CFD Online Logo CFD Online URL
www.cfd-online.com
[Sponsors]
Home > Forums > Main CFD Forum

Small Y+

Register Blogs Members List Search Today's Posts Mark Forums Read

Reply
 
LinkBack Thread Tools Display Modes
Old   September 18, 2009, 10:26
Default Small Y+
  #1
New Member
 
Shannon Galway
Join Date: Sep 2009
Posts: 4
Rep Power: 6
sgalway is on a distinguished road
Is there a disadvantage to a small Y+? I know for shear stress transport models I am looking for a Y+ less than approximately 2. Is there a disadvantage/error to having this number much smaller than that (like 0.2 for example)?
sgalway is offline   Reply With Quote

Old   September 25, 2009, 09:08
Default
  #2
New Member
 
federico
Join Date: Aug 2009
Posts: 8
Rep Power: 6
aatman is on a distinguished road
Quote:
Originally Posted by sgalway View Post
Is there a disadvantage to a small Y+? I know for shear stress transport models I am looking for a Y+ less than approximately 2. Is there a disadvantage/error to having this number much smaller than that (like 0.2 for example)?
Are you simulating a wall-bounded turbulent flow? if you use DNS or LES your first computational node off the wall must be at y1+ = 1 or even y1+=0.5, but this mean a very large ammount of nodes. With wall function (this means that you are not making DNS anymore) you can put your first node in the log zone, so y1+ > 30, and you can save the number of computational nodes.
aatman is offline   Reply With Quote

Old   September 25, 2009, 13:43
Default
  #3
New Member
 
kumar
Join Date: Sep 2009
Posts: 4
Rep Power: 6
cfd_lean is on a distinguished road
Hi For wall integration grids we need to have y plus around 0.2 to 1. where as with the low y plus values you will see some convergence issues. With the low y plus values there is possibility to predict wrong gradients. That will screw up your complete solution.
cfd_lean is offline   Reply With Quote

Old   September 25, 2009, 22:23
Default
  #4
New Member
 
Shannon Galway
Join Date: Sep 2009
Posts: 4
Rep Power: 6
sgalway is on a distinguished road
Quote:
Originally Posted by aatman View Post
Are you simulating a wall-bounded turbulent flow? if you use DNS or LES your first computational node off the wall must be at y1+ = 1 or even y1+=0.5, but this mean a very large ammount of nodes. With wall function (this means that you are not making DNS anymore) you can put your first node in the log zone, so y1+ > 30, and you can save the number of computational nodes.
I am modeling the leg of an offshore jackup drilling rig. Therefore, this is an open flow around an obstruction.

Quote:
Originally Posted by cfd_lean View Post
Hi For wall integration grids we need to have y plus around 0.2 to 1. where as with the low y plus values you will see some convergence issues. With the low y plus values there is possibility to predict wrong gradients. That will screw up your complete solution.
Can you expand on the "convergence issues". Why would low Y+ values give convergence problems?

Thanks for all the help. I really appreciate it.
sgalway is offline   Reply With Quote

Reply

Thread Tools
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are On
Pingbacks are On
Refbacks are On


Similar Threads
Thread Thread Starter Forum Replies Last Post
why did Time step is too small in stiffbs? satum FLUENT 1 December 10, 2012 09:15
Do small boats slow big boats? Richard Scott Main CFD Forum 13 August 3, 2004 03:51
SGS small scales == fluctuant scales ?? Bin Li Main CFD Forum 6 February 26, 2004 12:35
what happens if y+ is too large or too small Bo Jensen CD-adapco 12 February 19, 2003 23:31
small size holes simulation brian ko Phoenics 5 October 22, 2001 12:53


All times are GMT -4. The time now is 06:50.