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November 5, 2010, 05:53 
Reynolds number in 2D simulation

#1 
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Witold
Join Date: Nov 2010
Posts: 13
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First of all I am new here, so I'd like to say "hello" to all users. So: HELLO!!!
I am trying to get familiar with simulationg flows in ANSYS CFX, but my question concern some general physics problem: How do I fdefine the Reynolds number in an 2D case, so I can compare it to the one dafined for a circular pipe? As in CFX it is not possible to make a real 2D run, I at leat need to nodes depth, my idea was to make since my 2D case has a depth. But the front and rear walls are symmetric, so the area is infinite? Or do I just use the hight/length of my grid as the characteristic lenght in the Reynolds number? I used the search but none seems to have a similar problem. So, thank you all for your help! Geetz 

November 5, 2010, 10:37 

#2 
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Andrew
Join Date: Mar 2009
Location: Washington, DC
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I don't think you can use that to define the Reynold's number. Reynolds is the ratio of two forces. You might want to try
Re = (mdot*4)/(mu*pi*D) 

November 5, 2010, 12:35 

#3 
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Witold
Join Date: Nov 2010
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Hey mettler,
thanks for the equation. But am I right that D ist meant to be a diameter? Because that is the problem I am dealing with, I don't habe a circular pipe. The Reequation I gave I found in literature (D. Surek, Angewandte Strömungsmechanik) and in some other books also, so it is not made up by myself. But because the Re is a relation between two forces I must be able to choose a length on which the turbulence is growing. In case of a 2D layer it is the length of the layer an in case of a surrounded cylinder it should be the diameter f the cilinder, or not? 

November 5, 2010, 13:19 

#4 
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Andrew
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will the hydraulic diameter work?


November 5, 2010, 13:55 

#5 
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Witold
Join Date: Nov 2010
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It won't, cause it is 2 dimensional and i don't think I can use the depth of the grid.
But I have the clue I made some mistake in mind. I was trying to make Re equal in every case by choosing the right length. But of course I have to take the length of the layer in case 1 and the diameter of the cylinder in case 2. Than I have to choose the velocity, so both Re are equal. That I am able to compare both flows. I think I try this one. Thanks for your help! 

November 5, 2010, 14:53 

#6 
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Andrew
Join Date: Mar 2009
Location: Washington, DC
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maybe you should state exactly what you are trying to do. Are you trying to do a 2d simulation of flow in a pipe? If you are doing that, and your 2d plane is the centerline, you would use the pipe diameter in the Re number.


November 8, 2010, 04:19 

#7 
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Witold
Join Date: Nov 2010
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I simulate a layer flow of infinite depth. No circular pipe. So there is no diameter. The pic attatched shows my boundries. I will use the xlength as my Relength, because this is the way the turbulence flow accures.


November 10, 2010, 09:15 

#8 
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Witold
Join Date: Nov 2010
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Does anyone know, why ANSYS CFX defines the Reynolds number with [LaTeX Error: Syntax error]?
This leads to very strange values for y+. Greetings. 

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