How does Fluent RANS model work?

February 18, 2009, 20:50

I have a very basic question here.

Am i right that Fluent only run on RANS and LES models?

Why is it that the user does not need to input any temporal discretization when he uses a RANS viscous model, say K-epsilon?

Is it because RANS model take the average of the different flow components at steady state?

If so, am i right that one can only use LES to visualise unsteady flows?

February 18, 2009, 20:57

Hi,

You are right, RANS is time averaged so not really suitable for time-dependent problems. LES is the best in Fluent for this but you need ultra fine grids for complex 3D problems.

A compromise is to use DES or you can use URANS which is unsteady-RANS. This somehow uses RANS as unsteady and I think you can do this in the unsteady solver but I have only used RANS to date so not sure how it works.

Carlos.

February 18, 2009, 21:00

Thanks for the reply!

How do i use URANS?

February 18, 2009, 21:09

Not sure, think you select RANS model as normal then switch the solver from steady to unsteady. Thats all I know.

Carlos.

February 18, 2009, 21:10

I get it! Thanks dude

February 18, 2009, 21:21

No problem, good luck....

Ganesan Sethu · August 22, 2009, 15:08

hai,
is this possible the molar concentration value is similar to laminar flow in unsteady flow? and is this needed to run turbulent model before to run LES model?

February 18, 2009, 20:50	How does Fluent RANS model work?	#1
Tang Jingwei Guest Posts: n/a	I have a very basic question here. Am i right that Fluent only run on RANS and LES models? Why is it that the user does not need to input any temporal discretization when he uses a RANS viscous model, say K-epsilon? Is it because RANS model take the average of the different flow components at steady state? If so, am i right that one can only use LES to visualise unsteady flows?

February 18, 2009, 20:57	Re: How does Fluent RANS model work?	#2
Carlos Guest Posts: n/a	Hi, You are right, RANS is time averaged so not really suitable for time-dependent problems. LES is the best in Fluent for this but you need ultra fine grids for complex 3D problems. A compromise is to use DES or you can use URANS which is unsteady-RANS. This somehow uses RANS as unsteady and I think you can do this in the unsteady solver but I have only used RANS to date so not sure how it works. Carlos.

February 18, 2009, 21:00	Re: How does Fluent RANS model work?	#3
Tang Jingwei Guest Posts: n/a	Thanks for the reply! How do i use URANS?

February 18, 2009, 21:09	Re: How does Fluent RANS model work?	#4
Carlos Guest Posts: n/a	Not sure, think you select RANS model as normal then switch the solver from steady to unsteady. Thats all I know. Carlos.

February 18, 2009, 21:10	Re: How does Fluent RANS model work?	#5
Tang Jingwei Guest Posts: n/a	I get it! Thanks dude

February 18, 2009, 21:21	Re: How does Fluent RANS model work?	#6
Carlos Guest Posts: n/a	No problem, good luck....

August 22, 2009, 15:08	RNG k-epsilon model	#7
Ganesan Sethu New Member Ganesan Join Date: Aug 2009 Posts: 1 Rep Power: 0	hai, is this possible the molar concentration value is similar to laminar flow in unsteady flow? and is this needed to run turbulent model before to run LES model?

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