Wall Functions & Turbulence Models Query [y plus ranges]

TheMightySpartan · August 14, 2018, 16:37

Hi I'm trying to understand the wall functions and turbulence models and the requirements to satisfy each mathematical model via the mesh.

Please correct me if I'm wrong below.

Standard wall functions y+ range 30 < 300
Enhanced wall treatment, Fluent manual doesn't appear to be clear on this issue, some say < 1 others say <10.
For k-w models y+ < 1 for all.

Thanks.
TheMightySpartan

LuckyTran · August 14, 2018, 16:54

Standard wall functions can certainly be used in the y+<10 range. Standard wall functions have a low reynolds number treatment. What's special about enhanced wall functions, is the enhanced part.

Omega models are designed for low y+, but they can tolerate (they use wall functions) for high y+ meshes. Note that k-w SST model is a blended k-e and k-w model that tolerates high and low y+ meshes.

y+ < 10 or y+ < 1 is a matter of a lot of personal preference and experience. The point is to be low. The demarcation between viscous sublayer <5 versus linear region <10 only confounds the issue even more. If it matters to you, then do a grid sensitivity study.

What's more important than the wall y+ is the total # of grid points you have across the boundary layer and this is not captured by simply plotting y+.

TheMightySpartan · August 14, 2018, 17:02

So I should forget about y+ and do mesh sensitivity study?

LuckyTran · August 15, 2018, 04:57

You should always do a mesh sensitivity study. Especially if you ever plan on publishing anything.

TheMightySpartan · August 18, 2018, 03:48

I understand you are talking about the greater mesh region but there must be some kind of rule book regarding wall functions/turbulence model and y+ right?

August 14, 2018, 16:37	Wall Functions & Turbulence Models Query [y plus ranges]	#1
TheMightySpartan New Member Join Date: Oct 2017 Posts: 25 Rep Power: 9	Hi I'm trying to understand the wall functions and turbulence models and the requirements to satisfy each mathematical model via the mesh. Please correct me if I'm wrong below. Standard wall functions y+ range 30 < 300 Enhanced wall treatment, Fluent manual doesn't appear to be clear on this issue, some say < 1 others say <10. For k-w models y+ < 1 for all. Thanks. TheMightySpartan

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August 14, 2018, 16:54		#2
LuckyTran Senior Member Lucky Join Date: Apr 2011 Location: Orlando, FL USA Posts: 5,761 Rep Power: 66	Standard wall functions can certainly be used in the y+<10 range. Standard wall functions have a low reynolds number treatment. What's special about enhanced wall functions, is the enhanced part. Omega models are designed for low y+, but they can tolerate (they use wall functions) for high y+ meshes. Note that k-w SST model is a blended k-e and k-w model that tolerates high and low y+ meshes. y+ < 10 or y+ < 1 is a matter of a lot of personal preference and experience. The point is to be low. The demarcation between viscous sublayer <5 versus linear region <10 only confounds the issue even more. If it matters to you, then do a grid sensitivity study. What's more important than the wall y+ is the total # of grid points you have across the boundary layer and this is not captured by simply plotting y+.

August 14, 2018, 17:02		#3
TheMightySpartan New Member Join Date: Oct 2017 Posts: 25 Rep Power: 9	So I should forget about y+ and do mesh sensitivity study?

August 15, 2018, 04:57		#4
LuckyTran Senior Member Lucky Join Date: Apr 2011 Location: Orlando, FL USA Posts: 5,761 Rep Power: 66	You should always do a mesh sensitivity study. Especially if you ever plan on publishing anything.

August 18, 2018, 03:48		#5
TheMightySpartan New Member Join Date: Oct 2017 Posts: 25 Rep Power: 9	I understand you are talking about the greater mesh region but there must be some kind of rule book regarding wall functions/turbulence model and y+ right?