Y+

December 15, 2006, 09:25

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.

December 15, 2006, 09:39

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

December 15, 2006, 15:27

Thanks for your help Fusion .... I'm Using the SST Turbulence model..

Any Ideas..is it good or definitively wrong?

thanks

Andres.

December 15, 2006, 20:39

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.

December 16, 2006, 08:55

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 kε model is its inability to handle low turbulent Reynolds number computations. Complex damping functions can be added to the k-ε 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-ω model, making it more appropriate than the k-ε 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-ω 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 ω-equation (standard k-ω, Baseline k-ω, SST, ω 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?

December 15, 2006, 09:39	Re: Y+	#2
Fusion Guest Posts: n/a	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

December 15, 2006, 15:27	Re: Y+	#3
Andres Bernal Guest Posts: n/a	Thanks for your help Fusion .... I'm Using the SST Turbulence model.. Any Ideas..is it good or definitively wrong? thanks Andres.

December 15, 2006, 20:39	Re: Y+	#4
Johnny Guest Posts: n/a	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.

December 16, 2006, 08:55	Re: Y+	#5
Omer Guest Posts: n/a	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 kε model is its inability to handle low turbulent Reynolds number computations. Complex damping functions can be added to the k-ε 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-ω model, making it more appropriate than the k-ε 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-ω 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 ω-equation (standard k-ω, Baseline k-ω, SST, ω 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?

December 15, 2006, 09:25	Y+	#1
Andres Bernal Guest Posts: n/a	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.