CFD Online Logo CFD Online URL
www.cfd-online.com
[Sponsors]
Home > Forums > Software User Forums > ANSYS > FLUENT

Low-Re turbulence Model without wall-functions

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

Reply
 
LinkBack Thread Tools Search this Thread Display Modes
Old   April 21, 2012, 12:56
Default Low-Re turbulence Model without wall-functions
  #1
Member
 
bruce
Join Date: May 2009
Location: Germany
Posts: 42
Rep Power: 16
bruce is on a distinguished road
Dear All,

I am using Realizable k-epsilon low Re. turbulence model for a simple test case with y+~1-3. By default, the standard wall functions are selected in GUI. I want to disable all the wall functions because my grid resolves the boundary layer. I have tried fluent commands to disable by selection, but it seems it could not work as it is getting selected again automatically.

Although, it is better to include wall functions even for Low-Re simulations, I need to disable it for validation purpose.

Does any body help?

Thank You
bruce is offline   Reply With Quote

Old   April 21, 2012, 18:14
Default
  #2
Senior Member
 
Lucky
Join Date: Apr 2011
Location: Orlando, FL USA
Posts: 5,654
Rep Power: 65
LuckyTran has a spectacular aura aboutLuckyTran has a spectacular aura aboutLuckyTran has a spectacular aura about
Quote:
Originally Posted by bruce View Post
Dear All,

I am using Realizable k-epsilon low Re. turbulence model for a simple test case with y+~1-3. By default, the standard wall functions are selected in GUI. I want to disable all the wall functions because my grid resolves the boundary layer. I have tried fluent commands to disable by selection, but it seems it could not work as it is getting selected again automatically.

Although, it is better to include wall functions even for Low-Re simulations, I need to disable it for validation purpose.

Does any body help?

Thank You
hi bruce, if your grid is sufficiently fine then the standard wall function approach should only be resorting to the low-Re treatment. Doesn't that already do what you need it to do? Fluent uses y* ~ 11.225 as a switch between low-Re (linear) and high-Re (log) approximation. If your y* is about the same as y* but since your y+ is <3, y* is also approximately ~<3. Also, the enhanced wall function approach uses blending between the two.

You cannot run a simulation without some type of wall function, how else do you apply your wall boundary conditions?

Last edited by LuckyTran; April 21, 2012 at 18:33.
LuckyTran is offline   Reply With Quote

Old   April 23, 2012, 13:01
Default
  #3
Member
 
bruce
Join Date: May 2009
Location: Germany
Posts: 42
Rep Power: 16
bruce is on a distinguished road
Quote:
Originally Posted by LuckyTran View Post
hi bruce, if your grid is sufficiently fine then the standard wall function approach should only be resorting to the low-Re treatment. Doesn't that already do what you need it to do? Fluent uses y* ~ 11.225 as a switch between low-Re (linear) and high-Re (log) approximation. If your y* is about the same as y* but since your y+ is <3, y* is also approximately ~<3. Also, the enhanced wall function approach uses blending between the two.
hello Lucky, this is fine.

Quote:
Originally Posted by LuckyTran View Post
You cannot run a simulation without some type of wall function, how else do you apply your wall boundary conditions?
What i was looking was a user specified boundary conditions (for all field variables) at the wall instead evaluating from wall functions. In this case, we do not need any wall functions.is that possible?

Thanks
bruce is offline   Reply With Quote

Old   April 23, 2012, 13:44
Default
  #4
Senior Member
 
Lucky
Join Date: Apr 2011
Location: Orlando, FL USA
Posts: 5,654
Rep Power: 65
LuckyTran has a spectacular aura aboutLuckyTran has a spectacular aura aboutLuckyTran has a spectacular aura about
Quote:
Originally Posted by bruce View Post
hello Lucky, this is fine.

What i was looking was a user specified boundary conditions (for all field variables) at the wall instead evaluating from wall functions. In this case, we do not need any wall functions.is that possible?

Thanks
Bruce, a user specified wall boundary condition is in some sense still a wall function. What I mean is that you cannot specify boundary conditions without specify variables at (at least) some cells near the wall, thus making all methods of applying wall boundary conditions essentially wall functions (in some sense). Regardless of how well your grid resolves the boundary layer you still need to apply the boundary conditions this may be a wall or symmetry or other but it is still a boundary condition and has really not much to do with how well resolved your grid is. The boundaries do not care if there is even a grid at all.

