
[Sponsors] 
May 18, 2017, 20:29 
Graph Wall y+ and "y"

#1 
New Member
Join Date: Jul 2016
Posts: 6
Rep Power: 9 
It seems very nonsense to ask this one but I am little bit confused.
I am analyzing wall y+ from exporting data (paraview) so I am coding the y+ and u* by matlab. HVAC room model is 4*4*1.25 m and I take the data at y = 1.245 m from the wall and x = 1,2,3,4 for example. then from equation to get y+ is y*u_tau/nu. So I wonder what is y? What I calculate is 1.25x??? Which is wrong and I want to use "x" to calculate too. Could you please enlighten me what is y 

May 19, 2017, 01:22 

#2 
Senior Member
Uwe Pilz
Join Date: Feb 2017
Location: Leipzig, Germany
Posts: 744
Rep Power: 15 
y+ is the dimensionless wall distance. If you describe the behavior the velocity near the wall, very different settings lead to similar results if you use y+ instead of y. The velocity has to be normalized too, often it is expressed as a fraction of the free stream velocity, the inlet velocity or an other reference value.
The confusion is, that the y+ tool in openFoam gives the y+ value of the first cell center. But keep in mind, y+ is not a value, but a length measure. It differs from the real y (a coordinate normal to the surface) by a factor which depends on your geometry, and within your geometry, on the place of the wall you want to examine. If you have the y+ output and the "y value" of the first cell center you may calculate this factor and construct your y+ scale. I set "y value" in quotes: Please keep in mind that y in the sense of y+ is a coordinate normal to the surface. In most cases it has nothing to do with the y coordinate of your model if you built it cartesian.
__________________
Uwe Pilz  Die der Hauptbewegung überlagerte Schwankungsbewegung ist in ihren Einzelheiten so hoffnungslos kompliziert, daß ihre theoretische Berechnung aussichtslos erscheint. (Hermann Schlichting, 1950) 

May 19, 2017, 01:41 

#3 
Senior Member
Lucky
Join Date: Apr 2011
Location: Orlando, FL USA
Posts: 5,683
Rep Power: 66 
In the canonical definition of y+, y is the wallnormal distance. But in the canonical setup, there is only one wall and no ambiguity.
In more practical problems with many walls: y is the distance to the nearest wall. This of course leads to interesting questions about what the law of the wall should look in a corner for example when there are two nearby wall. 

May 19, 2017, 03:26 

#4  
Senior Member
Filippo Maria Denaro
Join Date: Jul 2010
Posts: 6,793
Rep Power: 71 
Quote:
It simply does not apply... 

May 19, 2017, 05:31 

#5 
New Member
Join Date: Jul 2016
Posts: 6
Rep Power: 9 
Theoretically, I understand what is y+ and y.
I would like to plot graph y+ and u+ like this and as you can see that I have a problem with coding numerical result. I am guessing that I would wrongly define y. I just want to know what is y in my case. 

May 19, 2017, 07:54 

#6 
Senior Member
Lucky
Join Date: Apr 2011
Location: Orlando, FL USA
Posts: 5,683
Rep Power: 66 
y should just be the distance to the nearest wall. Since I have no information on what your geometry looks like, I can't give you any hints on how to calculate this y.


May 19, 2017, 07:58 

#7 
New Member
Join Date: Jul 2016
Posts: 6
Rep Power: 9 
Here is my geometry.


Tags 
air condition simulation, cfd  post, hvac, paraview 4.2, wall y+ 
Thread Tools  Search this Thread 
Display Modes  

