Week 8 - BlockMesh Drill down challenge
Aim: To simulate an incompressible-laminar-viscous flow through the backward facing step geometry using paraFoam.
Given Inputs:
Boundary condition specification
Mesh specification
- Number of cells along the x direction (longer dimension) = 200
- Number of cells along the longer y direction = 20;
Desired output
- Measure the velocity profile at 0.085 m from the inlet of the geometry for both case studies and compare.
- Explain why Grading/Growth factor is essential for meshing.
Introduction:
- OpenFoam is an open source software to perform various simulations,the simulations performed can be views in the paraFoam.
- It consists of several tutorials which helps us to know about different solvers present in the openFoam.
- Run folder need to be created to run any solver present.
- There are many solvers available we just need to change the geomenty of the profile to run the desired simulation.
- For incompressible Flow there is a folder with incompressible flow having solvers to solve incompresssible flows.
Steps followed in the case set up:
- launch openFoam software by opening the terminal and typing openFoam ,go to Tutorials folder by typing shortcut tut which will direct to tutorials folder.
- Go into the incomplessible folder by cd incompressible ,there you can see icoFoam folder go into it.
- icoFoam is the solver used to solve the incompressible flows problems.now type cd cavity to enter into it.
- Type cp -r cavity/ $FOAM_RUN/cavity_test to create a copy of cavity folder in the run folder.because we should run the solver only in the run folder or else the original files get corrupted which is not advisable.
- in the run folder cavity_test will be created.type cd cavity_test and perform allrun command by typing ./Allrun .
- open cavity folder cd cavity and in here you will have system folder in which blockMeshdict ,controlDict,fvschems etc are present.Type cd system.
- we need to change the geomenty as per our need it can be performed inside the blockMeshDict. Type gedit blockMeshDict a file will be opened here we need to change the dimensions and click save and close.
- Type gedit controlDict and in this file we need to set the time limit and time step and it should satisfy (CFL <1).
- Now type cd .. to come back and type cd 0 .we can see P and U files present.
- Open both files by gedit and perform some changes.gedit p and gedit U.Now come back by typing cd ..
- Now perfom blockmesh command by typing blockMesh .This will create a Mesh.
- Now type icoFoam to start the,solver it will take some time to complete the simulation .now type checkMesh and everything is fine.
- Type ParaFoam to visulize the simulation.
blockMeshDict file:
/*--------------------------------*- 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)
(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
(
hex (0 1 2 3 11 12 13 14) (80 5 1) simpleGrading (1 1 1)
hex (3 2 4 5 14 13 15 16) (80 5 1) simpleGrading (1 1 1)
hex (2 7 6 4 13 18 17 15) (120 5 1) simpleGrading (1 1 1)
hex (1 8 7 2 12 19 18 13) (120 5 1) simpleGrading (1 1 1)
hex (10 9 8 1 21 20 19 12) (120 10 1) simpleGrading (1 1 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)
);
}
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 21 12 1)
(10 9 20 21)
);
}
);
mergePatchPairs
(
);
// ************************************************************************* //controlDict file:
/*--------------------------------*- 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 1e-4;
writeControl timeStep;
writeInterval 20;
purgeWrite 0;
writeFormat ascii;
writePrecision 6;
writeCompression off;
timeFormat general;
timePrecision 6;
runTimeModifiable true;
// ************************************************************************* //
Velocity File:
/*--------------------------------*- 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;
}
Noslipwalls
{
type noSlip;
}
frontAndBack
{
type empty;
}
}
// ************************************************************************* //Pressure File:
/*--------------------------------*- 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;
}
Noslipwalls
{
type zeroGradient;
}
frontAndBack
{
type empty;
}
}
// ************************************************************************* //Mesh of a backward step profile:
Velocity variation
pressure variation:
velocity profile:
Gradingfactor:
- Grading factor is the ratio of size of end cell to the size of starting cell .
- This helps in getting more detailing at some positions as per the grading factor.
- GF helps us to increase the cell number at start or end of the block.
- The change can be seen clearly with proper gradingfactor.
Simulate the flow with grading factor of 0.2
- There is no change in the controlDict file,P,U files there is only a small change in the grading in the blockMeshDict file.which is presented below.
- There is no change in the blockMesh file exept the folling block.
blocks
(
hex (0 1 2 3 11 12 13 14) (80 5 1) simpleGrading (5 5 1)
hex (3 2 4 5 14 13 15 16) (80 5 1) simpleGrading (5 0.2 1)
hex (2 7 6 4 13 18 17 15) (120 5 1) simpleGrading (5 0.2 1)
hex (1 8 7 2 12 19 18 13) (120 5 1) simpleGrading (5 5 1)
hex (10 9 8 1 21 20 19 12) (120 10 1) simpleGrading (5 5 1)
);
Mesh in the backward step:
Velocity variation:
pressure profile:
Velocity Profile:
Conclusion:
- There will not be a much difference in simulation after grading the curve and simulation looks a slight rough.
- Fine mesh gives more smoother simulations and graphs.
- the detailing at the more cells position will be more .