
[Sponsors] 
October 31, 2013, 10:26 
pisoFoam cavity case  k & epsilon values

#1 
New Member
RD
Join Date: Oct 2013
Location: UK
Posts: 18
Rep Power: 5 
Hi All,
Im a beginner in openFoam. So my question may be very silly to many people here. Please excuse me. In openFOAM tutorials, the 'k' value for pisoFoam cavity case is given as 0.00325 and epsilon=0.000765. Somebody please explain me how they got these values. I have already gone through this. HTML Code:
http://www.cfdonline.com/Wiki/Turbulence_freestream_boundary_conditions What about turbulence length scale? How it is selected here? And which equation to use for finding epsilon? one with cmu or cmu^0.75 ?? Thanks. 

November 1, 2013, 13:22 
No help yet!

#2 
New Member
RD
Join Date: Oct 2013
Location: UK
Posts: 18
Rep Power: 5 
No help yet!


November 1, 2013, 20:21 

#3  
Super Moderator
Bruno Santos
Join Date: Mar 2009
Location: Lisbon, Portugal
Posts: 9,838
Blog Entries: 39
Rep Power: 108 
Greetings Red Devil and welcome to the forum!
Near the beginning of that wiki page: http://www.cfdonline.com/Wiki/Turbu...ary_conditions  is this sentence: Quote:
So the question back to you is this: what Reynolds number and hydraulic diameter did you calculate? Best regards, Bruno
__________________


November 2, 2013, 13:21 
Thanks for your help Bruno

#4 
New Member
RD
Join Date: Oct 2013
Location: UK
Posts: 18
Rep Power: 5 
Thanks a lot for your reply Bruno.
I have already gone through the links you mentioned, but I couldnt understand it completely. I'm just trying to understand the pisoFoam cavity case  /tutorials/incompressible/pisoFoam/ras/cavity. I haven't made any changes to that. Values given are : U = 1 m/s, the dimensions are 1*1*0.1 and nu=1e05. That makes Dh=1 and Re=10^5. Hope I'm right here. Here, k has been given the value 0.00325 and epsilon = 0.000765. k=0.00325 and U =1 m/s ==> I = 0.047. How did they fix this I value? This is what I'm not able to understand. I=0.047, i.e between 1% and 5% implies Mediumturbulence case. Why? Also, when I varies from 1% to 5%, k varies from 0.00015 to 0.00375 for U=1 m/s. Which value to choose here? or how they fixed it as 0.00325? Can we randomly choose any value in the above range? Please explain this to me. Similarly, how to select the turbulent length scale? And after fixing that, which equation to use for calculating epsilon? or ??? Please help me on this also. Thanking you again for your help. 

November 2, 2013, 14:04 

#5 
Super Moderator
Bruno Santos
Join Date: Mar 2009
Location: Lisbon, Portugal
Posts: 9,838
Blog Entries: 39
Rep Power: 108 
Well, turbulence modelling is strange that way.
So what happens is this:
__________________


November 2, 2013, 15:35 
Thanks

#6 
New Member
RD
Join Date: Oct 2013
Location: UK
Posts: 18
Rep Power: 5 
Thanks for the quick reply Bruno.
So all values are randomly chosen! Ok then. Let me consider another example. Flow through a duct, U= 1m/s. dim  0.1*0.1*6 and nu=1e05 ==> Re=10000. Please tell me how to calculate k and epsilon for this case (for better solution/fast covergence). or what is intensity and length here? and which eq. for epsilon? Thanks. 

November 4, 2013, 06:50 
Help pls...

#7 
New Member
RD
Join Date: Oct 2013
Location: UK
Posts: 18
Rep Power: 5 
Somebody please help me on this.


November 4, 2013, 18:22 

#8  
Super Moderator
Bruno Santos
Join Date: Mar 2009
Location: Lisbon, Portugal
Posts: 9,838
Blog Entries: 39
Rep Power: 108 
Hi Red Devil,
Not randomly chosen, but instead you use an initial reference value and work with it from there... Quote:
Quoting from the wiki: http://www.cfdonline.com/Wiki/Turbu...ary_conditions Quote:
Quote:
Quote:
For this "pipe", check the sections "Fully developed pipe flow" on these pages:
Best regards, Bruno edit: Since this is a more basic CFD question, perhaps you can try asking this in the main forum, where the general CFD questions are asked: http://www.cfdonline.com/Forums/main/
__________________
Last edited by wyldckat; November 4, 2013 at 18:24. Reason: see "edit:" 

November 5, 2013, 12:34 
Some more queries...

#9  
New Member
RD
Join Date: Oct 2013
Location: UK
Posts: 18
Rep Power: 5 
Thanks again for your inputs Bruno.
I have some more queries which are more related to OpenFOAM. So I'm posting it here itself. Please find them below. 1). What about epsilon calculation? Which one of the below equations to use in case of OpenFOAM? From wiki: Quote:
Quote:
Below is my k file Quote:
4). What is the use of nuTilda file? Please help me on the above. Thanks. 

November 9, 2013, 17:58 

#10  
Super Moderator
Bruno Santos
Join Date: Mar 2009
Location: Lisbon, Portugal
Posts: 9,838
Blog Entries: 39
Rep Power: 108 
Quote:
Quote:
Quote:
Continue reading the thread for more information. Also calculated by the solver. This has been asked on the forum several times. Here's a thread that should explain it all about it: http://www.cfdonline.com/Forums/ope...tnutilda.html Best regards, Bruno
__________________


Tags 
cavity, epsilon, pisofoam, turbulence intensity, turbulence length scale 
Thread Tools  
Display Modes  


Similar Threads  
Thread  Thread Starter  Forum  Replies  Last Post 
k and epsilon values near the wall  hadishamsnia  Main CFD Forum  0  August 17, 2013 03:18 
Calculation of k and epsilon freezes  Nigirim  OpenFOAM Running, Solving & CFD  1  November 14, 2012 08:52 
k epsilon boundary conditions for pisofoam ras solver  chamoun  OpenFOAM  4  May 10, 2011 05:30 
k din't converge for Lid Driven Cavity case  Adrian  FLUENT  1  October 24, 2008 05:11 
Free surface boudary conditions with SOLAVOF  Fan  Main CFD Forum  10  September 9, 2006 12:24 