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November 28, 2012, 00:27 
Enhanced Wall Treatment

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Enhanced Wall Treatment is using by default for kw SST model.
What is it  Enhanced Wall Treatment? It's a kind of wall function, or some blended condition, that models boundary layer directly on meshes with y+=1 and uses some wall function, when y+>30. If it's so what kind of wall function uses Enhanced Wall Treatment? 

November 28, 2012, 03:03 

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Lucky Tran
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Take for example the wall models for velocity/momentum: In the twolayer approach; if y+ < 10 then the linear law of the wall is used: if y > 10 for the wall adjacent cell then the log law of the wall is used: In the Enhanced Wall Treatment approach, there is no "check" to see if y+ is greater/less than a certain value. The value of u+ is calculated from a single wall model. The enhanced wall function is simply: where is the blending function that allows the two different models to be smoothly blended. In fluent the blending function is Keep in mind there is an additional twolayer approach for the k equation (and ). The equation does not use a twolayer approach. Additional blending is performed for the turbulence quantities (k, ), and that is where the most significant differences are. Last edited by LuckyTran; November 28, 2012 at 04:13. 

November 28, 2012, 03:27 

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So if it's no separation and u+ calculated by single equation in all cases is there specific requirements for grid resolution in a near wall zone for proper using Enhanced Wall Treatment?


November 28, 2012, 04:06 

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Lucky Tran
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No, as long as you are sensible with the grid and have enough cells and your wall adjacent cell is not too far from the wall. The Enhanced Wall Treatment was developed to be flexible and work for all grids. It is one size fits all. But you will get more accurate results with finer grids.


November 28, 2012, 04:11 

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November 28, 2012, 04:38 

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Lucky Tran
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That is only for velocity (u+). You will have to dig up the ones for the turbulence quantities.
The Fluent manual goes into detail the EWT for the kepsilon models but leaves out a lot of details for the kwSST model. But you can find them on the CFDonline wiki section. 

November 28, 2012, 04:50 

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Sören Sander
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Hi paduchev,
you can find some additional information here: http://hpce.iitm.ac.in/website//Manu...ug/node514.htm 

November 28, 2012, 05:03 

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November 28, 2012, 05:03 

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November 28, 2012, 05:21 

#10 
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Sören Sander
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Hmmm, they should be the same, but you have a point. You should use the newest manual of fluent 13, of course.


November 28, 2012, 09:38 

#11  
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Lucky Tran
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Quote:
http://www.cfdonline.com/Wiki/Nearwall_treatment_for_komega_models 

