CFD Online Discussion Forums (http://www.cfd-online.com/Forums/)
-   Main CFD Forum (http://www.cfd-online.com/Forums/main/)
-   -   Easy question (http://www.cfd-online.com/Forums/main/7569-easy-question.html)

 Confused May 21, 2004 02:51

Easy question

Hello,

I have flow through a nozzle (circular pipe) and want to calculate Re-number. Is it correct to use the radius of the nozzle as the characteristic length?

In Fluent it seems like I should use the diameter and not the radius (6.2-2). In Malalasekera and Versteeg it is stated that radius is to be used (page 72).

Cheers!

 versi May 21, 2004 10:45

Re: Easy question

Both Radius and Diameter are OK. really simple.

 Confused May 21, 2004 22:26

Re: Easy question

Ok. Correct me if I'm wrong, but the radius is used when you have axial symmetry and the diameter is used otherwise?

 Reynolds May 22, 2004 00:01

Re: Easy question

In Re number you can set even length from your object to Moon, will be no problem. In this case just scale will be changed. If you want to use some empirical formula from literature, you need to know which dimenstion was used during those experiments.

 Confused May 22, 2004 02:14

Re: Easy question

Ok but this is kind of the problem here. As I stated in the first post there are two different values of L used for the same type of empirical formula.

1. Re = U*L/v 2. l = 0.07 L 3. I = 0.16(Re)^(-1/8)

4. k = 1.5(U*I)^2 5. e = Cnu^(3/4)*k^(3/2)/l

Here l is the turbulent length scale and L is the characteristic length. Using the above equations I will get two different BC values for k (and e). One value according to the diameter and one according to the radius....

This might seem all very simple to you, to me it's not. Please tell me what value you should have used in order to calculate the inlet BC for a pipe of radius r and diameter d.