You just need to distinguish what types of variables you like to apply at cells near the wall and only change the ones that Fluent does not do exactly the way you want it to do. So ask yourself, exactly what type of wall functions are you trying to implement? And does Fluent already do that or no?
LuckyTran is offline   Reply With Quote

Old   April 26, 2012, 15:37
Default
  #5
Member
 
bruce
Join Date: May 2009
Location: Germany
Posts: 42
Rep Power: 16
bruce is on a distinguished road
Dear Lucky

now i understand better. I have one more question.

As said in the user guide of fluent, is epsilon really computed at the near wall cells using a algebraic relation? I meant the equation no. (10.8-10) in the below link.

http://jullio.pe.kr/fluent6.1/help/html/ug/node451.htm

Because, when i calculate the value of epsilon near the wall i get different results,

for example

Cmu = 0.09
kP = 0.005
kappa = 0.41
yp = 1.92e-05

epsilon = 7.38

But, i found fluent reports it as 0.88. I wonder where does this difference comes from.

Do you have any idea for this?

Thank You
bruce is offline   Reply With Quote

Old   April 26, 2012, 16:00
Default
  #6
Senior Member
 
Lucky
Join Date: Apr 2011
Location: Orlando, FL USA
Posts: 5,654
Rep Power: 65
LuckyTran has a spectacular aura aboutLuckyTran has a spectacular aura aboutLuckyTran has a spectacular aura about
Quote:
Originally Posted by bruce View Post
Dear Lucky

now i understand better. I have one more question.

As said in the user guide of fluent, is epsilon really computed at the near wall cells using a algebraic relation? I meant the equation no. (10.8-10) in the below link.

http://jullio.pe.kr/fluent6.1/help/html/ug/node451.htm

Because, when i calculate the value of epsilon near the wall i get different results,

for example

Cmu = 0.09
kP = 0.005
kappa = 0.41
yp = 1.92e-05

epsilon = 7.38

But, i found fluent reports it as 0.88. I wonder where does this difference comes from.

Do you have any idea for this?

Thank You
Are you using standard wall functions or enhanced wall functions? If enhanced, did you take the blending/damping function into account?

That formula you referenced is correct for the low-Re approach (two-layer approach or standard wall function approach). For enhanced, there is a damping term.

Most importantly, in the realizable-k-e model, Cmu is no longer a constant. Cmu is constant only for the standard k-e model. Did you take the variation of Cmu into account? I noticed you took Cmu as 0.09, this is the constant Cmu from the standard k-e approach.
LuckyTran is offline   Reply With Quote

Old   April 26, 2012, 16:12
Default
  #7
Member
 
bruce
Join Date: May 2009
Location: Germany
Posts: 42
Rep Power: 16
bruce is on a distinguished road
Dear Lucky,

Sorry i forgot to mention that i used standard wall function. Since i had validation problems with enhanced wall function, i change it to standard k-epsilon model with standard wall treatment. And i switched off energy and make it constant density.

And as i calculated before using the relation for epsilon, i do not know why fluent result is 0.88 instead of 7.38 (at one particular cell). Do fluent use any other relation to calculate epsilon near wall? Do you see any problem?

Thank You
bruce is offline   Reply With Quote

Old   April 26, 2012, 16:57
Default
  #8
Senior Member
 
Lucky
Join Date: Apr 2011
Location: Orlando, FL USA
Posts: 5,654
Rep Power: 65
LuckyTran has a spectacular aura aboutLuckyTran has a spectacular aura aboutLuckyTran has a spectacular aura about
Quote:
Originally Posted by bruce View Post
Dear Lucky,

Sorry i forgot to mention that i used standard wall function. Since i had validation problems with enhanced wall function, i change it to standard k-epsilon model with standard wall treatment. And i switched off energy and make it constant density.

And as i calculated before using the relation for epsilon, i do not know why fluent result is 0.88 instead of 7.38 (at one particular cell). Do fluent use any other relation to calculate epsilon near wall? Do you see any problem?

Thank You
I see. In that case it is odd indeed. In that case Cmu of 0.09 is correct. yp is the distance to the nearest wall (m) correct?

Did you also check the calculation of turbulent kinetic energy? It probably is wrong also if dissipation is wrong. What is fluent reporting as the turbulent kinetic energy? While you are at it, also compare the production term, the production term should be approximately the same as the dissipation.

