# Uplus vs Yplus

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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
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 Uplus vs Yplus.jpg (18.2 KB, 67 views)

 November 7, 2012, 14:06 #2 Member   CC Join Date: Jun 2011 Posts: 52 Rep Power: 6 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*turb-kinetic-energy^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 Senior Member   Join Date: Mar 2010 Location: Cape Town, SA Posts: 153 Rep Power: 8 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:
 Originally Posted by C.C 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
Which wall function approximation are you using?

Quote:
 Originally Posted by Jonathan sorry, didnt see your last question - y+ / y* is only valid for wall adjacent cells ...
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 wall-adjacent cells.

Last edited by LuckyTran; November 14, 2012 at 04:52.

 November 14, 2012, 03:50 #5 Senior Member     Philipp Join Date: Jun 2011 Location: Germany Posts: 1,097 Rep Power: 16 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 Senior Member   Join Date: Mar 2010 Location: Cape Town, SA Posts: 153 Rep Power: 8 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 non-dimensional 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 Member   CC Join Date: Jun 2011 Posts: 52 Rep Power: 6 I'm using the stanrd wall function in the k-e turbulence model...

November 19, 2012, 10:00
#8
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CC
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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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