# How to set up BCs for k epsilon model

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 May 30, 2011, 14:32 #21 Senior Member     Join Date: Jan 2010 Posts: 347 Blog Entries: 2 Rep Power: 9 Hi Juan, I didn't face your problem before. Hi Eren, I have doubt about using free stream for both U and P. I haven't used this boundary too. Try using fixed value for U and zeroGradient for P and nut if possible in your case. i suggest to use settings same as tutorial settings. Any other suggestions for these two problems will be appreciated.

 May 30, 2011, 14:35 #22 Senior Member     Join Date: Jan 2010 Posts: 347 Blog Entries: 2 Rep Power: 9 Other possible for convergence problem of simpleFoam is running an unsteady case with this steady solver. i had this experience. Switching to pisoFoam solved my problem.

 June 22, 2011, 08:27 #23 Member   The True Join Date: Dec 2010 Posts: 80 Rep Power: 7 How important is the initial values for k and epsilon. I know that it doesn't require exact values but what is maximum variation ? Or is it enough if it converges well with the right yPlus values.

 June 22, 2011, 09:05 #24 Senior Member     Join Date: Jan 2010 Posts: 347 Blog Entries: 2 Rep Power: 9 Initial values are not important and your solution is independent on them but, you should set them exactly in boundaries. SailorLiu likes this.

June 22, 2011, 09:21
#25
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The True
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Quote:
 Originally Posted by maysmech Initial values are not important and your solution is independent on them but, you should set them exactly in boundaries.
This sounds as a contradiction !

 June 22, 2011, 09:48 #26 Senior Member     Join Date: Jan 2010 Posts: 347 Blog Entries: 2 Rep Power: 9 I don't see any contradiction. what is set in 0 folder is not initial condition only. you should set also boundary condition in this folder. if you see e.g case/0/U what is written after "Internal field" is initial value and after "Boundary field" is boundary condition. boundary condition effects solution in next iterations but initial value doesn't. vineet_bhola likes this.

June 22, 2011, 11:33
#27
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The True
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Quote:
 Originally Posted by maysmech I don't see any contradiction. what is set in 0 folder is not initial condition only. you should set also boundary condition in this folder. if you see e.g case/0/U what is written after "Internal field" is initial value and after "Boundary field" is boundary condition. boundary condition effects solution in next iterations but initial value doesn't.

Thank you ! I overlooked that. I have set that correct and now I have almost the same Cl. Only Cd is almost 2 times higher , y+ lies between 14-40 , average 29.5 . This is also not too bad.

Do you know what the cause of much higher Cd is ?

 June 21, 2012, 20:41 #28 Senior Member   Mihai Pruna Join Date: Apr 2010 Location: Boston Posts: 190 Blog Entries: 1 Rep Power: 8 This seems to have worked for me. Internal flow,compressible: SDuctOutlet { type inletOutlet; inletValue uniform 200; value uniform 200; } SDuctInlet { type compressible::turbulentMixingLengthDissipationRate Inlet; mixingLength 0.005; value uniform 200; } vol1face4 { type compressible::epsilonWallFunction; Cmu 0.09; kappa 0.41; E 9.8; value uniform 0; } however...what should I do for freestream conditions parallel to the flow? I have thins and I'm sure it's wrong rightZmax {type compressible::turbulentMixingLengthDissipationRate Inlet; mixingLength 0.005; value uniform 200; }

January 15, 2013, 11:08
#29
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Malik
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Quote:
 Originally Posted by wolle1982 inlet: U: fixed value p: zero gradient k/eps/omega: value outlet: U: zero ngradient p: fixed value (0) k/eps/omega: zero gradient walls: U: fixed value p: zero gradient k/eps/omega: zero gradient
Hi,
I tried to use those boundary conditions for my model.
I use a k epsilon turbulence model, and the simpleFoam (steady) solver.
At the inlet I fixed the value 1.5 for k since my flow's velocity is 10m/s et my turbulent intensity is 10%.

When I look at the k field and the epsilon field, they seem to be far from true as you can see in the picture I enclosed.

I think that this problem is related to the boundary condition at the outlet.
Do you have an idea about what could be the problem ?

Thx for all !
Attached Images
 kfield.png (23.7 KB, 89 views)

January 31, 2013, 13:05
Similar Problem
#30
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Suranga Dharmarathne
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I have a similar type of problem too. Why can not we set k=0 at wall boundaries. According to theory all the velocity fluctuations are zero at the wall boundaries hence k should be 0.

I gave this BC in one of my simulations and couldn't see k=0 at the wall in post processed results. Have attached the figure.

