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Old   January 1, 2012, 11:57
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James I
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Hi

I am having a question regarding the Y plus estiation and the Boundary layer thickness

http://www.cfd-online.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
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Old   January 2, 2012, 06:04
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Y+ need to be selected based on turbulence model and wall function. Refer user manual for model details.
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Old   January 2, 2012, 07:23
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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 5-10% of the physical length of your aircraft.
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Old   January 2, 2012, 11:52
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Quote:
Originally Posted by duri View Post
Y+ need to be selected based on turbulence model and wall function. Refer user manual for model details.
Duri, whick turbulence models needs Y+ ?? as i saw Spalart-Allmaras no need of that, but basically which turbulence model shall i use for a subsonic aerodynamic flow?



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?????
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Old   January 2, 2012, 13:42
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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.
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Old   January 2, 2012, 14:06
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Quote:
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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.
Duri, I am using Hypermesh, i am making the mesh of the aeroplane with a size of 100mm (my aeroplane is 100 meters long) but i can not see the y plus in hypermesh, i can see calculate the first boundary layer thickness with this equation

http://www.cfd-online.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
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Old   January 2, 2012, 23:45
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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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