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y+=1 or y+>30 in Spalart-Allmaras model

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Old   November 14, 2003, 13:00
Default y+=1 or y+>30 in Spalart-Allmaras model
  #1
Christian
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Hi.

The tutorial "Airfoil" in step 7 says that when using the Spalart-Allmaras model, the y+ is to be in the order of 1 or larger than 30, then the grid is fine. If not, the grid needs modification.

I understand that the y+ need to be in the order of 1. But why larger than 30? Wont the resolution be too coarse? And why is BOTH y+ values ok to choose from and not only one of them? Is there something in the model that states that y+ larger than 1 and smaller than 30 is unacceptable, or is there another thing with this range?

Hope someone can help me.

Cheers Christian
s.m and metmet like this.
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Old   November 14, 2003, 14:07
Default Re: y+=1 or y+>30 in Spalart-Allmaras model
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Jonas Larsson
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When y+ is larger than 30 fluent uses a wall-function approach (which is valid for y+ in the range between 30 and 200 or so). When y+ is below 30 the wall-function approach is not valid. Instead the equations have to be integrated all the way to the wall. However, this requires that the grid is sufficiently fine to resolve the inner parts of the boundary layers (y+ around 1). If the grid is not this fine you run into problems.

Hence, the problems is that there are two types of wall treatment - wall functions valid for y+ between 30 and 200 and low-Re integration down-to the wall which requires y+ around or below 1.

There are hybrid methods which are able to "mix" these two wall approaches and allow you to use any y+ you want. However, these hybrid methods aren't always that robust and they are certainly not as mature as the two original approaches (low-Re or wall-functions).

Btw, the non-equlibrium wall-functions in fluent are surprisingly good if you use a good y+ (between 30 and 200).
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Old   November 18, 2003, 12:47
Default Re: y+=1 or y+>30 in Spalart-Allmaras model
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Christian
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But..... The manual says that the wall law is initiated at y+ smaller than 11.something. Or am I reading it uncorrect?

And another question: The enhanced wall function can combine the enhanced features and the 2 zone model. I read that as no matter the y+, my calc will be correct. And the normal wall function starts at 11.something and if my y+ is above that but smaller than 30, my calc will be uncorrect. Is that correctly understood?

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Old   November 18, 2003, 21:29
Default Re: y+=1 or y+>30 in Spalart-Allmaras model
  #4
Alamgir
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11 is still of the order of 1, 10 to the power 1
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Old   November 19, 2003, 04:17
Default Re: y+=1 or y+>30 in Spalart-Allmaras model
  #5
Christian
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< 11 is still of the order of 1, 10 to the power 1

????

I dont understand your point. According to Jonas the Fluent wall function starts at y+ above 30, not above 10.

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Old   December 7, 2003, 23:27
Default Re: y+=1 or y+>30 in Spalart-Allmaras model
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richard
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hi,christian,i agree with jonas's opinion, and i also read the words about y+ euqals to 11, but i can't remember that clearly. would you please tell me where is the words? i am now also confused about that and the wall function just like you. and another words, did you do the calc test about y+ you mentioned above or you encountered that problem in practice or it is just what you deduced? thanks in advance.
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