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November 16, 2012, 03:34 
y+=1 vs Wall Function

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Does anybody knows how fluent chooses when to use wall function and when to model boundary layer directly when I'm using turbulenxe model KWSST?


November 16, 2012, 06:38 

#2 
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There is no separate option to use wall f/n in kw SST case. Just keep near wall cells fine enough to get y+=1 in FLUENT.


November 16, 2012, 13:57 

#3  
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Also, a bit of a technicality, but there are always wall functions used to model the wall boundary condition. Hence, it's not good practice to say wall function and modelling the boundary layer as you are saying. There is no option but it is there in the background. In other words, you don't have a choice but the wall functions are always there. You can use the kw SST model on a coarse grid and it will automatically "switch" to the loglaw. There actually is no switch, the blended wall function is just a smoothed version of linear and log laws. 

November 18, 2012, 01:19 

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Does it means that when i'm using kwSST model i must avoid meshes with y+ from 2 to 30? And the last. I have modeled experimental data  flow around conical nozzle and the "wall function mesh" y+ near 60 showed better match with test data, than mesh with y+ = 1 

November 18, 2012, 11:57 

#5 
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Sijal Ahmed Memon (turboenginner@gmail.com)
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In older versions there was switch which changes the formulation from integration to wall (ITW) approach to wall function (WF) at Y+ (Rather Y*) = 11.06. Below 11.06 ITW was used and above 11.06 WF was used irrespective the fact that WF were not valid in the buffer zone (from 530) and similarly ITW was used irrespective the fact the viscous sub layer requires Y+ = 1 or atleast Y +< 5.
Now Fluent is using the hybrid wall functions (similar to the apporach in reference give below and technology comes from CFX due to F. Menter) which is combination of ITW and WF. Upto Y+ = 6, it uses the ITW apporach and above Y+ > 30 it uses the Wall function (yes standard wall functions) and in the region between Y + 630 it uses the combination of both. Important points to remember: 1. Keep Y+ below 10 for viscous dominate flows with separation. 2. Use Y+ < 1 for the transition prediction and heat transfer calculations. 3. Use Y+ < 10 for SST model (omega based models in general) 4. Use KEpsilon model with scalable wall functions (default option in new version of Fluent). It will keep the Y+ above 11.06 so that it does not suffer the limitation of old standard wall functions. Moreover now you can carry the successive refinment of mesh, which was not possible with standard WFs. http://num.math.unigoettingen.de/ba...ings/knopp.pdf 

November 18, 2012, 12:01 
Matching the Experimental data is not the goal !

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Sijal Ahmed Memon (turboenginner@gmail.com)
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Moreover CFD is not done to match the experimental data, it can be done in MS Excel . You must ensure the best practices of CFD and there are many source of errors e.g. turbulence modelling, grid, boundary conditions, flow scheme and it is also possible that some source of error is cancelling other factor and you are getting good results. 

November 18, 2012, 22:30 

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November 18, 2012, 22:32 

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November 19, 2012, 02:57 

#9  
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Sijal Ahmed Memon (turboenginner@gmail.com)
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You must also understand that in the log region, you are solving the transformed KEpsilon model and it is good idea to use the original Kepsilon (which is designed for WFs) in combination with scalable wall functions for the reasons mentioned above and in previous post. In my limited experience Kepsilon model with scalable wall function is better choice than the SST model with hybrid functions for the Y+> 30. 

November 19, 2012, 03:15 

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Thanks a lot, I have writed you a privat message, could you answer on it?) 

November 19, 2012, 03:19 

#11  
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If one desired (and it seems paduchev definitely wants to) use a grid with y+ of wall adjacent cell to be >30, one can certainly do so. Will a low Re number grid do better? Probably. But I don't think any of the previous explanations were of any help. 

December 1, 2012, 10:08 

#12  
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Sijal Ahmed Memon (turboenginner@gmail.com)
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Please look at the document by Lars for details of SST model http://www.oscfd.ru/cfd_docs/chalme...5ebe3f5385c13b
And don't confuse the hybrid wall treatment with the hybrid turbulence model like SST model which is combination of Komega in inner and buffer layer. And Kepsilon model in outer and fully turbulent region. The hybrid wall treatment is used to relax the Y+ requirements on the Komega model in SST model. Which requires Y+<2 Moreover SST model was made by mentor in1994 and work on enhanced wall treatment (automatic wall treatment) was published in 20022003. Which means SST model was there and without hybrid wall functions. Now question arises what type of wall treatment was being used before the intorudction of hybrid wall functions. The answer is it is same as required by Komega model. This is also clearly mentioned in CFX theory guide where this model was implemented first by mentor. http://www.kxcad.net/ansys/ansys_cfx.../i1346068.html Quote:
Last edited by Far; December 1, 2012 at 10:24. 

December 2, 2012, 05:34 

#13  
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December 2, 2012, 12:05 

#14 
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François Grégoire
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Here's the complete and possibly fresher version of Lars Davidson's document that Far is referring to in post #12, very nice : http://www.tfd.chalmers.se/~lada/pos...modelling.pdf


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