Additionally, there were no convergence problems correct?

There are many other ways to solve for dissipation near the walls but I had thought that those were the equations used by Fluent. I will look into it more.
LuckyTran is offline   Reply With Quote

Old   April 26, 2012, 17:07
Default
  #9
Member
 
bruce
Join Date: May 2009
Location: Germany
Posts: 42
Rep Power: 16
bruce is on a distinguished road
Yes, yP is the cell center to boundary distance as by definition (half of the cell height in my case).

The turbulent kinetic energy calculation is correct indeed. turbulent kinetic energy = Cmu * k^2 / epsilon. When i calculate manually that i get correct value as done by fluent. I will check the production term soon.

And, since it is a flow over a flat plate, the convergence is very fast (1e-07 in 1500 iterations with second order upwind).

Indeed it looks odd.

Thanks
bruce is offline   Reply With Quote

Old   April 26, 2012, 17:10
Default
  #10
Senior Member
 
Lucky
Join Date: Apr 2011
Location: Orlando, FL USA
Posts: 5,654
Rep Power: 65
LuckyTran has a spectacular aura aboutLuckyTran has a spectacular aura aboutLuckyTran has a spectacular aura about
Quote:
Originally Posted by bruce View Post
Yes, yP is the cell center to boundary distance as by definition (half of the cell height in my case).

The turbulent kinetic energy calculation is correct indeed. turbulent kinetic energy = Cmu * k^2 / epsilon. When i calculate manually that i get correct value as done by fluent. I will check the production term soon.

And, since it is a flow over a flat plate, the convergence is very fast (1e-07 in 1500 iterations with second order upwind).

Indeed it looks odd.

Thanks
You can perhaps monitor the dissipation vs iterations just to double check the convergence but I doubt that is the case. Convergence of k and epsilon are usually very fast relative to momentum equation, that is why their under-relaxation factors are set so high.
LuckyTran is offline   Reply With Quote

Old   April 26, 2012, 17:12
Default
  #11
Member
 
bruce
Join Date: May 2009
Location: Germany
Posts: 42
Rep Power: 16
bruce is on a distinguished road
forgot to mention that epsilon was taken as 0.88 what was used by fluent.
bruce is offline   Reply With Quote

Old   April 27, 2012, 19:04
Default
  #12
Member
 
bruce
Join Date: May 2009
Location: Germany
Posts: 42
Rep Power: 16
bruce is on a distinguished road
Hi Lucky,

I was very unlucky. The epsilon relation was indeed correct. The real problem was i always used paraview to visual the fluent results, for some reason data translation went wrong (may be badly interpolated) So i had used wrong kinetic energy to calculate epsilon. Now it is fine.

Thank You
bruce is offline   Reply With Quote

Old   April 27, 2012, 22:04
Default
  #13
Senior Member
 
Lucky
Join Date: Apr 2011
Location: Orlando, FL USA
Posts: 5,654
Rep Power: 65
LuckyTran has a spectacular aura aboutLuckyTran has a spectacular aura aboutLuckyTran has a spectacular aura about
Quote:
Originally Posted by bruce View Post
Hi Lucky,

I was very unlucky. The epsilon relation was indeed correct. The real problem was i always used paraview to visual the fluent results, for some reason data translation went wrong (may be badly interpolated) So i had used wrong kinetic energy to calculate epsilon. Now it is fine.

Thank You
hmm, must be very unlucky indeed. yes paraview is a decent software for visualization. must have made a mistake somewhere during the translation. but all is well!
LuckyTran is offline   Reply With Quote

Reply

Thread Tools Search this Thread
Search this Thread:

Advanced Search
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 Off
Pingbacks are On
Refbacks are On


Similar Threads
Thread Thread Starter Forum Replies Last Post
Wall functions for near wall treatment : k - epsilon model fivos Main CFD Forum 4 April 27, 2011 08:44
low reynolds k omega / wall functions stawrogin OpenFOAM 1 March 30, 2011 07:04
postprocessing model with wall functions?????????? csertoi FLUENT 0 December 17, 2007 12:18
3-D turbulence model with wall function Henry Main CFD Forum 7 May 19, 2006 11:40
Wall functions John Grimm Main CFD Forum 13 February 1, 2002 19:34


All times are GMT -4. The time now is 00:54.