k-epsilon turbulence model

fijinx · January 25, 2010, 11:10

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, 19:33

Nobody knows?

matejfor · January 28, 2010, 03:14

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, 05:41

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, 20:01

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

JinBiao · January 28, 2010, 23:42

Quote:

Originally Posted by fijinx

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, 01:23

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, 02:19

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, 02:21

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

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?

January 25, 2010, 11:10	k-epsilon turbulence model	#1
fijinx Member James Baker Join Date: Dec 2009 Posts: 35 Rep Power: 16	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!

January 28, 2010, 05:41		#4
gwierink Senior Member Gijsbert Wierink Join Date: Mar 2009 Posts: 383 Rep Power: 18	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 0.05U, 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? Claudio87, FLZ, sbusmayer and 8 others like this. __________________ Regards, Gijs

January 29, 2010, 01:23		#7
gwierink Senior Member Gijsbert Wierink Join Date: Mar 2009 Posts: 383 Rep Power: 18	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, a_habib, gallon and 1 others like this. __________________ Regards, Gijs Last edited by gwierink; January 29, 2010 at 01:47. Reason: typo

January 29, 2010, 02:19		#8
matejfor Senior Member matej forman Join Date: Mar 2009 Location: Brno, Czech Republic Posts: 182 Rep Power: 17	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 a_habib and Ramzy1990 like this.

January 29, 2010, 02:21		#9
JinBiao Member Jinbiao Xiong Join Date: Oct 2009 Location: China/Japan Posts: 50 Rep Power: 16	Right. If you are considering the heat transfer, there should be an alphat considering the turbulence heat transfer. Jinbiao yalilou likes this.

January 27, 2010, 19:33		#2
fijinx Member James Baker Join Date: Dec 2009 Posts: 35 Rep Power: 16	Nobody knows?

January 28, 2010, 03:14		#3
matejfor Senior Member matej forman Join Date: Mar 2009 Location: Brno, Czech Republic Posts: 182 Rep Power: 17	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

January 28, 2010, 20:01		#5
fijinx Member James Baker Join Date: Dec 2009 Posts: 35 Rep Power: 16	Yes this information helps very much for k and epsilon. What are alphat and mut for?

May 16, 2015, 11:26	problem chosing the RAS turbulence models for compressible fluids	#10
Joao Lopes New Member Join Date: Apr 2015 Posts: 1 Rep Power: 0	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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