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November 7, 2012, 11:29 
Uplus vs Yplus

#1 
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CC
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Hi all,
I want represent Uplus as a function of Yplus in Fluent. I simulated the simple case of water flow in a pipe. But, the representation of Uplus as a function of Yplus doesn't agree with literature. What I'm doing wrong? Regards 

November 7, 2012, 14:06 

#2 
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I defined a Custom Field Function for Uplus and another for Yplus... Yplus I compared with the values of Ystar from Fluent and it is OK... For Uplus I used the expression: Uplus=V/(0.09^0.25*turbkineticenergy^0.5). As Yplus agree with the Ystar from fluent, the Uplus probably is not being well calculated, but I'm not sure about that.
I can calculate Yplus and Uplus for all domain or just for the points near the wall? I'm confused 

November 10, 2012, 05:33 

#3 
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hi bud,
a couple of things: 1) remember y+ is a log quantity, so make sure you are plotting on the correct axes ... 2) your first point looks like its at y+ = 5, so to see the laminar sub layer (if that is what you want in addition to the log region, you need to be calculating from Y+ = 1 at least ! cheers Jonathan edit: sorry, didnt see your last question  y+ / y* is only valid for wall adjacent cells ... 

November 14, 2012, 01:55 

#4  
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Lucky Tran
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Quote:
y+ can be calculated for any cell. The y in y+ is taken to be the distance to the nearest wall. It is valid for even non walladjacent cells. Last edited by LuckyTran; November 14, 2012 at 04:52. 

November 14, 2012, 03:50 

#5 
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Philipp
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y+ is the normalized distance to the wall. That's why it can be calculated for every location in the domain. For some reason it is commonly accepted to call the y+ value of the wall adjacent cells just "y+".


November 14, 2012, 07:41 

#6 
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hi Lucky / Rod,
well, yes technically i suppose you guys might be right  i guess it depends how you define distance y. for example, in most CFD contexts, y is defined as the distance between the wall and the centroid / node of the wall adjacent cell, and you will get zero for interior cells. but in a contiuum context if you define y as a distance from the wall to some interior point, yplus is simply a nondimensional distance from a surface. from the tone of the post, i thought it would easier to keep it simple, but i take your guys point ... cheers jonathan 

November 19, 2012, 09:53 

#7 
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I'm using the stanrd wall function in the ke turbulence model...


November 19, 2012, 10:00 

#8 
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I changed the expression that I was used to calculate u+ and y+... u+=V/((tauW/rho)^(1/2)) in which tauW is the value of wall shear stress and now the representation is good


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