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September 13, 2000, 17:58 
RNG KE

#1 
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Hi all
It possible for the RNG to by pass transition and it is formulated in such a manner that it can evaluate the Eddy viscosity, molecular viscosity depending on how close you are to the wall right down to through the viscous sublayer, which is fair enough. Fluent will not let you switch off all the wall functions, at least one has to be on. Had anyone similar difficulty with Fluent?. Regards oliver 

September 14, 2000, 02:41 
Re: RNG KE

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The RNG keps model in Fluent is not intended to be used as a lowRe model all down through the viscous sublayer. You should use either their twolayer model which uses a oneequation Wolfsteinmodel in the inner parts or the boundary layer or one of the lowRe models that are avalable through the textinterface for the standard keps model. Some of the lowRe models available in Fluent have been used succesfully to predict bypass transtition. I would recommend either the old LaunderSharma model or the YangShih model. However, predicting bypass transtition with twoequations models is very difficult and in most applications impossible. You really need to validate your simulation approach for your specific application in order to be able to trust your results. For most applications you will predict a too early transition and a transition which incorrectly is independent of the freestream turbulence level. If you want to predict bypass transition you also need to have a very good grid, both in the normal direction to the wall (y+<0.5) and in in the streamwise direction. I haven't done any bypass prediction simulations of this kind in Fluent som I'm not sure if Fluent's numerics are suitable for it  has anyone done this type of predictions with Fluent successfully?


September 14, 2000, 09:52 
Re: RNG KE

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hi Jonas
1 For 2D planar flow, such as flow over a blade, computational restrictions are near nonexistent even for a PC of even low spec. The RNG theory is based on a statistical approach, which uses its own formulation to calculate the effective viscosity right down to the wall. Therefore the implementation of a wall function only serves to corrupt results, by breaking the link in the RNG modeller and replacing it with data obtained from the implementation of a sophisticated wall function which either uses a one equation or 2 equation low Re model.The question now arises what was the model formulated for?. 2 On a similar note an according to your logic the RSM could be used to bypass transition by the implementation of an "appropaite" wall function. Therefore if one ran a RSM and noted a dip in transition which is similar to the WSS obtained experimentally, then the RSM model predicted transition, even though it not formulated to do so. Can you explain this?. 3 Can all wall functions be switched off in fluent?. Regards oliver 

September 14, 2000, 11:28 
Re: RNG KE

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When the original RNG keps model was presented by Yakhot and Orzag people thought that it would be possible to use this model through the viscous sublayer down to wall. However, later experience shows that the RNG keps model also needs special lowRe tuning in order to work well down through the vicous sublayer. I don't think that Fluent has any lowRe RNG keps model implemented, hence you can't use RNG keps down to the wall in Fluent  you have to use a wallfunction or a twolayer model. Fluent does have lowRe std keps models though, which you can use down to the wall without any wallfunction. These models might be able to predict transition.
A wallfunction approach can of course never predict bypass transition since it assumes a turbulent boundary layer. 

September 14, 2000, 12:57 
Re: RNG KE

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Hi Jonas
Thanks for the advice Jonas, It would be nice to see the omega formulation as optional closure models in Fluent. Regards Oliver 

September 15, 2000, 02:36 
Re: RNG KE

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Fluent is working on a UDF which implements the komega model. If you talk to support they might give you a copy of it. If you intend to try to predict bypass transition this might be a good idea  you could implement Wilcox's special transition version of the komega model and perhaps even tune it for your special application.


September 15, 2000, 07:36 
Re: RNG KE

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Hi Jonas
What omega model are they working on. Is it a first order closure turbulence model or is it the more elaborate Wilcox Multi Scale (WMS) turbulence model. Regards Oliver 

September 25, 2000, 09:18 
Re: RNG KE

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Hello Jonas,
just a question referring to your statement concerning the range of use of the RNGmodel. I am a little bit confused by your answer that RNG with its "differential viscosity option" is not suitable for resolving the near wall domain. Because this is just that what FLUENT proposed the RNGmodel to be able when it was introduced some years ago. Hence what for is the "differential viscosity option", unless for getting better results with a resolved boundary layer? 

September 25, 2000, 14:40 
Re: RNG KE

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Hi Volker
As with all models based upon epsilon formulation, the base RNG closure model is inherently weak. It cannot model the physics of the flow properly in the near wall region, therefore a Low Re variant of the RNG model is required which contains fudge factors (i.e. viscous damping functions) to ensure the viscous stress in the near wall region take over from the Reynolds stresses. However due to the strong coupling between all of the fluid flow regions, viscous sublayer, log law etc., which when numerically computed, are computed in tandem. If accurate results are obtained for the outer portion of the Boundary Layer, these results are bound to influence the weaker near wall results, hence overall producing a reasonably accurate "engineering" answer. Therefore it's logical to assume that the accuracy of the solution arises from the "differential viscosity" option. Regards Oliver 

September 17, 2009, 04:44 

#10 
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can i work with ke RNG model ,in a combustion problem?( can combustor of a gasturbine) .
best regards 

September 17, 2009, 06:50 

#11 
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Join Date: Mar 2009
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i see you are necroposter
Yes you can use rngke in a combustion problem. regards 

September 17, 2009, 17:42 

#12 
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thanks very much XDD
best regards 

May 15, 2010, 08:44 

#13 
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Claus Meister
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Hey All,
does anybody know how the eddyviscosity is calculated from RNGke model? Cheers 

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