# Graph Wall y+ and "y"

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May 18, 2017, 20:29
Graph Wall y+ and "y"
#1
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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 1.25-x??? Which is wrong and I want to use "x" to calculate too.
Could you please enlighten me what is y
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 Screen Shot 2017-05-19 at 02.23.12.png (5.0 KB, 8 views)

 May 19, 2017, 01:22 #2 Senior Member     Uwe Pilz Join Date: Feb 2017 Location: Leipzig, Germany Posts: 742 Rep Power: 14 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. LuckyTran likes this. __________________ 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: 5,252 Rep Power: 63 In the canonical definition of y+, y is the wall-normal 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
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Filippo Maria Denaro
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Quote:
 Originally Posted by LuckyTran In the canonical definition of y+, y is the wall-normal 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.

It simply does not apply...

May 19, 2017, 05:31
#5
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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.
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May 19, 2017, 07:54
#6
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Lucky
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Quote:
 Originally Posted by k-thie 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.
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
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Quote:
 Originally Posted by LuckyTran 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.
Here is my geometry.
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 4018.jpg (47.4 KB, 24 views)

 Tags air condition simulation, cfd - post, hvac, paraview 4.2, wall y+