CFD Online Logo CFD Online URL
www.cfd-online.com
[Sponsors]
Home > Forums > OpenFOAM Running, Solving & CFD

Running without wall function

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

Reply
 
LinkBack Thread Tools Display Modes
Old   February 12, 2013, 04:38
Exclamation Running without wall function
  #1
Member
 
Simone
Join Date: Sep 2012
Posts: 95
Rep Power: 3
batta31 is on a distinguished road
Hi to everyone! I'd like to run a case without wall function and the Spalart-Allmaras turbulence model. Searching around I didn't really figured out what to do regarding the boundary condition on the solid body for nut and nuTilda. I suppose this setting:

nut:
solidBody
{
type nutLowReWallFunction;
value uniform 0;
}

nuTilda:
solidBody
{
type fixedValue;
value uniform 0;
}

Am I correct or should I use also fixedValue for nut?
Any answer is really, really appreciated!

Cheers
Simone
batta31 is offline   Reply With Quote

Old   February 12, 2013, 05:26
Default
  #2
Member
 
Simone
Join Date: Sep 2012
Posts: 95
Rep Power: 3
batta31 is on a distinguished road
Using a dummy turbulence model I found out the the availlable turbulence models are:

(
LRR
LamBremhorstKE
LaunderGibsonRSTM
LaunderSharmaKE
LienCubicKE
LienCubicKELowRe
LienLeschzinerLowRe
NonlinearKEShih
RNGkEpsilon
SpalartAllmaras
kEpsilon
kOmega
kOmegaSST
kkLOmega
laminar
qZeta
realizableKE
)

So maybe only those two schemes can be used without wall functions? In the sense that I have to use wall function with SpalartAllmaras model?
batta31 is offline   Reply With Quote

Old   February 12, 2013, 16:43
Default
  #3
Senior Member
 
chegdan's Avatar
 
Daniel P. Combest
Join Date: Mar 2009
Location: St. Louis, USA
Posts: 488
Rep Power: 17
chegdan will become famous soon enough
There are other turbulence models that can be used without wall functions, for example LamBremhorst. Similar to high Re turbulence modles, low Re turbulence models requirements for y+ in order to be valid....so more stringent than others. I believe that LaunderSharmaKE has one near y+<=1 and LamBremhorst is about a y+ of 5. You might want to search the forum for the y+ values, and or read through Wilcox's book on turbulence modeling.

Lastly, you do not need use wall functions for the SpalartAllamaral. Some may argue what the BC's for SpalartAllmaras should be, but from a phenomenalogical perspective...zeroGradient works because there is diffusion of turbulent viscosity down through the turbulent boundary layer in course meshes. Ideally, it should be zero once it reaches the wall (if the turbulent boundary layer is assumed to be maintained and if the mesh is fine enough). i think one may run into stability problems with a nuTilda= 1e-20 (don't use zero exactly) at the wall. try the thread low reynolds re turbulence boundary and yPlus values for some more information. Good Luck
chegdan is offline   Reply With Quote

Old   February 13, 2013, 02:22
Default
  #4
Member
 
Simone
Join Date: Sep 2012
Posts: 95
Rep Power: 3
batta31 is on a distinguished road
Hi chegdan and thank you for your answer..
So you are suggesting me to use a setup like:

nut:
solidBody
{
type nutLowReWallFunction;
value uniform 0;
}

nuTilda:
solidBody
{
type fixedValue;
value uniform 1e-20;
}

And for the inlet/outlet and the other patch for the farfiel? Is it correct to use freestream BC? With a value that is a fraction of viscosity?

I read that for nut one should use about 0.1nu and for nuTilda 0.3nu.

Cheers
Simone
batta31 is offline   Reply With Quote

Old   February 14, 2013, 10:22
Default
  #5
Senior Member
 
chegdan's Avatar
 
Daniel P. Combest
Join Date: Mar 2009
Location: St. Louis, USA
Posts: 488
Rep Power: 17
chegdan will become famous soon enough
For nut wall, you can use the Low Re wall function like you have stated. It sets nut to zero and provides access to calculate a y+. For SA model, use waht you have , but you can also try zeroGradient to see if the results are closer to any validation results.

For inlet values, you can set nut to type calculated and then define nutTilda with either what you have or

\tilde{\nu}=\sqrt{\frac{3}{2}}||\vec{U}||Il

where ||\vec{U}|| is the inlet velocity magnitude, I is the turbulent intensity, and l is the turbulent length scale, see here. Making your own SA inlet BC to do this for you is relatively easy, you can derive one from the k or epsilon BCs dealing with defining scalar values based on turbulent intensities.

Last edited by chegdan; February 14, 2013 at 10:23. Reason: more words added
chegdan 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
foamToTecplot360 thomasduerr OpenFOAM Post-Processing 95 June 2, 2014 03:12
Enhanced Wall Treatment paduchev FLUENT 18 April 11, 2014 07:37
Radiation interface hinca CFX 15 January 26, 2014 17:11
channelFoam for a 3D pipe AlmostSurelyRob OpenFOAM 3 June 24, 2011 13:06
Need some wall function approaches! yka8150 Main CFD Forum 0 September 21, 2009 23:08


All times are GMT -4. The time now is 06:20.