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-   -   realizable k,e-model (https://www.cfd-online.com/Forums/fluent/27935-realizable-k-e-model.html)

Andreas Boemer October 9, 2000 07:47

realizable k,e-model
 
Hi collegues, what experiences do you have with the realizable k,e-model? I wonder if someone compared its results with those of the RNG model.

Wang October 9, 2000 11:27

Re: realizable k,e-model
 
Hi, I just finished a case with the k-e realizable. The case is a strong swirl flow. Comparison between RNG and realizable show that there are no obvious differences.

Phil October 9, 2000 12:01

Swirl factor
 
Hi I'm using RNG k-e model with swirl factor (default value) for a swirling flow. My question is how to make a good estimation for the swirl factor. Thank you Phil

Wang October 9, 2000 13:06

Re: Swirl factor
 
Could you detail about swirl factor? I choose RNG k-e model with swirl dominated flow. However, the results are different from experimental those. By the way, I have try several other models. I find no good model to compare with experimental data. Do you have good idea?

Phil October 9, 2000 14:56

Re: Swirl factor
 
I use the RNG Model with the default swirl factor. I use the Two layer zonal model but I accept that my grid at the wall adjacent zones is in some parts a little bit to coarse (y+ in the order of 5). I am using a unstructered 3 dim tet-hyb grid that I have generated in Gambit. I think for these swirling flows, in my geometry the swirl is induced by the geometry, need a fine mesh also in the core flow, because the flow fluctuations are very high hence the flow quantities change in these regions as well. My results are in good agreement with the experimental results when I simulate the isothermal flow. Unfortunately the errors are higher when I solve the non-isothermal case with heat transfer between the walls and the fluid. However the right trend is represented by my simulation, so if I compare different geometries of this heat exchanger the comparison between the results are right. So if you want to discuss some details you can also write an e-mail

Regards Phil

Rüdiger Schwarze October 10, 2000 04:06

Re: realizable k,e-model
 
I've investigated the flows in continuous casting tundishes with the k-e-, RLZ-k-e and the RNG-k-e model. I found that there are only small differences between the results of the k-e and RLZ-k-e model, whereas the RNG-k-e model estimates the turbulence of the flow field in a quite different way.

Andreas Boemer October 10, 2000 04:10

Re: realizable k,e-model
 
Interesting. Which calculation is closest to experiments?

Rüdiger Schwarze October 10, 2000 04:26

Re: realizable k,e-model
 
Unfortunately, it isn't possible to state that one models fits best the turbulence in ALL flow situations. However i found, that the RNG-ke-e model estimates the turbulence in complex flows better than the other models.

Andreas Boemer October 10, 2000 04:31

Re: realizable k,e-model
 
Of course not in all situations. Which was the best in the tundish?

Rüdiger Schwarze October 10, 2000 04:35

Re: realizable k,e-model
 
RNG-k-e (in tundishs with flow modifying devices) and k-e (in tundishs without such devices).

Phil October 10, 2000 08:51

Re: realizable k,e-model
 
Have you published something about the different results using k-epsilon and RNG-kepsilon models in complex flows, especially in swirling flows. I have also had good results with RNG-k-epsilon. Regards Phil

Rüdiger Schwarze October 10, 2000 09:08

Re: realizable k,e-model
 
I'm working on a publication.

Wang October 10, 2000 14:04

Re: Swirl factor
 
I also use RNG model. At most cases I use standard wall. Due to a big difference I try to use Two Layer to compare results. However, there is still a big errors. Because of memory problem it is impossible to lead a mesh to less than y+ of 5.

However, in non swirl flow, I test both RNG-Two Layer and standard model. There is a little difference.

I do not know whether the swirl flow leads a bigger difference compared with experimental data or the model leads errors

regards

Trac October 11, 2000 01:33

Re: realizable k,e-model
 
I recently ran some cases comparing various turbulence models for a NACA0012 airfoil, at 4 deg angle of attack. Using standard wall functions, the realizable model compared better with experimental results than the RNG, for both lift and drag. (RSM was better again).

Sung-Eun Kim October 12, 2000 11:04

Re: Swirl factor
 
Hi those who are interested in swirling flows

Although the RNG model tends to improve the results for weakly swirling flows, there's alimit to what it can do, since the RNG model is after all an isotropic eddy viscisoity based model and as such is less than adequate for swirling flows. If you have to deal with swirling flows most of the time, I would consider using the RSTM.

Sung-Eun Kim October 12, 2000 11:08

Re: realizable k,e-model
 
I though I could contribute my two cents.

In general, it's fair to say that the RKE and the RNG model give comparable results. The RNG model has been found to "often" perform better than the RKE for swirling flows. The RKE model works better than the RNG for jets and impinging flows (airfoil or hydrofoil at incidence). I prefer the RKE.

Phil October 12, 2000 13:11

Re: Swirl factor
 
You are right, but the problem with RSM is that you need to solve 7 additional equations which might be not a problem if you really have good computational ressources, but for me it is only possible with isotropic eddy viscosity models. So I have to live with less accurate results! Anyway thank you very much for your help

Phil

Sung-Eun Kim October 12, 2000 14:07

Re: Swirl factor
 
It's totally up to you to decide which turbulnece model to use. It depends on the purpose of you computation, computational resource, time constraints, etc. And that's why we offer a suite of turbulence models in FLUENT.

However, The RSTM is not as expensive as many people think. They think, as you did, in terms of the additional number of equations. In terms of memory requirement in FLUENT, the RSM requires only 20% more (a whole lot is already used for metrics, other solution variables, etc.). In terms of the CPU per iteration, it requires 60 % more at most. My point is that the RSTM is not "prohibitively" expensive as often blindedly quoted in the literature.

Phil October 13, 2000 06:28

I'l try to do it!!
 
Thank you for your advice. But should I start the simulation with my results I got with k-e model?

Regards Phil

Sung-Eun Kim October 13, 2000 09:38

Re: I'l try to do it!!
 
It's one good way of getting the RSTM sooution started. But you may start the RSTM solution from the start with less agressive under-relxation factors (e.g. 0.5 for turbulent quantities). The optimum solution strategy (solver settings, etc.) will depend upon the flow under consideration. But once you start trying the RSTM in FLUENT, you will be able to build your knowhow very quickly. One more thing I'd like to remind you is that you should choose high-order discretization (2nd-order at least) to have full benefits of using second-moment turbulence closure. First-order upwind, for instance, can easily plague your results, no matter how good the turbulence model is. Good luck!


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