Y+ for enhanced wall function in K-E model

ahmad_xjtu · May 6, 2019, 09:52

Hello,
I have modeled air flow passed a stationary cylinder. The cylinder dimensions are 18 m in dia and 20 m in height. The flow field is 100*100*100 m. I have achieved desired results and submitted the report. I have been asked about the y+ value and the wall functions. During simulation I opted for enhanced wall functions and chose k-e model. My Y+ values on the receiver wall lie the range 0-500. Are these acceptable?

P.S. mesh is dense near the wall and coarse as it moves towards the flow field boundaries.

LuckyTran · May 6, 2019, 10:58

A wall y+ of 500 is rather large, especially if you want to use enhanced wall functions. The enhanced part of the enhanced wall functions likes to be at a low y+, like <100. 500 is not completely wrong because wall functions are still wall functions, but there is certainly room for a lot of improvement. Even more so since nowadays even CFD newbies can produce meshes with a y+ globally < 0.0001 (a lot of sarcasm here).

ahmad_xjtu · May 6, 2019, 11:14

thanks a lot for the reply.

is there any ideal range? or an exact values range for enhance wall functions in K-e model? like for standard k-e I have read it should be between 3-30 and for SST <1 or so.

I am trying to improve the mesh. unfortunately (or fortunately) I am not a newbie to have such a smooth mesh.

I appreciate your help and taking time to answer my query.

LuckyTran · May 6, 2019, 15:29

The point of the two-layer approach is to switch to the one-equation Wolfstein model near walls and this model doesn't start to kick in until y+ <200. So you want several cells inside this region to truly get the benefit of the two layer approach. If your wall y+ is 100 then you probably only have 1 or 2 cells in this region, etc.

There are other reasons an even smaller y+ is desirable (accuracy, accuracy, accuracy, accuracy!) but this is simply based on the logic behind the two layer approach. The enhanced wall treatment option allows you to go for much coarser meshes and makes it more flexible, it allows you to use grids with y+ > 200. It is better to have a nice mesh, but enhanced wall treatment is your bandaid for this scenario. Your grid could be better, but enhanced wall treatment is designed exactly for this purpose of wide range of y+. You are using the EWT the way it was meant to be used. EWT is the medicine for your sick patient, but a better patient is one who has no illness.

ahmad_xjtu · May 7, 2019, 01:05

thank you. I got the point.
I will try again to have mesh with no illness now.

Thanks again.

May 6, 2019, 09:52	Y+ for enhanced wall function in K-E model	#1
ahmad_xjtu New Member ahmad Join Date: Mar 2018 Posts: 21 Rep Power: 8	Hello, I have modeled air flow passed a stationary cylinder. The cylinder dimensions are 18 m in dia and 20 m in height. The flow field is 100100100 m. I have achieved desired results and submitted the report. I have been asked about the y+ value and the wall functions. During simulation I opted for enhanced wall functions and chose k-e model. My Y+ values on the receiver wall lie the range 0-500. Are these acceptable? P.S. mesh is dense near the wall and coarse as it moves towards the flow field boundaries.

May 6, 2019, 10:58		#2
LuckyTran Senior Member Lucky Join Date: Apr 2011 Location: Orlando, FL USA Posts: 5,762 Rep Power: 66	A wall y+ of 500 is rather large, especially if you want to use enhanced wall functions. The enhanced part of the enhanced wall functions likes to be at a low y+, like <100. 500 is not completely wrong because wall functions are still wall functions, but there is certainly room for a lot of improvement. Even more so since nowadays even CFD newbies can produce meshes with a y+ globally < 0.0001 (a lot of sarcasm here). ahmad_xjtu and racing_student like this.

May 6, 2019, 15:29		#4
LuckyTran Senior Member Lucky Join Date: Apr 2011 Location: Orlando, FL USA Posts: 5,762 Rep Power: 66	The point of the two-layer approach is to switch to the one-equation Wolfstein model near walls and this model doesn't start to kick in until y+ <200. So you want several cells inside this region to truly get the benefit of the two layer approach. If your wall y+ is 100 then you probably only have 1 or 2 cells in this region, etc. There are other reasons an even smaller y+ is desirable (accuracy, accuracy, accuracy, accuracy!) but this is simply based on the logic behind the two layer approach. The enhanced wall treatment option allows you to go for much coarser meshes and makes it more flexible, it allows you to use grids with y+ > 200. It is better to have a nice mesh, but enhanced wall treatment is your bandaid for this scenario. Your grid could be better, but enhanced wall treatment is designed exactly for this purpose of wide range of y+. You are using the EWT the way it was meant to be used. EWT is the medicine for your sick patient, but a better patient is one who has no illness. racing_student likes this.

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May 6, 2019, 11:14		#3
ahmad_xjtu New Member ahmad Join Date: Mar 2018 Posts: 21 Rep Power: 8	thanks a lot for the reply. is there any ideal range? or an exact values range for enhance wall functions in K-e model? like for standard k-e I have read it should be between 3-30 and for SST <1 or so. I am trying to improve the mesh. unfortunately (or fortunately) I am not a newbie to have such a smooth mesh. I appreciate your help and taking time to answer my query.

May 7, 2019, 01:05		#5
ahmad_xjtu New Member ahmad Join Date: Mar 2018 Posts: 21 Rep Power: 8	thank you. I got the point. I will try again to have mesh with no illness now. Thanks again.