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Wotild November 5, 2010 05:53

Reynolds number in 2D simulation
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 Re = \frac{4 \cdot A}{\text{perimeter}} 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!


mettler November 5, 2010 10:37

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)

Wotild November 5, 2010 12:35

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 Re-equation 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?

mettler November 5, 2010 13:19

will the hydraulic diameter work?

Wotild November 5, 2010 13:55

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!

mettler November 5, 2010 14:53

maybe you should state exactly what you are trying to do. Are you trying to do a 2-d simulation of flow in a pipe? If you are doing that, and your 2-d plane is the centerline, you would use the pipe diameter in the Re number.

Wotild November 8, 2010 04:19

1 Attachment(s)
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 x-length as my Re-length, because this is the way the turbulence flow accures.

Wotild November 10, 2010 09:15

Does anyone know, why ANSYS CFX defines the Reynolds number with [LaTeX Error: Syntax error]?
This leads to very strange values for y+.


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