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

k-epsilon turbulence model

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

Like Tree8Likes
  • 6 Post By gwierink
  • 1 Post By gwierink
  • 1 Post By JinBiao

Reply
 
LinkBack Thread Tools Display Modes
Old   January 25, 2010, 12:10
Default k-epsilon turbulence model
  #1
Member
 
James Baker
Join Date: Dec 2009
Posts: 35
Rep Power: 7
fijinx is on a distinguished road
I have looked everywhere I know to look, but can not find what the constants in the k, e, alphat, mut files are actually for. From what I understand, the k is the kinetic energy, e is the dissipation rate, mut is the turbulent viscosity, and alphat has to do with the thermal diffusity. But those are all variable AFAIK. What do the constants represent in these files and how would they be calculated. Thank you for any light on this subject!
fijinx is offline   Reply With Quote

Old   January 27, 2010, 20:33
Default
  #2
Member
 
James Baker
Join Date: Dec 2009
Posts: 35
Rep Power: 7
fijinx is on a distinguished road
Nobody knows?
fijinx is offline   Reply With Quote

Old   January 28, 2010, 04:14
Default
  #3
Member
 
matej forman
Join Date: Mar 2009
Location: Brno, Czech Republic
Posts: 91
Rep Power: 8
matejfor is on a distinguished road
For example for me it is hard to answer as I do not know what constants you're talking about. Do you mean (a) turbulence model constants, (b) wall model constants (Cmu, kappa ...) or (c) some other numbers?


matej
matejfor is offline   Reply With Quote

Old   January 28, 2010, 06:41
Default
  #4
Senior Member
 
Gijsbert Wierink
Join Date: Mar 2009
Posts: 382
Rep Power: 9
gwierink is on a distinguished road
Hi James,

Well, k is the turbulent kinetic energy and e (or epsilon) is the dissipation rate of k. If you are wondering how to calculate basic initial values for a case, you can use 2.1.8.1 of the user guide. There is explained that you can use e.g. 5% turbulent intensity, so u' is the 0.05*U, so k = 1/2 * u' * u'. Epsilon is then C_mu^0.75 * k^1.5 / L, where C_mu is commonly 0.09 and L typically 20% of the characteristic length scale (e.g. pipe diameter). Is this of any help?
__________________
Regards, Gijs
gwierink is offline   Reply With Quote

Old   January 28, 2010, 21:01
Default
  #5
Member
 
James Baker
Join Date: Dec 2009
Posts: 35
Rep Power: 7
fijinx is on a distinguished road
Yes this information helps very much for k and epsilon. What are alphat and mut for?
fijinx is offline   Reply With Quote

Old   January 29, 2010, 00:42
Default
  #6
Member
 
Jinbiao Xiong
Join Date: Oct 2009
Location: China/Japan
Posts: 50
Rep Power: 7
JinBiao is on a distinguished road
Send a message via MSN to JinBiao
Quote:
Originally Posted by fijinx View Post
Yes this information helps very much for k and epsilon. What are alphat and mut for?
I am not sure what version are you using. In my case, I use nut instead of mut. I guess mut is eddy viscosity, and mut = nut*rho. But I have not idea what alphat is.
JinBiao is offline   Reply With Quote

Old   January 29, 2010, 02:23
Default
  #7
Senior Member
 
Gijsbert Wierink
Join Date: Mar 2009
Posts: 382
Rep Power: 9
gwierink is on a distinguished road
Ah, yes, forgot that . "mut" is the turbulent (eddy) viscosity. Basically, turbulence makes the fluid a bit more viscous. "alphat" is, I think, the thermal diffusivity. It probably plays a role in some wall functions etc.
Nikunj.R likes this.
__________________
Regards, Gijs

Last edited by gwierink; January 29, 2010 at 02:47. Reason: typo
gwierink is offline   Reply With Quote

Old   January 29, 2010, 03:19
Default
  #8
Member
 
matej forman
Join Date: Mar 2009
Location: Brno, Czech Republic
Posts: 91
Rep Power: 8
matejfor is on a distinguished road
If you're wondering what some variable means, you have several options where to look.
Most convenient is to use doxygen - documentation of the source codes.
For example you may use the online version at:
http://foam.sourceforge.net/doc/Doxygen/html/
and look for alphat. You will get a list of links. First you'll be lost, but using it will help you to understand better how foam works (at least happened to me).

for example here:
http://foam.sourceforge.net/doc/Doxy...ce.html#l00246
you will find how alphat is calculated for k-eps model.
You even see it in the source code, so you are pretty sure, that it really works like that in your computation. What a beauty!

hope this helps

matej
matejfor is offline   Reply With Quote

Old   January 29, 2010, 03:21
Default
  #9
Member
 
Jinbiao Xiong
Join Date: Oct 2009
Location: China/Japan
Posts: 50
Rep Power: 7
JinBiao is on a distinguished road
Send a message via MSN to JinBiao
Right. If you are considering the heat transfer, there should be an alphat considering the turbulence heat transfer.

Jinbiao
yalilou likes this.
JinBiao is offline   Reply With Quote

Old   May 16, 2015, 11:26
Default problem chosing the RAS turbulence models for compressible fluids
  #10
New Member
 
Join Date: Apr 2015
Posts: 1
Rep Power: 0
Joao Lopes is on a distinguished road
hello, i'm runnig a case on a 9 mm projectil. sould i use in RASpropreties as RAS turbulence models for compressible fluids compressibleRASModels, the kEpsilon model?
Joao Lopes 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
SimpleFoam k and epsilon bounded nedved OpenFOAM Running, Solving & CFD 13 November 4, 2013 15:13
Possible Incompressible Turbulence Model Bugs gocarts OpenFOAM Bugs 12 November 26, 2009 15:58
build your own turbulence model with buoyancy Thomas Baumann OpenFOAM 11 November 23, 2009 09:53
Use of k omega turbulence model john_w OpenFOAM Running, Solving & CFD 2 September 22, 2009 05:15
SimpleFoam k and epsilon bounded nedved OpenFOAM Running, Solving & CFD 1 November 25, 2008 21:21


All times are GMT -4. The time now is 20:17.