# why low reynolds number?

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 December 12, 2012, 06:13 why low reynolds number? #1 New Member   Join Date: Dec 2012 Posts: 10 Rep Power: 7 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=)

 December 12, 2012, 07:07 #2 Super Moderator   Sijal Join Date: Mar 2009 Location: Islamabad Posts: 4,358 Blog Entries: 6 Rep Power: 45 which Reynolds numbers you are talking about?

 December 12, 2012, 07:11 #3 New Member   Join Date: Dec 2012 Posts: 10 Rep Power: 7 The reynolds numbers in the whole pipe

 December 12, 2012, 07:13 #4 Super Moderator   Sijal Join Date: Mar 2009 Location: Islamabad Posts: 4,358 Blog Entries: 6 Rep Power: 45 Where you are seeing them? Reynolds number is a reference quantity and depends on the reference you are taking !!!

 December 12, 2012, 07:19 #5 New Member   Join Date: Dec 2012 Posts: 10 Rep Power: 7 I see them when I go to Results->Graphics and Animations->Contours and then Counters of Velocity-> cell Reynolds Number

 December 12, 2012, 07:25 #6 Super Moderator   Sijal Join Date: Mar 2009 Location: Islamabad Posts: 4,358 Blog Entries: 6 Rep Power: 45 I guess it is Peclet Number related to local cell velocity and cell length scale. No need to look on it.

December 14, 2012, 00:12
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Lucky Tran
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Quote:
 Originally Posted by Far 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 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.

December 14, 2012, 04:27
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Sijal
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Quote:
 Originally Posted by LuckyTran 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.

 December 14, 2012, 06:02 #9 New Member   Join Date: Dec 2012 Posts: 10 Rep Power: 7 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?

 December 14, 2012, 06:07 #10 Super Moderator   Sijal Join Date: Mar 2009 Location: Islamabad Posts: 4,358 Blog Entries: 6 Rep Power: 45 Real Reynolds number is what you have specified

 December 14, 2012, 06:11 #11 New Member   Join Date: Dec 2012 Posts: 10 Rep Power: 7 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.

 December 14, 2012, 06:13 #12 Super Moderator   Sijal Join Date: Mar 2009 Location: Islamabad Posts: 4,358 Blog Entries: 6 Rep Power: 45 Then calculate reynolds number from the formula

 December 14, 2012, 07:02 #13 Senior Member     Philipp Join Date: Jun 2011 Location: Germany Posts: 1,297 Rep Power: 20 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... __________________ The skeleton ran out of shampoo in the shower.

 December 18, 2012, 06:44 #14 New Member   Join Date: Dec 2012 Posts: 10 Rep Power: 7 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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