View source for OpenFOAM-version-7/C2/Turbulence-Modelling-in-OpenFOAM/English

'''Title of the script:''' Turbulence Modelling in OpenFOAM

'''Author:''' Padmini Priyadarshini

'''Keywords:''' OpenFOAM, Turbulence, YPlus, K-Omega, K-OmegaSST, Paraview, Video-Tutorial



{| border="1"
|- 
|| '''Visual Cue'''
|| '''Narration'''
|- 
|| Slide: Opening Slide
|| Welcome to this tutorial on '''Turbulence Modelling in OpenFOAM.'''
|- 
|| Slide: Learning Objective
|| In this tutorial, we will learn how to,
* Set up the '''blockMeshDict''' dictionary for a given '''YPlus''' value
* Implement '''k-omega''' and '''k-omega SST''' '''turbulence models'''
* '''Run '''the '''simulation'''
|- 
|| Slide: System Specifications
|| To record this tutorial, I am using,
* '''Linux Mint OS''' version 18.3
* '''OpenFOAM''' version 7
* '''ParaView''' version 5.6.0
* '''gedit Text Editor'''
However, you may use any other '''text editor''' of your choice.
The steps explained in this tutorial are identical in '''Ubuntu Linux OS'''.
|-
|| Slide: Prerequisites
|| As a prerequisite:
* You should have basic knowledge of '''geometric progression'''
* You should be familiar with '''simulating''' a''' turbulent flow''' through a''' channel '''and
* You should also be familiar with '''Multi-block Meshing of 2D Geometry'''
* If not, please go through the prerequisite '''OpenFOAM''' tutorial on this website
|-
|| Slide: Code Files
|| 
* The files used in this tutorial are available in the '''Code''' '''Files''' link on this tutorial page 
* Please download and extract them
* Make a copy and then use them while practising
|-
|| Slide: Solver detail
|| We will use the '''simpleFoam solver''' to '''simulate '''this problem.
'''simpleFoam''' is a '''steady-state solver''' for '''incompressible''', '''turbulent flow'''.
|-
|| Slide: Problem statement
|| The diagram shows a''' 2D channel '''of length '''65 m '''and width '''1 m'''.
The '''kinematic viscosity''' is '''1e-05 m<sup>2</sup>/s.'''
The '''Inlet velocity '''is '''20 m/s. '''
'''Outlet pressure '''is set to '''0 atmosphere.'''
|-
|| Slide: Flow Properties
|| '''Reynolds number''' is '''2 million'''.
The flow is '''turbulent.'''
|- 
|| Only Narration
|| First, We will use''' k-epsilon turbulence model '''to '''simulate''' this '''case'''.
Let us set up the '''case.'''
|-
|| Only Narration 
|| Download the '''kepsilon''' folder provided in the '''code files''' link and extract it.
|-
|| CTRL + ALT + T
|| Open the '''terminal''' by pressing '''Ctrl''', '''Alt''' & '''T''' keys.
|-
|| Only Narration
|| Here onwards, please remember to press the '''Enter''' key after typing each '''command''' in the '''terminal'''.
|-
|| [Terminal] Type: '''cd $FOAM_RUN '''
|| Open the '''Run directory '''by typing this '''command.''' 
|-
|| [Terminal] 
Type: '''cp -r ~/Downloads/kepsilon .'''
|| Copy the downloaded file into the '''Run directory'''.
To do that, type the following '''command.'''
|-
|| Only Narration 
|| I have downloaded the file into the '''Downloads directory.'''
Please change the '''command''' according to the '''file path''' on your machine.
|-
|| [Terminal] Type: '''cd kepsilon'''
|| Navigate to the '''kepsilon directory.'''
|-
|| [Terminal] Type:
'''gedit system/blockMeshDict'''
|| Open the '''blockMeshDict '''file in a '''text editor.'''
|-
|| [gedit -blockMeshDict]
Highlight '''blocks'''
|| Scroll down to the '''keyword “blocks”.'''
|-
|| [gedit -'''blockMeshDict''']
Highlight '''hex .... (1 0.18 1);'''
|| The '''domain''' has two equal-sized '''blocks.'''
|-
|| [gedit -'''blockMeshDict''']
Highlight '''75''' '''30'''
|| For each '''block''', the '''cell number''' is set to '''75 '''in the '''x-direction''', and '''30''' in the '''y-direction.'''
|-
|| Slide: Wall distance, yp
|| Let “'''yp'''” be the distance between the '''wall''' and the nearest '''cell centre'''.
For our '''case, yp''' is '''0.0031''' for '''YPlus''' value of '''200'''.
For''' channel flow''', the '''skin friction coefficient, Cf,''' is given by this formula.
|-
|| Slide: Cell width
|| The width of the''' cell '''near the '''wall''' is '''0.0062.'''
|-
|| Slide: Expansion ratio
|| The '''expansion ratio''' is the ratio of the last '''cell width''' to the first '''cell width.'''
|-
|| Slide: Expansion ratio calculation
|| It is calculated using '''geometric progression''' formula. 
Please refer to the '''Additional Reading Material''' for more details.
|-
|| Slide: Expansion ratio value
|| The '''expansion ratio''' in the '''+y direction''' is found to be 
* '''5.57 '''for '''block 1,''' and 
* '''0.18 '''for''' block 2'''
for a '''block''' of width of '''0.5 m''' and '''cell number 30.'''
Please refer to the '''Additional Reading Material''' on this tutorial page for details.
|-
|| Only Narration
|| I have already put these values into the '''blockMeshDict''' file.
|-
|| [gedit -blockMeshDict]
Highlight''' simpleGrading'''
|| '''simpleGrading''' describes the uniform '''expansion ratio '''in '''x, y''', and''' z-directions.'''
|-
|| Only Narration
|| For our '''case''', the''' expansion ratio''' in the '''y-direction''' is
|-
|| [gedit -'''blockMeshDict''']
Highlight '''5.57'''
|| '''5.57 '''for '''block 1'''
|-
|| [gedit -'''blockMeshDict''']

