Week 8 - Simulation of a backward facing step in OpenFOAM
AIM
To simulate an incompressible-laminar-viscous flow through the backward facing step geometry
OBJECTIVE
Based on the described physics of flow choose the appropriate solver and Perform a transient simulation
- Simulate for two different cases and compare the results
Case 1 - Simulate the flow without using any grading factor (i.e., GF = 1)
Case 2 - Simulate the flow with grading factor of 0.2. The cells should be finer near the walls (includig the step wall).
Mesh specification
- Number of cells along the x direction (longer dimension) = 200
- Number of cells along the longer y direction = 20;
Boundary condition specification
The domain specifications are provided in the following figure.
Desired output
- Measure the velocity profile at 0.085 m from the inlet of the geometry for both case studies and compare. You can use the plot over line tool for this.
- Explain why Grading/Growth factor is essential for meshing.
STEPS
- First we opens the terminal
- Then from the terminal we entered the openfoam
- By usinng 'tut' command in terminal we opened tutorials
- From the tutorials we finds the suitable one for this problem,ie;pisofoam
- We copied this tutorial in 'run' folder in the name of 'cavity' by using the command - cp -r cavity/ $FOAM_RUN/ cavity
- After that we opened blockmesh using terminal commands
- Then we added vertices and planes to build the geometry
- After that we runs the blockmesh in terminal using the blocMesh command
- Then we open paraview by using the command paraFoam
- Then we runs simulation using the solver pisofoam
- We runs it by the pisoFoam command in terminal
- Then we can see the simulation
- In order to maintain the value of CFL number we set the value of of 'dt' to 0.005 in controldict
- Then again we runs the simulation
First we simulates for mesh without grading
Mesh- Without grading
Block Mesh
Blockmesh-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) //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 -0.01 0) //9
(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 5 1) simpleGrading (1 1 1) //1
hex (3 2 4 5 14 13 15 16) (80 5 1 ) simpleGrading (1 1 1) //2
hex (2 7 6 4 13 18 17 15) (120 5 1) simpleGrading (1 1 1) //3
hex (1 8 7 2 12 19 18 13) (120 5 1) simpleGrading (1 1 1) //4
hex (10 9 8 1 21 20 19 12) (120 10 1) simpleGrading (1 1 1) //5
);
edges
(
);
boundary
(
inlet
{
type patch;
faces
(
(0 11 14 3)
(3 14 16 5)
);
}
fixedWalls
{
type wall;
faces
(
(4 5 16 15)
(6 4 15 17)
(0 1 12 11)
(10 9 20 21)
(12 1 10 21)
);
}
frontAndBack
{
type empty;
faces
(
(1 0 3 2)
(2 3 5 4)
(8 1 2 7)
(9 10 1 8)
(2 4 6 7)
(11 12 13 14)
(14 13 15 16)
(13 18 17 15)
(12 19 18 13)
(21 20 19 12)
);
}
outlet
{
type patch;
faces
(
(8 19 20 9)
(7 18 19 8)
(6 17 18 7)
);
}
);
mergePatchPairs
(
);
// ************************************************************************* //Controldict
we changed the value of 'dt' to 0.0005
And sets the start time and end time
Pressure
Velocity
Velocity contour
Velocity profile
Observations
- As the distance increases velocity increases and attains a peak value then in starts decreasing with increasing distance
Mesh- With grading (grading factor=0.2)
- We added grading factor of 0.2 here
Blockmesh
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) //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 -0.01 0) //9
(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 5 1) simpleGrading (0.2 5 1) //1
hex (3 2 4 5 14 13 15 16) (80 5 1 ) simpleGrading (0.2 0.2 1) //2
hex (2 7 6 4 13 18 17 15) (120 5 1) simpleGrading (5 0.2 1) //3
hex (1 8 7 2 12 19 18 13) (120 5 1) simpleGrading (5 5 1) //4
hex (10 9 8 1 21 20 19 12) (120 10 1) simpleGrading (5 5 1) //5
);
edges
(
);
boundary
(
inlet
{
type patch;
faces
(
(0 11 14 3)
(3 14 16 5)
);
}
fixedWalls
{
type wall;
faces
(
(4 5 16 15)
(6 4 15 17)
(0 1 12 11)
(10 9 20 21)
(12 1 10 21)
);
}
frontAndBack
{
type empty;
faces
(
(1 0 3 2)
(2 3 5 4)
(8 1 2 7)
(9 10 1 8)
(2 4 6 7)
(11 12 13 14)
(14 13 15 16)
(13 18 17 15)
(12 19 18 13)
(21 20 19 12)
);
}
outlet
{
type patch;
faces
(
(8 19 20 9)
(7 18 19 8)
(6 17 18 7)
);
}
);
mergePatchPairs
(
);
// ************************************************************************* //
Controldict
we changed the value of 'dt' to 0.00005
And sets the start time and end time
Pressure
Velocity
Transport Properties
Velocity Contour
Velocity profile
Observations
- As the distance increases velocity increases and attains a peak value then in starts decreasing with increasing distance
Conclusion
- Velocity profile in the first case ,ie; without grading ,the velocity profile is smooth
- But for mesh with grading velocity profile is not smooth as much the first case
- Using a single grading to describe mesh grading permits only one-way grading of the block.
- To reduce complexity and effort we can use multi-grading to control grading within separate divisions of a single block, rather than have to define several blocks which gives accurate result
All files is attached below