Attached Images
 k.jpg (31.2 KB, 91 views)

 February 1, 2013, 13:38 BC for k #32 Member   Suranga Dharmarathne Join Date: Jan 2011 Location: TX, USA Posts: 39 Rep Power: 7 Thanks for your reply. It seems that I have not expressed my question clearly in the previous post. I am very sorry about that. The boundary conditions are exactly same for U and p as in your post on Jan 15 2013. For k and epsilon I put fixedValue condition at the inlet zeroGradient condition at the outlet And at the walls for k , type fixedValue; value uniform 0; and for epsilon, type zeroGradient; for these conditions I got above diagram for k. You can see that I have explicitly set k=0 at the wall. That is the problem I have. Why doesn't the diagram show k=0. Is it a wrong BC for the k-epsilon model. Please give me a clue. Once again thank you in advance.

 February 3, 2013, 09:35 #33 Member   Malik Join Date: Dec 2012 Location: Austin, USA Posts: 52 Rep Power: 5 Hi, Could you tell me if you use wall function ? Maybe you should try to avoid using wall function for your case as you want to fix a value for k (which means not setting the BC nutWallFunction for nut). Tell me if it helped.

 August 19, 2013, 04:48 #34 Member   Vishal Join Date: Jul 2013 Posts: 73 Rep Power: 5 Hi Julian, Can you please share your files for the elbow tutorial if you still have them? I am also trying to simulate the elbow tutorial with turbulence but not able to get turbulence and everything just looks laminar.

 August 20, 2013, 13:42 setting of omega BC"s #35 Member   sonu Join Date: Jul 2013 Location: delhi Posts: 91 Rep Power: 5 helo I am trying to set up a case using rhopimplefoam solver and the kOmegaSST turbulance model model but I have difficulties defining the boundary conditions for omega. in my case i have inlet , axis , upperwall, outlet so hw to define BC's for that ?

 August 21, 2013, 06:03 #36 Senior Member     Ehsan Join Date: Oct 2012 Location: Iran Posts: 2,209 Rep Power: 19 what Eren?you have set both p and U in inlet by values and both zeroGradient in outlet? that's wrong obviously. you should set p be value in outlet and whether p or U be value in inlet and the other be zeroGradient in inlet. k and epsilon seems be correct. __________________ Injustice Anywhere is a Threat for Justice Everywhere.Martin Luther King. To Be or Not To Be,Thats the Question! The Only Stupid Question Is the One that Goes Unasked.

 February 5, 2014, 08:55 Boundary Conditions #37 Member   Mehdi GHOZALI Join Date: May 2013 Location: Dubai, UAE Posts: 65 Rep Power: 5 Hello Everyone, I see that you are quit familiar with all the BCs settings. I have a question for you all about the settings of U,P,k,epsilon and nut. I am trying to model a 3D free surface flow (flow goes in [inlet] and the same flow goes out [outlet]) and I'm having a hard time setting boundary conditions for allthe U,P... . I tried to follow some instructions suggested in this threads but I didn't really find what I need. So (as you can see in the pictures attached) I need to have in the Inlet a flow condition and outlet flow condition too. For the walls (k, Epsilon and nut) I used respectively : - kqRWallFunction uniforme 1; - epsilonWallFunction uniform 200; - nutkWallFunction uniform 0; (I copied the values from a tutorial) I am using a simpleFoam solver with RASModel ---> kEpsilon Can someone help me with th BCs it's the first time that I have to do the settings so I'm a little bit lost Thank you Dadou

 February 5, 2014, 09:10 #38 Senior Member   Mihai Pruna Join Date: Apr 2010 Location: Boston Posts: 190 Blog Entries: 1 Rep Power: 8 Dadou: I can't see the attachments. __________________ Mihai Pruna's Bio

February 5, 2014, 09:16
#39
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Mehdi GHOZALI
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Quote:
 Originally Posted by mihaipruna Dadou: I can't see the attachments.

I Apparently it didn't work.

And Now ?
Attached Images
 Screenshot from 2014-02-04 13_44_02.jpg (45.3 KB, 77 views) Screenshot from 2014-02-04 14_17_22.jpg (81.5 KB, 55 views)

 February 5, 2014, 09:23 #40 Senior Member   Mihai Pruna Join Date: Apr 2010 Location: Boston Posts: 190 Blog Entries: 1 Rep Power: 8 three things: did you actually type "uniforme" in the input file? try to set your free surface like freestream in the motorbike tutorial. if no typos in the file, I would suggest a K-omega model, look into the forums for definitions and initial conditions settings. __________________ Mihai Pruna's Bio

 Tags boundary conditions, eps, epsilon, initiallisation, turbulence

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