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-   -   why low reynolds number? (http://www.cfd-online.com/Forums/fluent/110484-why-low-reynolds-number.html)

 Lea December 12, 2012 06:13

why low reynolds number?

Hey,

I simulate a water stream through a pipe with 20mm bore. I chose the realizable k-e Model and enabled enhanced-wall -treatment. In boundary conditions I chose the ke model with k=e=1 as turbulence specification. The incomming flow rate is 2 m/s.

I calculated that there must be a turbulent flow with a flow rate of 0,0755m/s so that 2 m/s should bring reynolds numbers over 2300. But the reynoldsnumbers are maximal about 800.

Maybe you can help me and tell me what I do wrong.

Thank you=)

 Far December 12, 2012 07:07

which Reynolds numbers you are talking about?

 Lea December 12, 2012 07:11

The reynolds numbers in the whole pipe

 Far December 12, 2012 07:13

Where you are seeing them?

Reynolds number is a reference quantity and depends on the reference you are taking !!!

 Lea December 12, 2012 07:19

I see them when I go to Results->Graphics and Animations->Contours and then Counters of Velocity-> cell Reynolds Number

 Far December 12, 2012 07:25

I guess it is Peclet Number related to local cell velocity and cell length scale. No need to look on it.

 LuckyTran December 14, 2012 00:12

Quote:
 Originally Posted by Far (Post 397131) I guess it is Peclet Number related to local cell velocity and cell length scale. No need to look on it.
Pe=Re*Pr

It is not quite Peclet number but similar.

Quote:
 Originally Posted by Lea (Post 397129) I see them when I go to Results->Graphics and Animations->Contours and then Counters of Velocity-> cell Reynolds Number
The cell Reynolds number is different than the Reynolds number of the pipe.

This is a local Reynolds number. It uses the local cell velocity, length scale and properties. The length scale is the cube root of the volume of the cell.

 Far December 14, 2012 04:27

Quote:
 Originally Posted by LuckyTran (Post 397464) Pe=Re*Pr It is not quite Peclet number but similar. The cell Reynolds number is different than the Reynolds number of the pipe. This is a local Reynolds number. It uses the local cell velocity, length scale and properties. The length scale is the cube root of the volume of the cell.

Thanks :D It was a long time when I read them in CFD course. Thanks again.

 Lea December 14, 2012 06:02

Thank you both for your help=)

Lucky Train, can you tell me what I have to do to see the "real" reynolds number so that I can see if the flow is laminar or turbulent?

 Far December 14, 2012 06:07

Real Reynolds number is what you have specified

 Lea December 14, 2012 06:11

but I haven't specified the reynoldsnumber because that is my variable. I want to see how the reynoldsnumber changes when I have a different geometry.

 Far December 14, 2012 06:13

Then calculate reynolds number from the formula

 RodriguezFatz December 14, 2012 07:02

Re = density * velocity * length / viscosity.
Density and viscosity are material properties. Take them from the material you chose. Velocity is what you set as inlet velocity of your pipe.
"Lenght" is a typical length scale of your domain. For pipes one usually takes the diameter of the pipe. Now calculate.

What people here were trying to explain: Fluent can't tell you "the Reynold's number" because there is no single Reynold's number. It allways depends on the length-scale you choose. You could also take the radius for a pipe. It's more or less arbitrary what you take...

 Lea December 18, 2012 06:44

Thank you all for your help!

I found the solution: When you look on the turbulent reynolds number Re_y it is already turbulent with a value about 200.

So my flow is turbulent=)

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