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Old   December 15, 2006, 10:25
Default Y+
  #1
Andres Bernal
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Hi again..how do I interpret the Y+ value pleted over the airfoil..

1) the lower limit is 0.3679 2) the higuer limit is 8.145

I'm working with low reynolds number ('bout 600e3)

Thanks again in advance,

Andres.
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Old   December 15, 2006, 10:39
Default Re: Y+
  #2
Fusion
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The acceptability of Y+ values depends on the used turbulence model, probably you have too high values and the mesh needs to be refined near the airfoil solid boundaries.

Fusion
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Old   December 15, 2006, 16:27
Default Re: Y+
  #3
Andres Bernal
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Thanks for your help Fusion .... I'm Using the SST Turbulence model..

Any Ideas..is it good or definitively wrong?

thanks

Andres.
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Old   December 15, 2006, 21:39
Default Re: Y+
  #4
Johnny
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For SST your max Y+ should be less than 2, and you should have lots of nodes in the boundary layer (15-20). To get this many nodes in the boundary layer, your y+ may need to be even smaller.

SST is a pretty good model for predicting separation, so if you expect this in your flow it sounds like a good choice.
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Old   December 16, 2006, 09:55
Default Re: Y+
  #5
Omer
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I have just one question though, is it really required to get a Y+<2? Because from what I read from the manual I think for automatic wall treatment for SST models , CFX chooses between low-Re model for Y+<2 and the wall function approach for Y+>2, whichever appropriate.

It also says "A Y+< 200 is acceptable if you are using the automatic wall treatment" (Turbulence and Near-Wall Modelling: Modelling Flow Near the Wall,page 128)

and....

Wall-Functions:

Automatic near-wall treatment automatically switches from wall-functions to a low-Re near wall formulation as the mesh is refined. One of the well known deficiencies of the k&epsilon; model is its inability to handle low turbulent Reynolds number computations. Complex damping functions can be added to the k-&epsilon; model, as well as the requirement of highly refined near-wall grid resolution (y+ <0.2) in an attempt to model low turbulent Reynolds number flows. This approach often leads to numerical instability. Some of these difficulties may be avoided by using the k-&omega; model, making it more appropriate than the k-&epsilon; model for flows requiring high near-wall resolution (e.g., high wall heat transfer, transition). However, a strict low-Reynolds number implementation of the model would also require a near wall grid resolution of at least y+ <2. This condition cannot be guaranteed in most applications at all walls. For this reason, a new near wall treatment was developed by CFX for the k-&omega; based models that allows for a smooth shift from a low-Reynolds number form to a wall function formulation. This near wall boundary condition, named automatic near wall treatment in ANSYS CFX, is used as the default in all models based on the &omega;-equation (standard k-&omega;, Baseline k-&omega;, SST, &omega; Reynolds Stress). To take advantage of the reduction in errors offered by the automatic switch to a low-Re near wall formulation, you should attempt to resolve the boundary layer using at least 10 nodes when using these models.

So what do we conclude from here?

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