Week 8 - Simulation of a backward facing step in OpenFOAM
Aim:
To simulate the turbulent flow of air in a backward step for different base grid size and give the suitable conclusion
Theory:
The separate flow generated as fluid passes over a backward-facing step is of interest for a variety of reasons. First separated flows produced by an abrupt change in the geometry are of great importance in many engineering applications also the backward-facing step is an extreme example of separated floes that occur in aerodynamic devices such as high lift, airfoils at a large angle of attack. In these flows, separation may be created by a strong adverse pressure gradient rather than a geometric perturbation.
Objective: this project is to set up create a geometry buy editing blockMeshDict file to simulate a flow through backward-facing step for different grading factors.
To set up the geometry the figure was divided into 5 parts or blocks, a total number of cells along the x-direction is 120 which is divided as 80 no of cells for blocks no 0,1,2,3,4 for y-direction total number of cells is 10 which is divided as 5 cells each block in the y-direction.
Explanation of my simulation:
OpenFOAM code:
in OpenFOAM go to the tutorial file and incompressible file and then icoFoam, copy the cavity file to our required dictionary.
Block mesh dict: in the following code we have created the required geometry for grading factor 1 and 0.2 and we also assigned the boundary condition. This we did in the blockMeshDict file of the system in the cavity tutorial
/*--------------------------------*- 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) //0
(0.08 0 0) //1
(0.08 0.005 0) //2
(0 0.005 0) //3
(0.08 0.01 0) //4
(0 0.01 0) //5
(0.2 0.01 0) //6
(0.2 0.005 0) //7
(0.2 0 0) //8
(0.2 -0.005 0) //9
(0.08 -0.005 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 -0.005 0.01) //20
(0.08 -0.005 0.01) //21
);
blocks
(
hex (0 1 2 3 11 12 13 14) (120 5 1) simpleGrading (1 1 1)
hex (3 2 4 5 14 13 15 16) (120 5 1) simpleGrading (1 1 1)
hex (2 7 6 4 13 18 17 15) (80 5 1) simpleGrading (1 1 1)
hex (1 8 7 2 12 19 18 13) (80 5 1) simpleGrading (1 1 1)
hex (10 9 8 1 21 20 19 12) (80 5 1) simpleGrading (1 1 1)
);
edges
(
);
boundary
(
movingWall
{
type patch;
faces
(
(0 3 11 14)
(3 14 16 5)
);
}
fixedWalls
{
type patch;
faces
(
(7 6 17 18)
(8 7 18 19)
(9 8 19 20)
);
}
frontAndBack
{
type empty;
faces
(
(11 14 12 13)
(14 16 13 15)
(13 15 18 17)
(12 13 19 18)
(21 12 20 19)
(0 3 2 1)
(3 5 4 2)
(2 4 6 7)
(1 2 7 8)
(10 1 8 9)
);
}
Noslipwalls
{
type wall;
faces
(
(5 16 15 4)
(4 15 17 6)
(10 1 21 12)
(0 1 12 11)
(10 9 20 21)
);
}
);
mergePatchPairs
(
);
// ************************************************************************* //controlDict file:
in the following code we have assigned the start time, end time as well as time steps. This we did it in the controlDict file of system folder in cavity tutorial.
/*--------------------------------*- 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;
// ************************************************************************* //Velocity File:
In this folloeing code we have assign the value for velocity for boundary condition and also for different patch. This we did in the U file of constant folder in cavity tutorial
/*--------------------------------*- 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
{
movingWall
{
type fixedValue;
value uniform (3 0 0);
}
fixedWalls
{
type zeroGradient;
}
frontAndBack
{
type empty;
}
Noslipwalls
{
type noSlip;
}
}
// ************************************************************************* //Pressure file:
In the following code we have assign the value for preesure for boundary condition and also for different patch. This we did in P pressure file of constant folder in cavity.
/*--------------------------------*- 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
{
movingWall
{
type zeroGradient;
}
fixedWalls
{
type fixedValue;
value uniform 0;
}
frontAndBack
{
type empty;
}
Noslipwalls
{
type zeroGradient;
}
}
// ************************************************************************* //Results:
Following image shows the mesh with grid, and velocity mesh
For grading factor 1:
Velocity profile:
Line Plot velocity:
Mesh code for grding factor=0.5
blocks
(
hex (0 1 2 3 11 12 13 14) (120 5 1) simpleGrading (0.2 0.2 1)
hex (3 2 4 5 14 13 15 16) (120 5 1) simpleGrading (0.2 0.2 1)
hex (2 7 6 4 13 18 17 15) (80 5 1) simpleGrading (5.0 0.2 1)
hex (1 8 7 2 12 19 18 13) (80 5 1) simpleGrading (5.0 0.2 1)
hex (10 9 8 1 21 20 19 12) (80 5 1) simpleGrading (5.0 0.2 1)
);Same steps needs to follow but need to change grid factor as 0.5 in blockMeshDict file in blocks line
Velocity Profile:
Grading factor 1 Grading factor 0.2
https://drive.google.com/file/d/1Vzn668SDxAfBVE2RcFC-Mj_PlNcqLbX4/view?usp=sharing
Conclusion:
Line plot:
All above line plot is been obtained at x=0.085 and y=0.01, we different plot for different grading factor at same loction.
Velocity contour:
All above velocity contour is obtained for different grading factor at 1 and 0.5
And also after observing the above comparision we can conclude that grading factor affects the results. Finner grading factor at the wall create more clear simulation.