************************* //
/*--------------------------------*- 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;
}
fixedWalls
{
type zeroGradient;
}
frontAndBack
{
type empty;
}
}
// ************************************************************************* //
/*--------------------------------*- 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 0 0);
}
outlet
{
type zeroGradient;
}
fixedWalls
{
type noSlip;
value uniform (0 0 0);
}
frontAndBack
{
type empty;
}
}
// ************************************************************************* //
Mesh Grid with GF=0.2
Velocity Profile
Pressure Profile
Velocity Plot at 0.085
Blockmesh Dict for GF=0.2
/*--------------------------------*- 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 5 1) simpleGrading (0.2 5 1) //block1
hex (3 2 4 5 14 13 15 16) (80 5 1) simpleGrading (0.2 0.2 1) //block2
hex (2 7 6 4 13 18 17 15) (120 5 1) simpleGrading (5 0.2 1) //block3
hex (1 8 7 2 12 19 18 13) (120 5 1) simpleGrading (5 5 1) //block4
hex (10 9 8 1 21 20 19 12) (120 10 1) simpleGrading (5 5 1) //block5
);
edges
(
);
boundary
(
inlet
{
type wall;
faces
(
(0 11 14 3)
(3 14 16 5)
);
}
outlet
{
type wall;
faces
(
(9 8 19 20)
(8 7 18 19)
(7 6 17 18)
);
}
frontAndBack
{
type empty;
faces
(
(11 12 13 14)
(14 13 15 16)
(13 18 17 15)
(12 19 18 13)
(21 20 19 12)
(0 3 2 1)
(3 5 4 2)
(2 4 6 7)
(1 2 7 8)
(10 1 8 9)
);
}
fixedWalls
{
type wall;
faces
(
(5 16 15 4)
(0 1 12 11)
(4 15 17 6)
(10 9 20 21)
(10 21 12 1)
);
}
);
mergePatchPairs
(
);
// ************************************************************************* //
Conclusion
- By Changing the grading factor from the value 1 to 0.2 the mesh become finer and the solution become accurate.
- The small change in sine of grading factor(mesh size) the results remains approximately same.