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Required Y+ Range for kOmega Standard and SST 

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March 12, 2020, 15:48 
Required Y+ Range for kOmega Standard and SST

#1 
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Nian
Join Date: Jun 2019
Posts: 15
Rep Power: 6 
Hello,
This type of question might have appeared quite a lot in the forums, but I could not really find a definitive answer for it, so I would greatly appreciate it if anyone could help with the required y+ values for the turbulence models in Fluent: 1. Does komega Standard model use wall function at all? If so, does it mean that the y+ range should be between around 50  500? If not, what kind of range would the y+ value require? 2. As far as I am aware, komega SST does not use wall function and solves viscous layer directly, but some people have said it would require a y+ value of 1, some have said 510, so I was wondering if there might be a more certain answer for it? Thank you! 

March 12, 2020, 15:58 
Range of y+

#2 
Senior Member

As far as Fluent is concerned, each model uses Enhanced Wall Treatment. This is a two layer model and in reality is insensitive. So, it can work with all values up to 500 or so. However, is a low Re model, i.e., it's applicability is more in the region with low Re. So, it is recommended that the should be of the order 1, i.e., between 1 and 10.
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March 12, 2020, 16:05 

#3 
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Nian
Join Date: Jun 2019
Posts: 15
Rep Power: 6 
Quote:
Would the previous one (1500) be applying for komega standard, and latter (110) for komega SST? Also, would you mind me asking what kind of range of Reynolds number would usually be considered as low Re simulation for external flow? Thank you very much! 

March 13, 2020, 04:36 
Turbulence Model and Wall Function

#4 
Senior Member

has got nothing to do with . So it doesn't care how fine or coarse the mesh is. It is the wall treatment. And the wall treatment used by all models in Fluent is EWT. And EWT is insensitive, i.e., it can work with high as well as low values of . However, if you wish to resolve the viscous sublayer, most likely because the objective demands that, you should maintain below 10.
is nothing but Reynolds number based on the mesh size. And when it is of the order 1, this implies that the viscous forces are comparable to inertial forces, i.e., the equations can resolve the flowfield without any special treatment. Therefore, it is supposed to be of order 1. Do note that order 1 does not imply 1, rather values up to 9.
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March 13, 2020, 10:01 

#5 
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Lucky
Join Date: Apr 2011
Location: Orlando, FL USA
Posts: 5,676
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Standard komega does have wall functions that kick in around wall y+ > 100. However, it is against the philosophy of the komega model to be using wall functions in the first place. The wall function is there just to make the komega model usable for coarse meshes. The wall function for omega is also quite garbage and not as robust compared to wall functions in epsilon models.
This wall function also exists in komega SST which blends a kepsilon with a komega solution. But because the blending favors kepsilon far from boundaries and komega near boundaries, the wall function that exists for the omega equation practically has no effect. In either case, the philosophy of using either the komega or komega SST model is to resolve viscous layers directly, so the recommendation is always going to be super low wall y+ when either of these models are involved. What's much more important than achieving the target y+ on only the first grid cell is resolution. You want to pack 10/30/100 cells across the boundary layer, etc. 

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turbulence modelling, yplus 
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