
[Sponsors] 
November 7, 2012, 11:29 
Uplus vs Yplus

#1 
Member
CC
Join Date: Jun 2011
Posts: 55
Rep Power: 7 
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 
Member
CC
Join Date: Jun 2011
Posts: 55
Rep Power: 7 
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 
Senior Member
Join Date: Mar 2010
Location: Cape Town, SA
Posts: 156
Rep Power: 9 
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  
Senior Member
Lucky Tran
Join Date: Apr 2011
Location: Orlando, FL USA
Posts: 1,372
Rep Power: 20 
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 
Senior Member
Philipp
Join Date: Jun 2011
Location: Germany
Posts: 1,297
Rep Power: 19 
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: 156
Rep Power: 9 
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 
Member
CC
Join Date: Jun 2011
Posts: 55
Rep Power: 7 
I'm using the stanrd wall function in the ke turbulence model...


November 19, 2012, 10:00 

#8 
Member
CC
Join Date: Jun 2011
Posts: 55
Rep Power: 7 
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


Thread Tools  
Display Modes  


Similar Threads  
Thread  Thread Starter  Forum  Replies  Last Post 
uplus values  C.C  FLUENT  0  October 31, 2012 07:45 
Compressible yPlus and uPlus for LES  owayz  OpenFOAM Running, Solving & CFD  0  August 13, 2012 19:03 
which yplus (SST)  sanchezz  CFX  17  January 11, 2010 05:45 
Who stole my Yplus?  Chebeba  CFX  4  March 23, 2006 05:18 
heat transfer coefficients with "bad" yplus  Andrew  Main CFD Forum  4  April 8, 1999 04:43 