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why low reynolds number?

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Old   December 12, 2012, 06:13
Default why low reynolds number?
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Lea
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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=)
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Old   December 12, 2012, 07:07
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which Reynolds numbers you are talking about?
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Old   December 12, 2012, 07:11
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The reynolds numbers in the whole pipe
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Old   December 12, 2012, 07:13
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Where you are seeing them?

Reynolds number is a reference quantity and depends on the reference you are taking !!!
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Old   December 12, 2012, 07:19
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I see them when I go to Results->Graphics and Animations->Contours and then Counters of Velocity-> cell Reynolds Number
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Old   December 12, 2012, 07:25
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I guess it is Peclet Number related to local cell velocity and cell length scale. No need to look on it.
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Old   December 14, 2012, 00:12
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Quote:
Originally Posted by Far View Post
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.

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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.
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Old   December 14, 2012, 04:27
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Quote:
Originally Posted by LuckyTran View Post
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 It was a long time when I read them in CFD course. Thanks again.
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Old   December 14, 2012, 06:02
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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?
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Old   December 14, 2012, 06:07
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Real Reynolds number is what you have specified
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Old   December 14, 2012, 06:11
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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.
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Old   December 14, 2012, 06:13
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Then calculate reynolds number from the formula
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Old   December 14, 2012, 07:02
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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...
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Old   December 18, 2012, 06:44
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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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