Highlight''' 0.18'''
|| '''0.18''' for '''block 2'''
|-
|| [gedit - '''blockMeshDict'''] Close Text Editor
|| Close the '''blockMeshDict''' file.
|- 
|| Only Narration 
|| The '''case''' is now ready to be''' run. '''
|-
|| [Terminal] Type: '''blockMesh'''
|| On the '''terminal''', type '''blockMesh''' to '''mesh''' the '''geometry'''.
|-
|| [Terminal] Type:
'''simpleFoam'''
|| Then type '''simpleFoam''' in the '''terminal '''to '''run '''the '''simulation'''.
|-
|| Only Narration 
|| The '''simulation '''will take some '''time''' depending on your computer’s '''hardware'''.
|-
|| [Terminal] Highlight''' End'''
|| The''' simulation''' is complete.
|-
|| Only Narration 
|| Let's check the final value of '''YPlus.'''
|- 
|| [Terminal] Type:
'''simpleFoam -postProcess -func "yPlus" -latestTime'''
|| Now type the following '''command''' on the '''terminal''' to get the '''YPlus '''value on the '''walls.'''
|- 
|| [Terminal]
Highlight''' Patch to 213.433'''
|| We can see that the '''YPlus''' value stays below 300. 
|-
|| [Terminal] Type: '''cd ..'''
|| Let us go back to the '''Run directory''' using the '''cd command'''.
|- 
|| Only Narration 
|| Let us '''simulate''' the same problem using the k-omega''' turbulence model '''now'''.'''
|- 
|| Slide: K-Omega turbulence model
|| '''k-omega '''is a '''two-equation model'''.
It solves-
* The '''turbulent kinetic energy''' '''transport equation''', and
* The '''specific''' '''turbulent dissipation rate transport equation'''
|-
|| Slide: Inlet Boundary Condition - omega
|| Let's look at the '''boundary''' and '''initial conditions''' for '''omega.'''
We will use '''turbulentMixingLengthFrequencyInlet'''
It calculates '''omega''' using '''kappa '''and 
user-specified '''mixing length. '''
'''Mixing length '''refers to the''' turbulent length scale.'''
'''Kappa''' value is automatically put in by the '''solver.'''
Here''', '''the value of''' turbulent length scale''' is '''0.07 m'''
The '''specific''' '''turbulent dissipation rate '''value is '''16.67 1/s'''
Where''' empirical constant, cmu '''is equal to '''0.09'''
|-
|| Slide: Wall & Outlet Boundary Condition - omega
|| '''Wall boundary condition''' is set to '''omegaWallFunction.'''
'''zeroGradient boundary condition''' is imposed at the '''outlet. '''
Please refer to the '''Additional Reading Material''' on this tutorial page for details.
|-
|| Only Narration 
|| Download the '''komega '''folder provided in the '''Code files''' link and extract it.
|-
|| [Terminal] Type:
'''cp -r ~/Downloads/komega .'''
|| Copy the downloaded file into '''Run directory.'''
