Week 8 - Simulation of a backward facing step in OpenFOAM
Simulation of a backward-facing step in OpenFOAM
Aim
we are going to perform a transient simulation on incompressible-laminar-viscous flow through the backward-facing step geometry.
Cases:
Case 1 - Simulate the flow without using any grading factor (i.e., GF = 1)
Case 2 - Simulate the flow with a grading factor of 0.2. The cells should be finer near the walls (including the step wall).
Desired output:
Measure the velocity profile at 0.085 m from the inlet of the geometry for both case studies and compare the results.
Procedure:
- The solver can be chosen based on the described physics of the flow. use this https://www.openfoam.com/documentation/user-guide/standard-solvers.php
- The physics of our problem is Transient incompressible, laminar flow of Newtonian fluids so icoFoam solver has chosen.
- After choosing the right solver for our problem search for tutorials files of that solver copy the tutorial files of icoFoam solver to the RUN folder.
- open terminal(command-line-based interface) in Linux OS.
- type 'tut' shortcut for $FOAM_TUTORIALS
- cd incompressible/icoFoam/cavity/ ==> navigation to required tutorial file
- cp -r cavity/ $FOAM_RUN ==> this command copies the cavity tutorial file to the run folder
- type 'run' it will directly navigate to run folder 'run' is shortcut of $FOAM_RUN
- cd cavity/system/ ==>navigation to system folder
- gedit blockMeshDict ==>edit file according to the geometry.
- gedit controlDict ==>edit file according to the time dt.
- don't edit fvSchemes, fvSolution files leave as it is they are. they are finite volume-based schemes and solutions to solve our problem
- in constant folder edit transport-Properties give proper kinematic viscosity value as the fluid medium in our problem
- in '0' folder edit initial, final boundary conditions & all walls are have no-slip boundary condition. initial velocity 1.5m/s, & final pressure is 1bar.
- after changing all parameters in the tutorial file according to our problem run the simulation using the icoFoam command.
- use paraFoam command to post-process & visualize the results in ParaView. use cell data to point data filter for smoother results.
- Create proper graphs.
Geometry:
splitting of geometry
blockMeshDict file code:
/--------------------------------- 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 1;
vertices
(
(0 0 0) //00
(0.08 0 0) //01
(0.08 0.005 0) //02
(0 0.005 0) //03
(0.08 0.01 0) //04
(0 0.01 0) //05
(0.2 0.01 0) //06
(0.2 0.005 0) //07
(0.2 0 0) //08
(0.2 -0.01 0) //09
(0.08 -0.01 0) //10
(0 0 0.01) //11
(0.08 0 0.01) //12
(0.08 0.005 0.01) //13
(0 0.005 0.01) //14
(0.08 0.01 0.01) //15
(0 0.01 0.01) //16
(0.2 0.01 0.01) //17
(0.2 0.005 0.01) //18
(0.2 0 0.01) //19
(0.2 -0.01 0.01) //20
(0.08 -0.01 0.01) //21
);
blocks
(
hex (0 1 2 3 11 12 13 14) (80 10 1) simpleGrading (1 1 1) //block1
hex (3 2 4 5 14 13 15 16) (80 10 1) simpleGrading (1 1 1) //block2
hex (2 7 6 4 13 18 17 15) (120 10 1) simpleGrading (1 1 1) //block3
hex (1 8 7 2 12 19 18 13) (120 10 1) simpleGrading (1 1 1) //block4
hex (10 9 8 1 21 20 19 12) (120 20 1) simpleGrading (1 1 1) //block5
);
edges
(
);
boundary
(
inlet
{
type patch;
faces
(
(11 14 3 0)
(14 16 5 3)
);
}
outlet
{
type patch;
faces
(
(19 20 9 8)
(18 19 8 7)
(17 18 7 6)
);
}
front_back
{
type empty;
faces
(
(16 14 13 15)
(14 11 12 13)
(15 13 18 17)
(13 12 19 18)
(12 21 20 19)
(0 3 2 1)
(3 5 4 2)
(2 4 6 7)
(1 2 7 8)
(10 1 8 9)
);
}
top_bottom_step
{
type wall;
faces
(
(16 15 4 5)
(15 17 6 4)
(11 0 1 12)
(21 10 9 20)
(21 12 1 10)
);
}
);
mergePatchPairs
(
);
//*******************//controlDict code :
/--------------------------------- 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 1.2;
deltaT 0.00005;
writeControl timeStep;
writeInterval 100;
purgeWrite 0;
writeFormat ascii;
writePrecision 6;
writeCompression off;
timeFormat general;
timePrecision 6;
runTimeModifiable true;
// ************************* //fvSchemes:
/--------------------------------- 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 fvSchemes;
}
// * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //
ddtSchemes
{
default Euler;
}
gradSchemes
{
default Gauss linear;
grad(p) Gauss linear;
}
divSchemes
{
default none;
div(phi,U) Gauss linear;
}
laplacianSchemes
{
default Gauss linear orthogonal;
}
interpolationSchemes
{
default linear;
}
snGradSchemes
{
default orthogonal;
}
// ************************* //
fvSolution:
/--------------------------------- 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 fvSolution;
