Week 8 - Simulation of a backward facing step in OpenFOAM
Project Objective:
-
To simulate the fluid flow through a backward facing step in openFoam using icoFoam solver.
-
To predict the magnitude of velocity at a distance of 0.085 m from the inlet section of the geometry & grid dependency is also tested for this prediction by imposing various mesh grading (0.2 & 0.5) with respect to the wall respectively.
Scope of the Project:
The work is carried out with the aid of tutorial named cavity under the icoFoam solver and the appropriate Geometry, Mesh, Boundary and initial conditions are created by modifying the script files from the respective solver.
Modelling Information:
The solver here is used is icoFoam which solves for a combination of transient, laminar, incompressible and Newtonian fluid flow problems.
Since this is a mere flow problem, energy equations are not required and it solves for only the mass balance (continuity eqn) and Force balance (Momentum balance) respectively.
The flow is assumed to be incompressible (water is used as a working fluid) and the flow is assumed to be a two dimensional flow (i.e.) not a function of z axis.
Boundary conditions:
The inlet velocity is set to be 1m/s and boundary condition at the outlet is pressure outlet.
Initial conditions:
The initial conditions for the above flow problem is defined in accordance with the velocity and size of the mesh.
Keeping the velocity constant (m/s), for the ungraded mesh, size of the mesh is 0.001m. So to keep the CFL number less than 1 (here we fixed as 0.5), the time step should be 5e-04. Similarly, for the grading factor of 0.2, the minimum size of the mesh is 0.0002m (20% of the ungraded), so the time step for this case setup is 1e-04. In this fashion, the time step is to be fixed for the next case.
-
For grading = 0.5, the least size of the mesh is 0.0005 (50%), so del t = 2.5e-04
blockMeshDict file
========= |
\\ / F ield | OpenFOAM: The Open Source CFD Toolbox
\\ / O peration | Website: https://openfoam.org
\\ / A nd | Version: 9
\\/ M anipulation |
\*---------------------------------------------------------------------------*/
FoamFile
{
format ascii;
class dictionary;
object blockMeshDict;
}
// * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //
convertToMeters 1;
vertices
(
(0 0 0)
(0.08 0 0)
(0.08 0.005 0)
(0 0.005 0)
(0 0.01 0)
(0.08 0.01 0)
(0.2 0.01 0)
(0.2 0.005 0)
(0.2 0 0)
(0.2 -0.01 0)
(0.08 -0.01 0)
(0 0 0.001)
(0.08 0 0.001)
(0.08 0.005 0.001)
(0 0.005 0.001)
(0 0.01 0.001)
(0.08 0.01 0.001)
(0.2 0.01 0.001)
(0.2 0.005 0.001)
(0.2 0 0.001)
(0.2 -0.01 0.001)
(0.08 -0.01 0.001)
);
blocks
(
hex (0 1 2 3 11 12 13 14) (80 5 1) simpleGrading (1 1 1)
hex (3 2 5 4 14 13 16 15) (80 5 1) simpleGrading (1 1 1)
hex (2 7 6 5 13 18 17 16) (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
(
frontAndBack
{
type empty;
faces
(
(0 3 2 1)
(11 12 13 14)
(3 4 5 2)
(14 13 16 15)
(2 5 6 7)
(13 18 17 16)
(1 2 7 8)
(12 19 18 13)
(10 1 8 9)
(21 20 19 12)
);
}
noSlipWall
{
type wall;
faces
(
(15 16 5 4)
(16 17 6 5)
(0 1 12 11)
(10 21 12 1)
(10 9 20 21)
);
}
inlet
{
type patch;
faces
(
(0 11 14 3)
(3 14 15 4)
);
}
outlet
{
type patch;
faces
(
(9 8 19 20)
(8 7 18 19)
(7 6 17 18)
);
}
);
mergePatchPairs
(
);
controlDict file
/*--------------------------------*- C++ -*----------------------------------*\
========= |
\\ / F ield | OpenFOAM: The Open Source CFD Toolbox
\\ / O peration | Website: https://openfoam.org
\\ / A nd | Version: 9
\\/ M anipulation |
\*---------------------------------------------------------------------------*/
FoamFile
{
format ascii;
class dictionary;
location "system";
object controlDict;
}
// * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //
application icoFoam;
startFrom startTime;
startTime 0;
stopAt endTime;
endTime 10;
deltaT 0.01;
writeControl timeStep;
writeInterval 20;
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: 9
\\/ M anipulation |
\*---------------------------------------------------------------------------*/
FoamFile
{
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: 9
\\/ M anipulation |
\*---------------------------------------------------------------------------*/
FoamFile
{
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;
}
// ************************************************************************* //p file
/*--------------------------------*- C++ -*----------------------------------*\
========= |
\\ / F ield | OpenFOAM: The Open Source CFD Toolbox
\\ / O peration | Website: https://openfoam.org
\\ / A nd | Version: 9
\\/ 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;
}
noSlipWall
{
type zeroGradient;
}
frontAndBack
{
type empty;
}
}
U file
/*--------------------------------*- C++ -*----------------------------------*\
========= |
\\ / F ield | OpenFOAM: The Open Source CFD Toolbox
\\ / O peration | Website: https://openfoam.org
\\ / A nd | Version: 9
\\/ 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 (0.02 0 0);
}
outlet
{
type zeroGradient;
}
noSlipWall
{
type noSlip;
}
frontAndBack
{
type empty;
}
}
// ************************************************************************* //OUTPUT
UNGRADED
hex (0 1 2 3 11 12 13 14) (80 5 1) simpleGrading (1 1 1)
hex (3 2 5 4 14 13 16 15) (80 5 1) simpleGrading (1 1 1)
hex (2 7 6 5 13 18 17 16) (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)
GRADED 0.2
hex (0 1 2 3 11 12 13 14) (80 5 1) simpleGrading (1 5 1)
hex (3 2 5 4 14 13 16 15) (80 5 1) simpleGrading (1 0.2 1)
hex (2 7 6 5 13 18 17 16) (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) 
GRADED 0.5
hex (0 1 2 3 11 12 13 14) (80 5 1) simpleGrading (1 2 1)
hex (3 2 5 4 14 13 16 15) (80 5 1) simpleGrading (1 0.5 1)
hex (2 7 6 5 13 18 17 16) (120 5 1) simpleGrading (2 0.5 1)
hex (1 8 7 2 12 19 18 13) (120 5 1) simpleGrading (2 2 1)
hex (10 9 8 1 21 20 19 12) (120 10 1) simpleGrading (2 2 1) 
