CFD Online Logo CFD Online URL
www.cfd-online.com
[Sponsors]
Home > Forums > OpenFOAM Pre-Processing

Problem with K omega boundary conditions

Register Blogs Members List Search Today's Posts Mark Forums Read

Reply
 
LinkBack Thread Tools Display Modes
Old   February 19, 2013, 11:59
Default Problem with K omega boundary conditions
  #1
Member
 
Malik
Join Date: Dec 2012
Location: Austin, USA
Posts: 52
Rep Power: 4
malaboss is on a distinguished road
Hi FOAMers !
I am studying some airfoils in 2D and before applying any turbulence model to them I test the model on a cylinder.

I tested the komega model which gave me good results with my drag and my strouhal.

However, when I want to see the k field in the domain, I see a high value at the inlet (the one I fixed) and then a fast decrease.
Do you know why I observe it ? Do you know how to fix this ?




My Reynolds is 1000
Here are my boundary conditions :

inlet
U fixed value 0.015
p zerogradient
k fixedValue 0.00000375
epsilon fixedValue 0.0000000112
nut calculated value 0
omega fixedValue 0.037

outlet
U zeroGradient
p fixedValue 0
k zeroGradient
epsilon zeroGradient
nut calculated value 0
omega zeroGradient

up
U symmetryPlane
p symmetryPlane
k symmetryPlane
epsilon symmetryPlane
nut symmetryPlane
omega symmetryPlane

down
U symmetryPlane
p symmetryPlane
k symmetryPlane
epsilon symmetryPlane
nut symmetryPlane
omega symmetryPlane

cylinder
U fixedvalue 0
p zeroGradient
k kqWallfunction 0.00000375
epsilon epsilonWallFunction 0.0000000112
nut nutkWallfunction 0
omega omegaWallFucntion 0.037

frontAndBack
U empty
p empty
k empty
epsilon empty
nut empty


I enclose a screenshot of the kfield with komega so you can see the problem.


Thanks a lot for your help
Attached Images
File Type: jpg komega.jpg (67.7 KB, 77 views)
malaboss is offline   Reply With Quote

Old   February 21, 2013, 10:52
Default
  #2
Member
 
Malik
Join Date: Dec 2012
Location: Austin, USA
Posts: 52
Rep Power: 4
malaboss is on a distinguished road
Hi all,
since my previous post, I have tried to look at the same case but with komega SST turbulenceModel. I applied the same boundary conditions and found that we had the same jump at the inlet.

However this time we have some example of OpenFoam Cases with kOmega SST turbulence model. For example the motorbike.
Looking at the inlet, we find the same jump (see attached picture). In fact, what bothers me is that the solution depends on the size of the domain.

I don't think we should have this, should we ?

There is something that makes me a little more comfortable with this solution which is that the jump is very small.

One could say me that we fixed the inlet value quite arbitrarily and that there are no reason why the internalfield would have the same value. But should not we have zero Gradient Boudnary Condition instead ?

Thanks for your help
Attached Images
File Type: jpg motorbikekomegasSST.jpg (17.1 KB, 41 views)
malaboss is offline   Reply With Quote

Old   February 28, 2013, 06:18
Default
  #3
Member
 
Roland
Join Date: Mar 2009
Location: Netherlands
Posts: 62
Rep Power: 8
sylvester is on a distinguished road
Hi,

Does reducing turbulent dissipation (epsilon) help?

regards,
Sylvester
sylvester is offline   Reply With Quote

Old   February 28, 2013, 09:43
Default
  #4
Senior Member
 
Olivier
Join Date: Jun 2009
Location: France, grenoble
Posts: 237
Rep Power: 9
olivierG is on a distinguished road
hello,

Your k/epsilon ratio isn't ok.

But why don't you try :
- turbulentIntensityKineticEnergyInlet for k,
- turbulentMixingLengthDissipationRateInlet for epsilon,
- turbuelntMixingLengthFrequencyInlet for omega
?

regards,
olivier
olivierG is offline   Reply With Quote

Old   March 1, 2013, 11:56
Default
  #5
Member
 
Malik
Join Date: Dec 2012
Location: Austin, USA
Posts: 52
Rep Power: 4
malaboss is on a distinguished road
Hi and thanks for your answers.
Actually I choosed my Espilon Value at the inlet with this formula :

epsilon = k^(3/2) * c mu / l
with l, the turbulence length scale. I choosed l = 0,05*D where D is my cylinder's diameter.

