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October 31, 2013, 09:26 
pisoFoam cavity case  k & epsilon values

#1 
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RD
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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, 12:22 
No help yet!

#2 
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No help yet!


November 1, 2013, 19:21 

#3  
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Bruno Santos
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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
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November 2, 2013, 12:21 
Thanks for your help Bruno

#4 
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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, 13:04 

#5 
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Well, turbulence modelling is strange that way.
So what happens is this:
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November 2, 2013, 14:35 
Thanks

#6 
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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, 05:50 
Help pls...

#7 
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Somebody please help me on this.


November 4, 2013, 17:22 

#8  
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Bruno Santos
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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/
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Last edited by wyldckat; November 4, 2013 at 17:24. Reason: see "edit:" 

November 5, 2013, 11:34 
Some more queries...

#9  
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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, 16:58 

#10  
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Bruno Santos
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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
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Tags 
cavity, epsilon, pisofoam, turbulence intensity, turbulence length scale 
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