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January 1, 2012, 11:57 
Yplus ....

#1 
New Member
James I
Join Date: Jan 2012
Posts: 3
Rep Power: 6 
Hi
I am having a question regarding the Y plus estiation and the Boundary layer thickness http://www.cfdonline.com/Tools/yplus.php I want to simulate an Aeropalne on 250 m/sec, i know the freestream velocity which is 250 m.sec and the air density and dynamic visconsity But, what is te boundary layer thickness? does it means the lenght from airplane to the laminar flow? but we o not know that now...!!! Also the desire Y+ how to knwo the correct number??? maybe 10 or 50... i need a small answer so that to be able to make the mesh with the boundaries thanks 

January 2, 2012, 06:04 

#2 
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duri
Join Date: May 2010
Posts: 160
Rep Power: 8 
Y+ need to be selected based on turbulence model and wall function. Refer user manual for model details.


January 2, 2012, 07:23 

#3 
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Peter Jones
Join Date: Jan 2009
Posts: 684
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You need to specify a boundary layer length (not thickness) where you want the y+ value you set to be fullfilled. The length should be based on the physical length of the object you simulate. I'd recommend you to use something like 510% of the physical length of your aircraft.


January 2, 2012, 11:52 

#4  
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James I
Join Date: Jan 2012
Posts: 3
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Quote:
pete, my models has a lenght of 10 meters and each element on the surface has a leaght of 10 to 50 mm, so the leght of these elements descrbes the Y+? how much shallt he first boundary layer thickness should be and how to count the Y+ from the elements thickness and leght? is there any tutorial with pictures to understant of that????? 

January 2, 2012, 13:42 

#5 
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duri
Join Date: May 2010
Posts: 160
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If you are using SA model. Fluent uses either log law or viscous sub layer law. Either go for y+>30 and y+<50 for better results in log law region or go y+<5 for better results in viscous sub layer law. Still confused on selecting y+ values? it on your discretion, compromise on computational resource or accuracy.


January 2, 2012, 14:06 

#6  
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James I
Join Date: Jan 2012
Posts: 3
Rep Power: 6 
Quote:
http://www.cfdonline.com/Tools/yplus.php the boundary layer lenght i suppoce is the surface element lenght of the aeroplane, isn't? wha tis the estimate wall distance??? where do we need of that? thanks for the help , i really need of that 

January 2, 2012, 23:45 

#7 
Senior Member
duri
Join Date: May 2010
Posts: 160
Rep Power: 8 
James, Y+ will not be constant across the boundary layer. For incompressible flow without heat transfer it depends only on the shear stress (for given first cell height). Formula given in the link is for boundary layer on flat plate and this is fair enough for initial mesh (you may need to adapt later on).
Estimate wall distance  first cell height (for first cell y+ input) Boundary layer length  location where do you want to do this calculation from the origin of boundary layer. suppose you want to estimate first cell height at upper TE of airfoil then enter distance between TE and LE along the upper surface (not the chord). Y+ is a turbulence quantity you can't see this in any meshing software. Only way to control y+ is first cell height. 

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