As sylvester suggested it, I lowered the epsilon value at the inlet and all disappeared. I divided my previous espilon inlet value by 100 (and my omega by 100)to get the first enclosed result. However the nut Field has significantly increased, which seems quite weird to me (2nd enclosed result)
Sylvester may be right, I should have chosen a turbulent length scale value lower than 5%.



For the boundary conditions, I did not use the turbulent BC simply because I did not know they existed. I just tried to implement it and it required to set the value for k, epsilon and omega, in addition of the turbulent intensity and mixinglength. Furthermore It does not change anything in the solution compared to the case where I only implemented the fixedValue BC.


From what you said and what i just tested I really think that I don't know how to define the turbulent length scale. Do you know some experimental formulas I could use ? (I already have looked for it on google but I found no result for external aerodynamic flows).

Thanks for your help !
Attached Images
File Type: jpg k_lowepsilon.jpg (34.6 KB, 43 views)
File Type: jpg Nut_fixedValue.jpg (38.4 KB, 34 views)
File Type: jpg Nut_lowepsilon.jpg (46.3 KB, 40 views)
malaboss is offline   Reply With Quote

Old   April 18, 2015, 17:27
Default
  #6
New Member
 
bassam djedi
Join Date: Apr 2015
Posts: 2
Rep Power: 0
bassamdjedi is on a distinguished road
I am doing simulation using open-foam on 2D aerofoil. I used komega model and put all boundary conditions and started the simulation. However, at Time= 26 i receive an error, I hope i can get help with this issue.
Thank you
#0 Foam::error:rintStack(Foam::Ostream&) at ??:?
#1 Foam::sigFpe::sigHandler(int) at ??:?
#2 in "/lib/x86_64-linux-gnu/libc.so.6"
#3 Foam::GaussSeidelSmoother::smooth(Foam::word const&, Foam::Field<double>&, Foam::lduMatrix const&, Foam::Field<double> const&, Foam::FieldField<Foam::Field, double> const&, Foam::UPtrList<Foam::lduInterfaceField const> const&, unsigned char, int) at ??:?
#4 Foam::GaussSeidelSmoother::smooth(Foam::Field<doub le>&, Foam::Field<double> const&, unsigned char, int) const at ??:?
#5 Foam::smoothSolver::solve(Foam::Field<double>&, Foam::Field<double> const&, unsigned char) const at ??:?
#6 Foam::fvMatrix<double>::solveSegregated(Foam::dict ionary const&) at ??:?
#7 Foam::fvMatrix<double>::solve(Foam::dictionary const&) at ??:?
#8 Foam::SolverPerformance<double> Foam::solve<double>(Foam::tmp<Foam::fvMatrix<doubl e> > const&) at ??:?
#9 Foam::incompressible::RASModels::kOmega::correct() at ??:?
#10
at ??:?
#11 __libc_start_main in "/lib/x86_64-linux-gnu/libc.so.6"
#12
at ??:?
Floating point exception (core dumped)
bassamdjedi is offline   Reply With Quote

Old   April 19, 2015, 23:25
Default Answer
  #7
Senior Member
 
tareqkh's Avatar
 
Khamlaj
Join Date: Nov 2010
Location: United States
Posts: 116
Rep Power: 6
tareqkh is on a distinguished road
HeyBassam,

Just zip your files up, and I will fix your problem.

Regards,

tareqkh is offline   Reply With Quote

Reply

Thread Tools
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are On
Pingbacks are On
Refbacks are On


Similar Threads
Thread Thread Starter Forum Replies Last Post
Radiation interface hinca CFX 15 January 26, 2014 18:11
ribbed channel / simpleFoam / boundary conditions beeo OpenFOAM Pre-Processing 20 July 17, 2013 08:39
An error has occurred in cfx5solve: volo87 CFX 5 June 14, 2013 17:44
k and omega boundary conditions. A.D.E OpenFOAM 6 October 31, 2011 06:01
Problem with using periodic boundary conditions Sun FLUENT 0 January 14, 2011 10:47


All times are GMT -4. The time now is 05:14.