To do that, type the following '''command '''in the '''terminal.'''
|-
|| [Terminal] Type:
'''cd komega'''
|| Navigate to the '''komega''' directory using the '''cd command.'''
|-
|| Only Narration
|| Let’s take a look at the '''specific''' '''turbulent dissipation rate '''file.
|-
|| [Terminal] Type: '''gedit 0/omega'''
|| Open the '''omega''' file in a '''text editor.'''
|-
|| [gedit -'''omega'''] 
Highlight '''internalField ... 16.67;'''
|| The '''internalField''' value is initialised as '''16.67'''
|-
|| [gedit -'''omega''']
Highlight''' type turbulentMixingLengthFrequencyInlet'''
|| The '''inlet '''boundary '''type''' is set to '''turbulentMixingLengthFrequencyInlet'''
|-
|| [gedit -'''omega'''] 
Highlight''' mixingLength 0.07;'''
|| The '''keyword''' '''mixingLength '''represents the '''Turbulent length scale''', and is set to '''0.07'''
|-
|| [gedit -'''omega'''] 
Highlight''' value $internalField;'''
|| We have passed the '''internalField''' value to the '''patch field '''value.
|-
|| [gedit -'''omega'''] Highlight '''zeroGradient'''
|| The '''outlet patch type''' is set to '''zeroGradient.'''
|-
|| [gedit -'''omega'''] 
Highlight '''omegaWallFunction'''
|| The '''wall patch type''' is set to '''omegaWallFunction.'''
|-
|| [gedit -'''omega'''] Close Text Editor
|| Close the '''omega''' file.
|-
|| [Terminal] Type:
'''gedit constant/turbulenceProperties'''
|| Open the '''turbulenceProperties''' file in a '''text editor.'''
|-
|| [gedit -'''turbulenceProperties'''] 
Highlight''' kOmega'''
|| '''kOmega '''is selected as the '''RASModel.'''
Note,''' O '''is capitalized here.
|-
|| [gedit -'''turbulenceProperties'''] 
Highlight''' on'''
|| Ensure the '''keyword''' '''turbulence''' is set to '''On'''.
|-
|| [gedit -'''turbulenceProperties''']
Close Text Editor
|| Close the '''turbulenceProperties '''file.
|-
|| Only Narration 
|| The '''case''' is now ready to be '''run.'''
|-
|| [Terminal] Type: '''blockMesh'''
|| On the '''terminal''', type '''blockMesh '''to '''mesh '''the '''geometry.''' 
|-
|| [Terminal] Type: '''simpleFoam'''
|| Now, type the following '''command''' to '''run''' the '''case.'''
|-
|| [Terminal] Highlight''' End'''
|| The''' simulation''' is over.
|-
|| [Terminal] Type: '''cd ..'''
|| Let us go back to the '''Run directory''' using the '''cd command.'''
|-
|| Only Narration 
|| Now, let’s '''run''' the same '''case''' with the '''k-omegaSST''' '''turbulence model.'''
|-
|| Slide: K-omegaSST turbulence model
|| The''' K-omega SST turbulence model''' is a '''hybrid '''model.
In the '''near-wall '''region, '''K-omegaSST''' uses the '''K-omega turbulence model.'''

