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September 15, 2006, 02:52 
KatoLaunder model

#1 
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can any body explain was is katolaunder modification.
1. It is use for turbomachinery if yes then why.? 2. What we gain in terms of solution accuracy? 3. Will this take more time than standard turbulence models? 4. what is value of y+ for this model if exits any?. 5. Is this low reynolds number model if yes then why ? 

September 15, 2006, 05:54 
Re: KatoLaunder model

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Did you check out CFDWiki? You can find a basic introduction of the KatoLaunder model here:
http://www.cfdonline.com/Wiki/Kato...r_modification 

September 15, 2006, 15:16 
Re: KatoLaunder model

#3 
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ya i have already refered it. It is very good reference . and after reading i have these question. plz could u answer them and tell me which turbulence model is best for turbomachinery


September 16, 2006, 03:06 
Re: KatoLaunder model

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Okay, here comes the answers:
1. The KatoLaunder modification is often used in turbomachinery applications. It was very common about 1015 years ago. Lately it has been replaced by more modern variants though, I'm thinking of various nonlinear models and realizability constraints that inherently avoid the problems with stagnation regions and regions with strong acceleration or decelleration. 2. You avoid the excessive overproduction of turbulent energy that classical kepsilon models tend to produce in stagnation regions, shock regions and regions with very large normal strain. 3. The KatoLaunder modification usually makes the simulation slightly more stable since it avoids the excessive overproduction of k in problematic regions. 4. Which y+ you need depends on what model that you use as basis before you add the KatoLaunder modification. 5. See question 4. It all depends on which model you use as a basis. The KatoLaunder modification can be used both with a highRe wallfunction simulation and with a lowRe simulation. 

September 16, 2006, 07:04 
Re: KatoLaunder model

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which model is best for cfd of turbomachinry. My advisor always prefer the SA model...??????


September 16, 2006, 08:01 
Re: KatoLaunder model

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The SA model has become very popular in the last few years. The main reason is that it is a fairly simple and robust model which rarely leads to completely unhysical results. People are tired of getting completely unphysical results with the kepsilon models and instead turn to the more simple SA model. However, the SA model is a oneequation model which is not that good at predicting convection and diffusion of turbulent properties. The SA model is often used for cases with fairly low freestream turbulence models, like fans, wings etc. A validated and well tuned twoequation model should be better than the SA model. However, the SA model is always good to start with since it gives you a fairly quick and fairly reliable result before you decide if you need to also test a more advanced model.
I don't think that you can say which turbulence model is best for turbomachinery applications. There are so many different applications  separation prediction, heattransfer, performance prediction, secondary flow predictions, ... In the last few years Menter's SST kw model has become more and more popular, as has the more simple SA model as you suggested. In terms of turbulence modeling for turbomachinery I'd say that one clear trend in the last years has been to start to do transient simulations, perhaps running a DES model, instead. The original DES model is based ona a SA variant, but there are also several other hybrid models that use twoequation modesl like SST komega as a basis. 

September 17, 2006, 10:33 
Re: KatoLaunder model

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right. But SST model requires y0 = 1 which means atleast 1 milion modes for one components in other words for one stage of compressor we need atleast 2 millions nodes to apply SST.
now question is that if we need to only find out performance not the secondary or seprated flow, then is it advisible to use y + = 1 or sst model?? waiting for ur preciuos reply regards sam 

September 17, 2006, 11:33 
Re: KatoLaunder model

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First, both the SST kw model and the SA model are lowRe models and as such they both require y+ to be around 1 for the first cell. However, these models can also be used together with a wallfunction approach at the walls in order to save grid points. Hence, you can't select between these models based on their lowRe capabilities only.
If you are predicting ondesign performance for a fan or compressor using a code and models that you have validated against similar applications you can most likely get fairly good results with a highRe model using wall functions. With todays computers it is not a problem to run a steady state 2million cell simulation though. What is really important is that you have a validated code and model and use it approriately, otherwise both a lowRe and a highRe simulation might produce bad results. My experience is that when you have fairly complex separations and shapes, with wallcurvature etc., you most often get better results with a lowRe simulation, whereas with a simple ondesign simulation of a compressor cascade with no complex shock interactions etc. you might also get good performance from a highRe simulation. 

September 17, 2006, 11:40 
Re: KatoLaunder model

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I am using Fluet and I have used KE RNG and SA. They both produce same results.
Unfortunely i have no test data. so switching to different models is time vast? 

September 17, 2006, 11:46 
Re: KatoLaunder model

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thanks jonas. please tell me more abt kato launder


September 17, 2006, 13:29 
Re: KatoLaunder model

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Nope, it is not a vaste of time to switch models. That you get similar results with different models is one indication that your choice of model is not that important. You can of course get good results with an unvalidated code, but then you of course need to have done all the details right (right mesh density, right boundary conditions, right convergence criteria, right discretization schemes, ...). You also need some form of information that the models and your code you is suitable for the cases you predict.
About Fluent models. I'd only use the RNG version for strongly rotating flows (cyclons etc.). For most turbomachinery application I would prefer either the simpler SA model or the more advanced Realizable kepsilon model or the SST komega model (only well implemented if Fluent versions 6.2 and later though) 

September 17, 2006, 14:05 
Re: KatoLaunder model

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Kato launder is only used with standard kepslon? or with SST as well? Where can I find out results of different turbulence models and thier discussion as applied to typical turbomachinery.
One request with jonas: Can I get geometry of nasa rotor 67 and 35 along with their validation data(i.e test data) 

September 18, 2006, 03:56 
Re: KatoLaunder model

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Yep, you can use the KatoLaunder modification also with the SST komega model. However, this isn't very common since the SST komega model doesn't have as big problems as the standard kepsilon model has in flows with large normal strain. For tricky cases with strong stagnations etc. also the SST komega can produce too much turbulent energy though, and then it might be good to use the KatoLaunder modification. I'd always recommend using another type of limiter though (some realizability contraint from Shih's works or perhaps Durbin's limiter). I think that the KatoLaunder modifcation is too crude and basically turns off the turbulence model outside of the boundary layers.
About good references. There aren't that many complete references. A good classical turbulence modeling book is Wilcox's (see http://www.cfdonline.com/Books/show_book.php?book_id=1 ). Otherwise your best bet to stay on top is to read papers in journals like ASME Journal of Turbomachinery, AIAA Journal etc. About the NASA rotors. There are versions of this data avilable I think. You have to ask the ones who owns the data if you can get a copy. It is a great way to start. Perhaps someone else with more information about this can help you here also. 

September 21, 2006, 11:15 
Re: KatoLaunder model and a query to Jonas

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Jonas,
since I have you on the line, a quick question... in "twoequation turbulence modles for turbine blade heat transfer simulations" you state f_mu = 1  exp ( 3.4/(1 + Rt/50)^2) this gives f_mu = (approx) 1.0 at Rt = 0.0 and f_mu = 0.0 at Rt = infinity... in "an experimental and numerical CFD study of turbulence in tundish container" P. Gardin, M. Brunet, J.F. Domgin, K.Pericleous use... f_mu = exp ( 3.4/(1 + Rt/50)^2) (reversing f_mu values) amongst CFD papers there is probably a 60%/40% split! It seems logical to me that f_mu tends to zero as Rt tends to zero (i.e. you are dampening your dissipation term in the dissipation equation as you approach the wall). So who is right? I am guessing you have repeated a typo probably from the original Launder and Sharma paper? Iain 

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