Challenge

Week 8 - Simulation of a backward facing step in OpenFOAM

AAkash N8 Mar 2021

Aim: To simulate an incompressible-laminar-viscous flow through the backward-facing step geometry in OpenFOAM.

Objective:

We are going to simulate an incompressible-laminar-viscous flow through the backward-facing step geometry. We should
perform a transient simulation. The solver can be chosen based on the described physics of the flow. We need to explain the
entire simulation procedure (how we set up the case). We should also provide explanations for every change we make inside the
reference case files obtained from OpenFOAM tutorials. We need to find out:-
1. How does the velocity magnitude profile change as a function of the mesh grading factor? (factors, 0.2, 0.5, 0.8)
2. Measure the velocity profile at 0.085 m from the inlet of the geometry?
3. Compare velocity magnitude contours near the step region for the above-mentioned grading factors.
Mesh Specification
1. Number of cells along the x-direction (longer dimension) = 200
2. Number of cells along with the y-direction = 10
3. Use a graded mesh. The grading factor near all walls should be 0.2

OpenFoam Script
1. First open the Linux Operating system.
2. Now type 'tut' and press enter. It will take you to the in-built tutorial files. Now type 'ls' to check the files in this folder.
3. We are solving the incompressible problem. So we are going to the incompressible folder by the 'cd incompressible'
command. It takes you to the incompressible directory.
4. Now type 'cd icoFoam' that takes you to the icofoam directory. In this directory, there are two folders elbow and cavity.
5. Now we are going to copy the cavity file by 'cp -r cavity/ $FOAM_RUN'. This will copy all the cavity files and paste them into the run
folder.
6. Now type 'run' it will take you to the run folder. Now type ls to check the files in that folder. There we will find a cavity folder.
Now open that folder. There another folder name 'cavity'. Now open that folder by 'cd cavity'.
7. Now we are going to type 'cd system' it will take you to the system directory. now when we type 'ls' it shows what is in this
directory.
8. Now we are going to create Mesh for our problem in the blockMeshDict. So in order to edit that we are going to use
'gedit blockMeshDict'. It will open the text editor file.
9. so we do the following edit in the editor:-
blockMeshDict

/*--------------------------------*- 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 0.05) //11
(0.08 0 0.05) //12
(0.08 0.005 0.05) //13
(0 0.005 0.05) //14
(0.08 0.01 0.05) //15
(0 0.01 0.05) //16
(0.2 0.01 0.05) //17
(0.2 0.005 0.05) //18
(0.2 0 0.05) //19
(0.2 -0.01 0.05) //20
(0.08 -0.01 0.05) //21
);
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 (10 9 8 1 21 20 19 12) (120 10 1) simpleGrading (1 1 1)
hex (1 8 7 2 12 19 18 13) (120 5 1) simpleGrading (1 1 1)
hex (2 7 6 4 13 18 17 15) (120 5 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)
);
}
fixedWalls
{
type wall;
faces
(
(16 15 4 5)
(15 17 6 4)
(10 21 12 1)
(0 1 12 11)
(10 9 20 21)
);
}
frontAndBack
{
type empty;
faces
(
(3 2 1 0)
(5 4 2 3)
(1 8 9 10)
(2 7 8 1)
(4 6 7 2)
(11 12 13 14)
(14 13 15 16)
(21 20 19 12)
(12 19 18 13)
(13 18 17 15)
);
}
);
mergePatchPairs
(
);
// ************************************************************************* //

Explanation
We are giving the co-ordinance in the meter already. So we will use convert meters to be 1.
Now we are going to define the vertices of the blocks. There are 21 vertices. To define the vertices we are going to give the
x, y, and z co-ordinance for every vertices.
Now we are going to define the blocks of the mesh. Each block definition is a compound entry consisting of a list of vertex
labels.
We are giving the number of the grid points different-different so that one grid be in 1 mm.
Now we need to define the edges. This is optional. Each edge joining 2 vertex points is assumed to be straight by default.
However, any edge may be specified to be curved by entries in a list named edges.
First, we are taking grad factor to be 1 so we are typing 'simpleGrading(1 1 1)'.
Now we are going to define the boundaries that I have explained above.

We have created Mesh with the 5 blocks. So now we have to merge them. We are going to use mergePatchPairs to do that.
Now we are going to save this.
Now in the terminal, we are going to give the command 'blockMesh' to check that the Mesh is good or not. If there is any
mistake OpenFOAM will give us a warning.
10. Now we are going to type 'gedit controlDict'. It will open the editor file where we can edit the time
interval. The controlDict dictionary sets input parameters essential for the creation of the time database.

controlDict

Explanation
First, we are defining that by which application we are solving our problem.
Now we give the time at which the simulation starts. I have taken the start time to be 0.
Now we are giving the simulation end time. At which simulation is going to stop. I have taken the end time to be 0.1.

Now we have to give the values. We have to give the like this so that the maximum courant number don't go
above 1. OpenFoam automatically calculates the courant number and show that in the terminal window. If the values go
above it gives us an error.
Courant number
writeControl, Controls the timing of write output to file.
Timestep, Writes data every writeInterval time steps.
We have given the write interval timestep to be 20.
We have given purge write to be 0, pure write Integer representing a limit on the number of time directories that are
stored by overwriting time directories on a cyclic basis
We have taken the ASCII format of the data files.
writeCompression, Specifies the compression of the data files.
After changing the values you have to save the file.