}
// * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //
solvers
{
p
{
solver PCG;
preconditioner DIC;
tolerance 1e-06;
relTol 0.05;
}
pFinal
{
$p;
relTol 0;
}
U
{
solver smoothSolver;
smoother symGaussSeidel;
tolerance 1e-05;
relTol 0;
}
}
PISO
{
nCorrectors 2;
nNonOrthogonalCorrectors 0;
pRefCell 0;
pRefValue 0;
}
// ************************* //
Initial & final boundary conditions:
Pressure:
/--------------------------------- 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;
}
front_back
{
type empty;
}
top_bottom_step
{
type zeroGradient;
}
}
// ************************* //
Velocity:
/--------------------------------- 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;
}
front_back
{
type empty;
}
top_bottom_step
{
type noSlip;
}
}
// ************************* //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;
}
// * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //
nu [0 2 -1 0 0 0 0] 1.48e-5;
// ************************* //NOTE:- for various simple gradings we have to modify lines 47-51 in blockMeshDict code according to the grading factor
Grading factor 0.2:
hex (0 1 2 3 11 12 13 14) (80 10 1) simpleGrading (0.2 5 1) //block1
hex (3 2 4 5 14 13 15 16) (80 10 1) simpleGrading (0.2 0.2 1) //block2
hex (2 7 6 4 13 18 17 15) (120 10 1) simpleGrading (5 0.2 1) //block3
hex (1 8 7 2 12 19 18 13) (120 10 1) simpleGrading (5 5 1) //block4
hex (10 9 8 1 21 20 19 12) (120 20 1) simpleGrading (5 5 1) //block5
Grading factor 0.4:
hex (0 1 2 3 11 12 13 14) (80 10 1) simpleGrading (0.4 5 1) //block1
hex (3 2 4 5 14 13 15 16) (80 10 1) simpleGrading (0.4 0.4 1) //block2
hex (2 7 6 4 13 18 17 15) (120 10 1) simpleGrading (5 0.4 1) //block3
hex (1 8 7 2 12 19 18 13) (120 10 1) simpleGrading (5 5 1) //block4
hex (10 9 8 1 21 20 19 12) (120 20 1) simpleGrading (5 5 1) //block5
Grading factor 0.5:
hex (0 1 2 3 11 12 13 14) (80 10 1) simpleGrading (0.2 5 1) //block1
hex (3 2 4 5 14 13 15 16) (80 10 1) simpleGrading (0.2 0.2 1) //block2
hex (2 7 6 4 13 18 17 15) (120 10 1) simpleGrading (5 0.2 1) //block3
hex (1 8 7 2 12 19 18 13) (120 10 1) simpleGrading (5 5 1) //block4
hex (10 9 8 1 21 20 19 12) (120 20 1) simpleGrading (5 5 1) //block5
Grading factor 0.6:
hex (0 1 2 3 11 12 13 14) (80 10 1) simpleGrading (0.6 5 1) //block1
hex (3 2 4 5 14 13 15 16) (80 10 1) simpleGrading (0.6 0.6 1) //block2
hex (2 7 6 4 13 18 17 15) (120 10 1) simpleGrading (5 0.6 1) //block3
hex (1 8 7 2 12 19 18 13) (120 10 1) simpleGrading (5 5 1) //block4
hex (10 9 8 1 21 20 19 12) (120 20 1) simpleGrading (5 5 1) //block5
Grading factor 0.8:
hex (0 1 2 3 11 12 13 14) (80 10 1) simpleGrading (0.8 5 1) //block1
hex (3 2 4 5 14 13 15 16) (80 10 1) simpleGrading (0.8 0.8 1) //block2
hex (2 7 6 4 13 18 17 15) (120 10 1) simpleGrading (5 0.8 1) //block3
hex (1 8 7 2 12 19 18 13) (120 10 1) simpleGrading (5 5 1) //block4
hex (10 9 8 1 21 20 19 12) (120 20 1) simpleGrading (5 5 1) //block5
Grading factor 1:
hex (0 1 2 3 11 12 13 14) (80 10 1) simpleGrading (1 1 1) //block1
hex (3 2 4 5 14 13 15 16) (80 10 1) simpleGrading (1 1 1) //block2
hex (2 7 6 4 13 18 17 15) (120 10 1) simpleGrading (1 1 1) //block3
hex (1 8 7 2 12 19 18 13) (120 10 1) simpleGrading (1 1 1) //block4
hex (10 9 8 1 21 20 19 12) (120 20 1) simpleGrading (1 1 1) //block5
Temperature profiles for various mesh gradings
Glyph profiles for various mesh gradings
Graphs of velocity, pressure V/S length of the domain at 0.25second of simulation
Simulation videos:
https://drive.google.com/file/d/12JkhRmmPxQNTKj1vlPzkibKoRIKuV7Yt/view?usp=sharing
https://drive.google.com/file/d/1Mft4kKXuE1lFQeg5OFGLa8p6tP00Y-G4/view?usp=sharing
Simple grading:
The accuracy of the solution mainly depends on the size of the mesh. if we reduce mesh size continuously computation time required to solve the problem increases so we cant go finer and finer mesh leads to more computation time. sample grading on such technique used to create finer mesh where required and the coarse mesh was not required. if we want to analyze a problem which contains the sudden change in geometry and some obstacles. the solution divers at the location so we need finer mesh at that location for good results and coarse mesh at other locations.
simple grading stretches cushes the mesh where required. The simple description specifies uniform expansions in the local x, y, and z directions respectively with only 3 expansion ratios,
simpleGrading(x y z)
conclusions:
As we decrease the grading ratio we are getting smoother results at backward-facing step.