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May 30, 2011, 15:32 |
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#21 |
Senior Member
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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. |
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June 22, 2011, 09:27 |
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#23 |
Member
The True
Join Date: Dec 2010
Posts: 80
Rep Power: 15 |
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.
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June 22, 2011, 10:21 |
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#25 |
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The True
Join Date: Dec 2010
Posts: 80
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June 22, 2011, 10:48 |
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#26 |
Senior Member
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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.
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June 22, 2011, 12:33 |
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#27 | |
Member
The True
Join Date: Dec 2010
Posts: 80
Rep Power: 15 |
Quote:
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 ? |
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June 21, 2012, 21:41 |
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#28 |
Senior Member
Mihai Pruna
Join Date: Apr 2010
Location: Boston
Posts: 195
Rep Power: 16 |
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; } |
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January 15, 2013, 11:08 |
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#29 | |
Member
Malik
Join Date: Dec 2012
Location: Austin, USA
Posts: 53
Rep Power: 13 |
Quote:
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 ! |
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January 31, 2013, 13:05 |
Similar Problem
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#30 |
Member
Suranga Dharmarathne
Join Date: Jan 2011
Location: TX, USA
Posts: 39
Rep Power: 15 |
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. Please help me to figure this out. |
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February 1, 2013, 03:58 |
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#31 |
Member
Malik
Join Date: Dec 2012
Location: Austin, USA
Posts: 53
Rep Power: 13 |
I just forgot to tell that I found what was my problem : I has inverted my inlet boundary conditions with my outlet boundary conditions.
However, for your case, it seems like you have fixed a k value at the inlet and put zero gradient at the outlet. Maybe you should try to see if your velocity boundary conditions are OK. For your second question : there is no reason why k would be zero at the wall because the boundary condition is zeroGradient. You use this boundary condition when you work with a wall function (see simpleFoam/pitzDaily tutorial for example). And when you don't use wall function, you can put a value near 0 on the wall. For example you can see the lowRekEpsilon model LaunderSharma tutorials. It does not answer your question but it shows that these boundary conditions are classical. The reason why we use zero gradient when we have a wall function was explained in another thread here : what i understood was that on the first cell the wall function creates a profile of k, espilon, U ... You do not see that because on your simulation there is only one value in one cell. But you should not actually worry about it. This is all I know on the subject and there must be a lot of things I do not understand, so be careful with what i said ! |
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February 1, 2013, 13:38 |
BC for k
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#32 |
Member
Suranga Dharmarathne
Join Date: Jan 2011
Location: TX, USA
Posts: 39
Rep Power: 15 |
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. |
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February 3, 2013, 09:35 |
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#33 |
Member
Malik
Join Date: Dec 2012
Location: Austin, USA
Posts: 53
Rep Power: 13 |
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. |
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August 19, 2013, 05:48 |
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#34 |
Member
Vishal
Join Date: Jul 2013
Posts: 73
Rep Power: 13 |
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. |
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August 20, 2013, 14:42 |
setting of omega BC"s
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#35 |
Member
sonu
Join Date: Jul 2013
Location: delhi
Posts: 92
Rep Power: 13 |
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 ? |
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August 21, 2013, 07:03 |
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#36 |
Senior Member
Ehsan
Join Date: Oct 2012
Location: Iran
Posts: 2,208
Rep Power: 27 |
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.
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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. |
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February 5, 2014, 08:55 |
Boundary Conditions
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#37 |
Member
Mehdi GHOZALI
Join Date: May 2013
Location: Dubai, UAE
Posts: 65
Rep Power: 13 |
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 |
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February 5, 2014, 09:10 |
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#38 |
Senior Member
Mihai Pruna
Join Date: Apr 2010
Location: Boston
Posts: 195
Rep Power: 16 |
Dadou: I can't see the attachments.
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February 5, 2014, 09:16 |
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#39 |
Member
Mehdi GHOZALI
Join Date: May 2013
Location: Dubai, UAE
Posts: 65
Rep Power: 13 |
I Apparently it didn't work. And Now ? |
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February 5, 2014, 09:23 |
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#40 |
Senior Member
Mihai Pruna
Join Date: Apr 2010
Location: Boston
Posts: 195
Rep Power: 16 |
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.
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Tags |
boundary conditions, eps, epsilon, initiallisation, turbulence |
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