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March 11, 2011, 13:01 
"LRR Turbulent Model" Problems

#1 
Senior Member

Dear experts,
By changing KEpsilon to LRR in constants/RASProprties OpenFOAM works fine apparently but the problem is in this model: k and epsilon which is set for inlet has a sharp decrease toward internal domain for example in my case which is a backward facing step it reach from k=7.5 to 0.0001, also same for epsilon. I think it is a reason which it is not possible to reach true answer. Settings are same as pitzDaily tutorial. i used pisoFoam. What is the problem? Any suggestion will be appreciated. Regards. 

March 12, 2011, 04:43 

#2 
Senior Member
Claus Meister
Join Date: Aug 2009
Location: Wiesbaden, Germany
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Hard to day: Did u set the inlet conditions for R correctly, meaning everywhere zero apart from the main diagonal and on the k=... I haven't the conditions formula on me, but you can loop up in Versteeg.


March 12, 2011, 05:46 

#4 
Senior Member
Claus Meister
Join Date: Aug 2009
Location: Wiesbaden, Germany
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Rep Power: 9 
You must set the BC for R correctly:
Compute k=3/2*(U_ref*Ti)^2 For U_ref take the velocity at the inlet, look in "U", set Ti = 0.05 (for other cases you muss adjusted it). If you have concrete value for k than set for R at the inlet: type fixedValue; value uniform (k/2 0 0 k/2 0 k); as you can see the kvalues are divided by 2; one is untouched, this one points in flow direction (here zaxis). Since you integrate a epsilon equation with these, you must set the BC suitable. Somewhere in the user guide or programmer's guide is an entry, you gotta go for it. 

March 12, 2011, 07:55 

#5  
Senior Member

Quote:
Thanks for your suggestion. i have calculated k and epsilon. k=7.5 and epsilon=500. But by seeing incompressible/simpleFoam/pitzdaily tutorial decide to use (0 0 0 0 0 0) for inlet R. By changing inlet R to what you told answers are seemed to be better. Two questions: 1 How can R be calculated to what value you told? i mean if x direction be streamwise or if it be 3D and ... 2 Is T=(a b c d e f) means: a d e d b f e f c or it has another format for symmetric tensors. i searched in user guide and programmers guide but nothing found about it. 

March 12, 2011, 11:36 

#6 
Senior Member
Claus Meister
Join Date: Aug 2009
Location: Wiesbaden, Germany
Posts: 241
Rep Power: 9 
(1) Basically, when you consider a canonical reference frame where the inlet is pointing into the xdirection than you must set:
R=(k 0 0 k/2 0 k/2), i.e. in flow direction k and into the others k/2. The reason why k appears here is due to the relationship trace(R)=2*k (look Versteeg). R is a symmetric tensor; to reduce memory consumption the symmetry property is exploited, i.e. 6 entries instead of 9, so (2) is right! R is called the Reynolds Stress Tensor and appearce by reynolds averaging procedure. My suggestion: Try to get Versteeg. Many times posted here and by simply entering it in google; you find in google books, definitely. Cheers and good luck 

March 12, 2011, 12:31 

#7 
Senior Member

I have Versteeg. thanks for your suggestion.
if trace of reynolds stress tensor should be 2k and if your told R could be true, format of 6 elements tensor instead of 9 elements couldn't be what i told. it should be: T=(a b c d e f) means: a b c b d e c e f Isn't it? 

March 12, 2011, 13:58 

#8 
Senior Member
Claus Meister
Join Date: Aug 2009
Location: Wiesbaden, Germany
Posts: 241
Rep Power: 9 
You got it, foamer!


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