# y+ calculation (for no wall function walls) (and its BC)

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 March 4, 2011, 05:59 y+ calculation (for no wall function walls) (and its BC) #1 Senior Member     Join Date: Jan 2010 Posts: 347 Blog Entries: 2 Rep Power: 9 Dear experts, When i use wall function i can see y+ by yPlusRAS and yPlusLES command. it calculate min, max and mean of y+ and write it on terminal. But when y+<5 i don't need wall function and i set wall as zerograDient. but by above commands no y+ would be written on terminal: Code: ```Time = 0.47 Calculating wall distance Writing wall distance to field y Reading field U Reading/calculating face flux field phi Selecting incompressible transport model Newtonian Selecting RAS turbulence model LRR LRRCoeffs { Cmu 0.09; Clrr1 1.8; Clrr2 0.6; C1 1.44; C2 1.92; Cs 0.25; Ceps 0.15; sigmaEps 1.3; couplingFactor 0; } no nutWallFunction patches Writing yPlus to field yPlus End``` in yPlus file it is: Code: ``` lowerWall { type calculated; value uniform 0;``` How can i see y+ now? it may because it is zeroGradient nut, K, epsilon, R so what BC should be used when no need to wall function. U is set value 0 not zeroGradien. Regards

 March 4, 2011, 07:27 #2 Senior Member   Steven van Haren Join Date: Aug 2010 Location: The Netherlands Posts: 149 Rep Power: 8 Use wallGradU and formula 7.24 from pope: tau_wall = rho*nu*(dU/dy)_wall However, this will only work if first y+ is really small, say around 1. You can use this is an wall-resolved LES or DNS.

 March 4, 2011, 07:31 #3 Senior Member   Steven van Haren Join Date: Aug 2010 Location: The Netherlands Posts: 149 Rep Power: 8 sorry, double post Last edited by stevenvanharen; March 4, 2011 at 08:35.

 March 4, 2011, 08:09 #4 Senior Member   Steven van Haren Join Date: Aug 2010 Location: The Netherlands Posts: 149 Rep Power: 8 sorry, double post Last edited by stevenvanharen; March 4, 2011 at 08:35.

March 4, 2011, 09:02
#5
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Quote:
 Originally Posted by stevenvanharen Use wallGradU and formula 7.24 from pope: tau_wall = rho*nu*(dU/dy)_wall However, this will only work if first y+ is really small, say around 1. You can use this is an wall-resolved LES or DNS.
Thanks.

But this formula won't give me y+.

i want see y+ value by yPlusRAS or yPlusLES

 March 4, 2011, 09:12 #6 Senior Member   Steven van Haren Join Date: Aug 2010 Location: The Netherlands Posts: 149 Rep Power: 8 yPlusLES works fine for me. Why does it not work for you?

 March 4, 2011, 10:58 #7 Senior Member     Join Date: Jan 2010 Posts: 347 Blog Entries: 2 Rep Power: 9 yPlusRAS works when the patch setting be nutWallFunction. When it be set as zeroGradient above output is seen.

March 7, 2011, 05:39
#8
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Vesselin Krastev
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Quote:
 Originally Posted by maysmech Thanks. But this formula won't give me y+. i want see y+ value by yPlusRAS or yPlusLES
Not true. The formula is indeed suitable for giving you the y+ value at the wall, as:

y+=(y/nu)*sqrt(tau_wall/rho)

Youll'have simply to know the height y of your first near wall node and then you can calculate y+ wherever you want, by sampling wallGradU or directly wallShearStress (both of these utilities work also without wall functions as bcs)

Regards

V.

 March 7, 2011, 07:17 #9 Senior Member     Join Date: Jan 2010 Posts: 347 Blog Entries: 2 Rep Power: 9 My problem is yPlusRAS dosen't work for me.

March 7, 2011, 08:18
#10
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Vesselin Krastev
Join Date: Jan 2010
Location: University of Tor Vergata, Rome
Posts: 364
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Quote:
 Originally Posted by maysmech How can i see y+ now?
Is this you or not? However, yPlusRAS doesn't work if you don't use a wallfunction for nut...Sorry

V.

 March 7, 2011, 10:01 #11 Senior Member     Join Date: Jan 2010 Posts: 347 Blog Entries: 2 Rep Power: 9 Thanks Vesselin It is strange because yPlusLES works for a LES case without wall function but yPlusRAS doesn't work for a LRR case without wall function.

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