Now we are going to give the initial values of the pressure and velocity. So we have to go to the folder '0'. So first go again in
the cavity folder then use 'cd 0' to go to that folder.
Now type 'gedit p' to edit the pressure values for the initial condition.
Pressure

fixedWalls
{
type zeroGradient;
}
frontAndBack
{
type empty;
}
}
// ************************************************************************* //

Explaination
The unit of the pressure is `
The internal field is uniform So we have given uniform 0.
Now we are defining the boundary field.
For inlet we have given zeroGradient type boundary condition. This boundary condition applies a zero-gradient condition
from the patch internal field onto the patch faces. A zero-gradient condtion is applied for outflow.
At the outlet we have taken the pressure to be fixedValue type. This boundary condition supplies a fixed gradient
condition, such that the patch values are calculated using. We have taken the value to be 0.
At the fixed wall we have given the zeroGradient type boundary condition.
At the front and back we have given the empty type boundary condition. This boundary condition provides an ’empty’
condition for reduced dimensions cases

Now type 'gedit U' to edit the velocity values for initial condition.

outlet
{
type zeroGradient;
}
fixedWalls
{
type noSlip;
}
frontAndBack
{
type empty;
}
}
// ************************************************************************* //

Explaination
The unit of the velocity is .
The internal field is uniform So we have given uniform 0.
Now we are defining boundary field.
At the inlet we have taken the velocity to be fixedValue type. This boundary condition supplies a fixed gradient condition,
such that the patch values are calculated using. We have taken the value to be uniform(1 0 0).
For outlet we have given zeroGradient type boundary condition. This boundary condition applies a zero-gradient condition
from the patch internal field onto the patch faces. A zero-gradient condtion is applied for outflow.
At the fixed wall we have given the no slip type boundary condition.
At the front and back we have given the empty type boundary condition. This boundary condition provides an ’empty’
condition for reduced dimensions cases

Now we are going to outside the folder '0' by command 'cd ..' and go to the folder 'constant' by command 'cd constant'.

Now we are going to use command 'gedit transportProperties'. In the transportProperties we can edit the viscosity of the
flow We have given viscosity to be 001 The unit of the viscosity

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] 0.01;

// ************************************************************************* //

Now we are going to the cavity folder by 'cd ..' command.

To calculate the value we are going to type 'icoFoam'. Now when it solve the values it appear on the terminal window. So to
check the effect of the pressura and velocity on the mesh we are going to see the mesh in paraview. To do that we are going to
use 'paraFoam' command.

Now paraview window is open, now by applying properties on the mesh we can see the mesh's paraview. After playing the
frame it shows the frame at many different time.

Now we are going to plot the graph of the effect of the velocity on the Mesh at the line 0.085 m. We have givent the graph
title and axis label.

Now to save the velocity data we are going to use 'Export Scene..' and save the file.
case 2 Grading factor 0.2

/*--------------------------------*- 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 0.05) //11
(0.08 0 0.05) //12
(0.08 0.005 0.05) //13
(0 0.005 0.05) //14
(0.08 0.01 0.05) //15
(0 0.01 0.05) //16
(0.2 0.01 0.05) //17
(0.2 0.005 0.05) //18
(0.2 0 0.05) //19
(0.2 -0.01 0.05) //20
(0.08 -0.01 0.05) //21
);
blocks
(
hex (0 1 2 3 11 12 13 14) (80 5 1) simpleGrading (1 5 1)
hex (3 2 4 5 14 13 15 16) (80 5 1) simpleGrading (1 0.2 1)
hex (10 9 8 1 21 20 19 12) (120 10 1) simpleGrading (5 5 1)
hex (1 8 7 2 12 19 18 13) (120 5 1) simpleGrading (5 5 1)
hex (2 7 6 4 13 18 17 15) (120 5 1) simpleGrading (5 0.2 1)
);
edges
(
);
boundary
(
inlet
{
type patch;
faces
(
(

(0 11 14 3)
(3 14 16 5)

Explanation
Now in this case everything is the same the only difference is to give the grading factor to the blocks. we have to use the formula
Grading factor =
where
l = size of the last boundary
f = size of the first boundary
We have to take the grid to be small at the boundary where the boundary of the block is at the boundary of the mesh. We have
to be careful while giving the grading to any boundary. The mesh should be in alignment.
In the case when we have taken grading factor 0.2 We have written the blocks like this:-

hex (0 1 2 3 11 12 13 14) (80 5 1) simpleGrading (1 5 1)
hex (3 2 4 5 14 13 15 16) (80 5 1) simpleGrading (1 0.2 1)
hex (10 9 8 1 21 20 19 12) (120 10 1) simpleGrading (5 5 1)
hex (1 8 7 2 12 19 18 13) (120 5 1) simpleGrading (5 5 1)
hex (2 7 6 4 13 18 17 15) (120 5 1) simpleGrading (5 0.2 1)
Other than this we don't need to change anything.

Output
case 1
Mesh

Pressure effect

Velocity effect

Graph

case 2
Mesh

Pressure effect

Velocity effect

Graph

Week 8 - Simulation of a backward facing step in OpenFOAM

Ship work like this
in your own course.

Talk to a career counsellor

Ship work like thisin your own course.

Talk to a career counsellor

Ship work like this
in your own course.