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 fijinx January 25, 2010 12:10

k-epsilon turbulence model

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 January 27, 2010 20:33

Nobody knows?

 matejfor January 28, 2010 04:14

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

 gwierink January 28, 2010 06:41

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? :)

 fijinx January 28, 2010 21:01

Yes this information helps very much for k and epsilon. What are alphat and mut for?

 JinBiao January 29, 2010 00:42

Quote:
 Originally Posted by fijinx (Post 244151) 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.

 gwierink January 29, 2010 02:23

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.

 matejfor January 29, 2010 03:19

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

 JinBiao January 29, 2010 03:21

Right. If you are considering the heat transfer, there should be an alphat considering the turbulence heat transfer.

Jinbiao

 Joao Lopes May 16, 2015 11:26

problem chosing the RAS turbulence models for compressible fluids

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?

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