In the region away from the '''wall''', it switches to the '''K-epsilon''' '''turbulence model'''.

Please refer to the '''Additional Reading Material''' on this tutorial page for details.
|-
|| Only Narration 
|| Download the '''komegasst '''folder provided in the '''Code files''' link and extract it.
|-
|| [Terminal] Type:
'''cp -r ~/Downloads/komegasst .'''
|| Copy the downloaded file into the '''Run directory.'''
To do that, type the following '''command '''in the '''terminal.'''
|-
|| [Terminal] Type: '''cd komegasst'''
|| Navigate to the '''komegasst '''directory using the '''cd command.''' 
|-
|| Only Narration 
|| '''komega''' and '''komegasst models''' use the same''' turbulence variables'''.
Hence, the files in the '''0''' directory are the same.
|-
|| [Terminal] Type:
'''gedit constant/turbulenceProperties'''
|| Open the '''turbulence''' properties file in a '''text editor.'''
|-
|| [gedit -'''turbulenceProperties''']
Highlight '''kOmegaSST'''
|| '''RASModel field''' entry is set to '''kOmegaSST.'''
Note,''' O''' and''' SST''' are capitalized here.
|-
|| [gedit -'''turbulenceProperties''']
Close Text Editor
|| Close the '''turbulenceProperties''' file.
|-
|| [Terminal] Type:
'''gedit system/fvSchemes'''
|| Open '''fvSchemes dictionary''' in a '''text editor.'''
|-
|| Only Narration
|| Scroll down to the end of the document.
|-
|| [gedit -'''fvSchemes''']
Highlight '''wallDist ... ;}'''
|| '''wallDist sub-dictionary''' is included in the '''fvSchemes dictionary'''.
|-
|| Only Narration
|| This enables the calculation of the '''distance '''between the '''cell centre''' and the nearest '''wall.'''
|-
|| [gedit -'''fvSchemes'''] Close Text Editor
|| Close the '''fvSchemes''' file.
|-
|| Only Narration 
|| The setup is now ready. 
Let us '''run''' the '''simulation.'''
|-
|| [Terminal] Type: '''blockMesh'''
|| '''Mesh''' the '''geometry''' using the '''blockMesh command '''on the '''terminal.'''
|-
|| [Terminal] Type: '''simpleFoam'''
|| Type '''simpleFoam''' in the '''terminal.'''
|-
|| [Terminal] Highlight''' End'''
|| The '''simulation''' is finished successfully.
|-
|| Show slide
Exit velocity profile - comparison
|| The slide shows the''' exit velocity''' '''profile''' for all three '''turbulence models'''
|-
|| Only Narration
|| With this, we have come to the end of the tutorial.
Let’s summarize.
|- 
|| Slide: Summary
|| In this tutorial, we have learnt to,
* Set up the '''blockMeshDict''' file for a given '''YPlus''' value
* Implement '''k-omega '''and''' k-omega SST turbulence models'''
* '''Run''' the '''simulation'''
|-
|| Slide: Assignment
|| As an assignment:
* Repeat the '''simulation''' for a '''YPlus''' value of '''250 '''using '''k-epsilon''' '''model'''
* Repeat the''' simulation '''for a '''velocity''' of '''40 m/s''' using '''k-omega''' and '''k-omega SST model'''
|-
|| Slide: About the Spoken Tutorial Project
|| The video at the following link summarises the Spoken Tutorial project.
Please download and watch it.
|-
|| Slide: Spoken Tutorial Workshops
|| We conduct workshops using Spoken Tutorials and give certificates.
Please contact us.
|-
|| Slide: Spoken Tutorial Forum
|| 
* Please post your timed queries in this forum.
|-
|| Slide: FOSSEE Forum
|| 
* Do you have any general/technical questions?
* Please visit the forum given in the link.
|-
|| Slide: FOSSEE Case Study Project
|| 
* The FOSSEE team coordinates solving feasible CFD problems of reasonable complexity using OpenFOAM.
* We give honorarium and certificates to those who do this.
* For more details, please visit these sites.
|-
|| Slide: Spoken Tutorial 
|| The Spoken Tutorial project is supported by MHRD, Govt. of India.
The script for this tutorial is contributed by Padmini.
And this is Swetha Sridhar from IIT Bombay signing off.
Thanks for joining.
|-
|}