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-   -   Uplus vs Yplus (http://www.cfd-online.com/Forums/fluent/109027-uplus-vs-yplus.html)

 C.C November 7, 2012 11:29

Uplus vs Yplus

1 Attachment(s)
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

 C.C November 7, 2012 14:06

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 :(

 Jonathan November 10, 2012 05:33

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 ...

 LuckyTran November 14, 2012 01:55

Quote:
 Originally Posted by C.C (Post 390824) 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 (Post 391370) 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.

 RodriguezFatz November 14, 2012 03:50

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+".

 Jonathan November 14, 2012 07:41

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

 C.C November 19, 2012 09:53

I'm using the stanrd wall function in the k-e turbulence model...

 C.C November 19, 2012 10:00

1 Attachment(s)
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 goodhttp://www.cfd-online.com/Forums/dat...AooooAKKKKAP/Z

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