wall y+ value in buffer layer. what to do?

ruzfactor · December 3, 2010, 18:07

I'm using K-epsilon model and I have converged thee solution using all three of the wall functions. But for all three cases the wall y+ falls in the buffer layer( 5<y+<30). It fluctuates between 15-25 for different wall functions. What can I do to improve the wall y+? please suggest.

Jonny6001 · December 3, 2010, 18:51

change the cell size.

ruzfactor · December 3, 2010, 20:10

@jonny6001: Is it possible to change the cell size in fluent? if so how? I didn't create the mesh. I'm using it because it's given to me for my project. Also don't know how create a mesh.

Jonny6001 · December 4, 2010, 05:38

Y+ is proportional to the normal distance of the cell centroid from the wall. To change the Y+ you would need to reduce the cell length perpendicular to the wall.
I have never needed to do this in Fluent so I am not sure if it's possible. There is a nice built in function in Fluent that adapts the mesh based on user inputted criterion but I have never user it for boundary layer meshes. If I can remember, it just splits the cell in half or even a quarter for refinement. Now it might be possible for you to use this if the mesh adaptor splits your boundary cells like you need it to to reduce your Y+.

It is certainly not how I would do it but it might work.

Good luck.

ruzfactor · December 4, 2010, 08:25

I'm posting a y+ curve for you to see whether its a wrong curve or not. There is a mesh refine option in FLUENT but the problem is there are plenty of options to be adapted. which one to choose?

dst · December 8, 2010, 09:47

In general, you can try to live with y+ 5..30
Depending on the simulation, k-e Realizable Scalable wall function can give a good results.

in the command line:
define-mod*-visc*-near*-sc*
(without starts - you just do not need to finish the command... you know)

I have compared this model with some others in a certain range of y+ wider than 5..30 with some empirical solutions (known as correct). And it gave results much closer to theoretical solution in comparison to k-e Realizable Enhanced w.t. and many others two-equation based models.

But... This was one case. I can't be sure about different...

December 3, 2010, 18:07	wall y+ value in buffer layer. what to do?	#1
ruzfactor New Member RUZ Join Date: Oct 2010 Posts: 25 Rep Power: 15	I'm using K-epsilon model and I have converged thee solution using all three of the wall functions. But for all three cases the wall y+ falls in the buffer layer( 5<y+<30). It fluctuates between 15-25 for different wall functions. What can I do to improve the wall y+? please suggest.

Thread Tools	Search this Thread
Show Printable Version Email this Page	Search this Thread: Advanced Search
Display Modes
Linear Mode Switch to Hybrid Mode Switch to Threaded Mode

Similar Threads
Thread	Thread Starter	Forum	Replies	Last Post
How to make boundary layer mesh moving with wall	wayne	FLUENT	3	June 11, 2008 23:23
Question about bcdefw.f for wall temperature bc.	Jimmy	Siemens	10	March 18, 2008 15:28
wall function	Alex	CFX	9	May 19, 2005 10:05
When I use the wall functions....!	maximus	Main CFD Forum	7	January 20, 2003 09:35
atmospheric roughness height	Alain BASTIDE	Siemens	5	December 30, 2002 05:26

December 3, 2010, 18:51		#2
Jonny6001 Member Jonny Join Date: Aug 2009 Posts: 72 Rep Power: 16	change the cell size.

December 3, 2010, 20:10		#3
ruzfactor New Member RUZ Join Date: Oct 2010 Posts: 25 Rep Power: 15	@jonny6001: Is it possible to change the cell size in fluent? if so how? I didn't create the mesh. I'm using it because it's given to me for my project. Also don't know how create a mesh.

December 4, 2010, 05:38		#4
Jonny6001 Member Jonny Join Date: Aug 2009 Posts: 72 Rep Power: 16	Y+ is proportional to the normal distance of the cell centroid from the wall. To change the Y+ you would need to reduce the cell length perpendicular to the wall. I have never needed to do this in Fluent so I am not sure if it's possible. There is a nice built in function in Fluent that adapts the mesh based on user inputted criterion but I have never user it for boundary layer meshes. If I can remember, it just splits the cell in half or even a quarter for refinement. Now it might be possible for you to use this if the mesh adaptor splits your boundary cells like you need it to to reduce your Y+. It is certainly not how I would do it but it might work. Good luck.

December 8, 2010, 09:47		#6
dst New Member Join Date: Oct 2010 Location: Germany Posts: 25 Rep Power: 15	In general, you can try to live with y+ 5..30 Depending on the simulation, k-e Realizable Scalable wall function can give a good results. in the command line: define-mod-visc-near-sc (without starts - you just do not need to finish the command... you know) I have compared this model with some others in a certain range of y+ wider than 5..30 with some empirical solutions (known as correct). And it gave results much closer to theoretical solution in comparison to k-e Realizable Enhanced w.t. and many others two-equation based models. But... This was one case. I can't be sure about different...