November 28, 2012, 10:39 

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Thanks a lot


December 1, 2012, 05:07 

#13  
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Sijal Ahmed Memon (turboenginner@gmail.com)
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If this is true then they would have used the simple elseif command which was used in the previous version of SA and KOmega model. Still you can observe in the theory guide of Fluent (I am talking about SA model) they are using the integration to wall approach for the Y+< 2 and wall function approach for the Y+>30 and strongly recommend to make the meshes either with Y+< 2 or Y+> 30 so that they can use the either IWT or wall function approach. Although the actual switch is implemented at the intersection of two profiles i.e. 11.225 (previously it was implemented at 11.06 in version 6.3). Now lets discuss the theory behind the enhanced wall treatment for Kepsilon and Komega models. First question comes into mind why two approaches used for the same effect i.e. implementing the smooth transition between the loglaw and viscous sublayer implementation. This is because: 1. Kepsilon models were not designed for the near wall flow, therefore they require the damping functions to simulate the near wall effects. 2. Komega based models were designed originally for the near wall region and therefore does not require the damping functions, hence the hybrid wall functions (blending of near wall and log law function) were implemented directly and same is true for SA model. You can find the details of latest work here for the komega and SA model with hybrid wall functions here http://num.math.unigoettingen.de/ba...ings/knopp.pdf http://num.math.unigoettingen.de/ba...ngs/alrutz.pdf But whether it is two layer approach (Kepsilon) or single model implementation approach (Komega or SA model) the purpose is same i.e. to remove the short comings of the both models. i.e. the low reynolds number is valid for the Y+ < 0.2 (low reynolds number Kepsilon model) or Y+<2 (Komega model, I am not writing low reynolds number komega becuase Komega is originally a low Reynolds number model, so no need to define the Rose) and similarly the Y+ > 30 for high Reynolds number Komega and KEpsilon model. To be continued.... Now consider this http://en.wikipedia.org/wiki/Law_of_the_wall It is clearly written that U+ = Y+ for the Y+ < 5 (you can consider the sublayer up 11.225 but at the Y+ = 12 the error is around 25%) http://en.wikipedia.org/wiki/Law_of_the_wall Log law is for Y+> 30. Buffer zone is Y+ = 5 to Y+ = 30 This is problem area where both models (low Reynolds number and high Reynolds number ) don't work. This is the reason why the hybrid or enhanced wall treatment model was came into existence. Here is the some material from Fluent user guide: Quote:
"The Velocity and Temperature Distribution of OneDimensional Flow with Turbulence Augmentation and Pressure Gradient. Int. J. Heat Mass Transfer, 12:301318, 1969." Put in simple words: 1. With Y+~1 , you are solving the low Reynolds number Kepsilon model 2. In original form Wolfstein model is not applicable for the Y+ > 0.2 3. So to over come this we have to use the hybrid wall functions. 4. Enhanced wall treatment is method to implement the hybrid (or enhanced) wall functions for the varying Y+ in the CFD model. 5. Enhanced wall treatment is not needed to implement hybrid (enhanced) wall functions in komega model because they are already applicable upto viscous sublayer. Now the question is how does the enhanced (hybrid) wall function work. They work like Uplus = (1blending function) * Uplus (of viscous sublayer) + blending function * Uplus (of log law) Blending function = 0 for y+ < 6 Blending function =~ 1 for Y+ > 3040 So for Y+< 6 you have viscous sub layer and you are using the low Reynolds number model for Y+> 30 You are using the log law implement ion (aka wall functions) Between Y+ ~ 6 and 30 one is using the linear some of both profiles according to the relative weightage. For example in reference http://num.math.unigoettingen.de/ba...ings/knopp.pdf Blending functions has the following values (Equation 7 of reference http://num.math.unigoettingen.de/ba...ings/knopp.pdf) Y+ = 1 , BF = 0 Y+ = 10, BF = 0.018 Y+ = 12, BF = 0.038 Y+ = 15, BF = 0.094 Y+ = 20, BF = 0.2922 Y+ = 25, BF = 0.626 Y+ = 27, BF = 0.761 Y+ = 30 , BF = 0.909 Y+ = 35, BF = 0.9929 Y+ = 38, BF = 0.9992 Y+ = 40, BF = 0.9998 Blending function is different for different terms. For example in above example, BF was calculated for U+ and Y+. But which ever function is used the basic theory is same. PS : I have already mentioned in one thread that the enhanced wall treatment is good for the Y+ < 10, because for higher values you have increasing weitage of log law and which is not good at predicting the separation. Last edited by Far; December 1, 2012 at 06:25. 

December 1, 2012, 07:16 

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Enhanced wall treatment is method to implement the enhanced wall function for the two layer model like Kepsilon model. It uses the normal viscous sub layer (Y+= 15) and log law formulae (wall function with Y+ >30). Only difference is that it uses the blending function to make it useful in the region Y+ from 530 (buffer zone). 

December 21, 2012, 15:01 

#16 
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Chris Dorer
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Hi,
i am modelling a turbulent flow in a finned tube. I am very interested in the near wall behaviour and my aim is to get values for pressure drop and the surface heat transfer coefficient. I am using the kw SST model. As far as i understand this model it combines the advantages of the kE model for fully turbulent flow far away from the wall and the kw modell near the wall. As proposed in the fluent 13.0 tutorial 6 for turbulence modelling i set my y+ value to y+=0,25 and i meshed a boundary layer with 15 cells for the inner region flow which is governed by viscous effects. so my questions are: 1. can i enable the "low Re corrction"? the kw model does already solve the flow close to the wall? 2. what does the "viscous heating" mean? thanks 

December 9, 2013, 12:01 

#17  
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Meimei Wang
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Is the Enhanced wall treatment of Fluent use the same idea as the CFX's Automatic wall treatment? Do they have any difference besides the coefficient values?
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Best regards, Meimei 

December 16, 2013, 06:55 

#18  
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Sijal Ahmed Memon (turboenginner@gmail.com)
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In the case of the komega and sa model family we dont need special treatment for the boundary layer modelling as these models are valid upto viscous sublayer. So AWT is the method to ease the restrictions of yplus on the IWT approach with the almost same results and reliability. 

April 11, 2014, 07:37 

#19  
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federico
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Thank you very much for your clarifying posts. I wish the ansys fluent manual was so clear. Can you please justify your assertion (see quote) by providing a reference or something that can be cited within an article? 

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