Y plus value

mohan dey · April 20, 2017, 12:03

Hlw Everyone. I need one help. I want to know, how to calculate the Wall Y Plus from CFD-Post, please teach me. I know how to calculate Y plus value and also how to generate Y plus mesh but I want to check, the Y plus value in CFD-Post please help me.

evcelica · April 20, 2017, 15:24

Make a contour plot on the wall of Yplus.

mohan dey · April 20, 2017, 22:13

Quote:

Originally Posted by evcelica

Make a contour plot on the wall of Yplus.

Thank you so much for your reply sir. Is it possible to check the Y plus value for 3D model. My model is boiling tube model.

urosgrivc · April 21, 2017, 01:17

You can only see Y+ value in the 1st mesh element from any of the surfaces you have.

1 You can make a plane, and plot y+ but everithing will be grey exept 1st elements near wals you will se your y+ there.

2 As Evcelica wrote make a contour on the suface of you interest and plot y+ contour

advantage of plane over contour is that you can (render shov mesh lines) and actualy see the thicknes or other properties of your mesh)) i think it is usefull.

AS_Aero · April 27, 2017, 02:57

Hi All

Sorry to ask you, I am not aware of this Y+ calculation at my wall. But I have a complex geometry and when I check the contour plot for a mid plane or wall in CFD-Post I can see the values goes from 0.03 to 23. So now am bit scared if its ok or not. And also the problem is this higher y+ is on some sharp corners and uniform surfaces have around 2 as my y+. But I dint do any y+ calculation when I did the mesh. I just took an average mesh size and resolved the wall with an isolation layer thickness.

ghorrocks · April 27, 2017, 07:17

It is normal for y+ to vary over a range.

So do a sensitivity analysis and see if the mesh you have is OK or not.

AS_Aero · April 27, 2017, 07:59

But then according to the theory the y+ should be 1 right, for k-w model ? And by sensitivity study you mean to try with different meshes right ?

ghorrocks · April 27, 2017, 08:05

Yes, sensitivity study means trying different meshes.

Note that if the larger y+ happens in a region where nothing much happens then it won't matter too much. But you are right, for k-w you will need to keep y+ to around 1 in important areas for good accuracy.

Why are using k-w? SST is a better general choice as it allows automatic wall functions which can handle a wider range of y+. It also has many other benefits and includes many interesting sub-models not available on other 2-equation models (curvature correct, turbulence transition, DES, SAS)

AS_Aero · April 27, 2017, 08:18

Well my geometry has a sharp corner and there comes the y+ around 23 but other regions its 2 or less than that...I believe thats because of the geometry that the inflation layer is not able to keep that spacing constant. I am not sure though. And regarding k-w, as it was proposed by my professor, that k-w is better than k-e although he is not aware of the cfx options.But I was using the k-w RANS value to restart my LES- WALE model , where am using the same mesh.

ghorrocks · April 27, 2017, 18:20

y+ really only applies to where a boundary layer is growing and is important to the model. Boundary layers are not really important (usually) at sharp edges and recirculations.

I would keep an open mind about turbulence models. Even better, try a few of them and see how they go in comparison to each other.

AS_Aero · April 28, 2017, 02:49

Ok Glenn

Thanks a lot for your recommendations.

April 20, 2017, 12:03	Y plus value	#1
mohan dey New Member Mohan Kumar Dey Join Date: Mar 2017 Location: Busan, South Korea Posts: 8 Rep Power: 9	Hlw Everyone. I need one help. I want to know, how to calculate the Wall Y Plus from CFD-Post, please teach me. I know how to calculate Y plus value and also how to generate Y plus mesh but I want to check, the Y plus value in CFD-Post please help me.

April 27, 2017, 08:05		#8
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,728 Rep Power: 143	Yes, sensitivity study means trying different meshes. Note that if the larger y+ happens in a region where nothing much happens then it won't matter too much. But you are right, for k-w you will need to keep y+ to around 1 in important areas for good accuracy. Why are using k-w? SST is a better general choice as it allows automatic wall functions which can handle a wider range of y+. It also has many other benefits and includes many interesting sub-models not available on other 2-equation models (curvature correct, turbulence transition, DES, SAS) ahmad_xjtu likes this.

April 20, 2017, 15:24		#2
evcelica Senior Member Erik Join Date: Feb 2011 Location: Earth (Land portion) Posts: 1,171 Rep Power: 23	Make a contour plot on the wall of Yplus.

April 21, 2017, 01:17		#4
urosgrivc Senior Member urosgrivc Join Date: Dec 2015 Location: Slovenija Posts: 365 Rep Power: 11	You can only see Y+ value in the 1st mesh element from any of the surfaces you have. 1 You can make a plane, and plot y+ but everithing will be grey exept 1st elements near wals you will se your y+ there. 2 As Evcelica wrote make a contour on the suface of you interest and plot y+ contour advantage of plane over contour is that you can (render shov mesh lines) and actualy see the thicknes or other properties of your mesh)) i think it is usefull.

April 27, 2017, 02:57		#5
AS_Aero Senior Member Join Date: Mar 2013 Location: Germany Posts: 357 Rep Power: 14	Hi All Sorry to ask you, I am not aware of this Y+ calculation at my wall. But I have a complex geometry and when I check the contour plot for a mid plane or wall in CFD-Post I can see the values goes from 0.03 to 23. So now am bit scared if its ok or not. And also the problem is this higher y+ is on some sharp corners and uniform surfaces have around 2 as my y+. But I dint do any y+ calculation when I did the mesh. I just took an average mesh size and resolved the wall with an isolation layer thickness.

April 27, 2017, 07:17		#6
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,728 Rep Power: 143	It is normal for y+ to vary over a range. So do a sensitivity analysis and see if the mesh you have is OK or not.

April 27, 2017, 07:59		#7
AS_Aero Senior Member Join Date: Mar 2013 Location: Germany Posts: 357 Rep Power: 14	But then according to the theory the y+ should be 1 right, for k-w model ? And by sensitivity study you mean to try with different meshes right ?

April 27, 2017, 08:18		#9
AS_Aero Senior Member Join Date: Mar 2013 Location: Germany Posts: 357 Rep Power: 14	Well my geometry has a sharp corner and there comes the y+ around 23 but other regions its 2 or less than that...I believe thats because of the geometry that the inflation layer is not able to keep that spacing constant. I am not sure though. And regarding k-w, as it was proposed by my professor, that k-w is better than k-e although he is not aware of the cfx options.But I was using the k-w RANS value to restart my LES- WALE model , where am using the same mesh.

April 27, 2017, 18:20		#10
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,728 Rep Power: 143	y+ really only applies to where a boundary layer is growing and is important to the model. Boundary layers are not really important (usually) at sharp edges and recirculations. I would keep an open mind about turbulence models. Even better, try a few of them and see how they go in comparison to each other.

April 28, 2017, 02:49		#11
AS_Aero Senior Member Join Date: Mar 2013 Location: Germany Posts: 357 Rep Power: 14	Ok Glenn Thanks a lot for your recommendations.