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January 18, 2011, 03:38 
Calculation of Reynolds Number In CFX

#1 
Senior Member
Ashkan Javadzadegan
Join Date: Sep 2010
Posts: 230
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Hi all
I simulated a steady flow in a pipe. Mean velocity at inlet of pipe is 0.02 [m/s]. The pipe diameter is 20 [mm]. The following data has been taken from CFX solver output file. Global Length = 5.4614E02 Minimum Extent = 1.9994E02 Maximum Extent = 5.4000E01 Density = 1.0600E+03 Dynamic Viscosity = 3.7100E03 Velocity = 2.0012E02 Advection Time = 2.7290E+00 Reynolds Number = 3.1227E+02 When I calculate the Reynolds number with above information. It becomes 114 while as it shown above, CFX calculated 312 for Reynolds number. Does anybody know how CFX calculated the Reynolds Number? regards Ashtonj 

January 18, 2011, 03:49 

#2 
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Lance
Join Date: Mar 2009
Posts: 522
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CFX use Volume^(1/3) to calculate the length scale, that why you dont get the same Reynolds number.
From your data: ((0.02^2*pi/4)*(5.4e12e2))^(1/3)*1060*2e2/3.7e3 = 313 

January 25, 2011, 06:12 
mean Velocity at Inlet

#3 
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Hi,
In my output file the Velocity is shown to be 70 but if i caluculate it in CFD Post as ave(Velocity)@Inlet then it is much higher than that.It is 129.Can anyone please tell me what is the problem.I want to know this to calculate Reynolds number manualy in CFDPost. Last edited by rskrishna87; January 25, 2011 at 06:37. 

January 25, 2011, 06:13 

#4 
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Lance
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Have you tried areaAve(Velocity)@inlet ?


January 25, 2011, 06:35 

#5 
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yup...Its showing 150.659 which is more higher


January 25, 2011, 07:00 

#6 
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Does anyone know how to calculate Reynolds number manually in CFDPost using an expression??


January 25, 2011, 17:05 

#7 
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Glenn Horrocks
Join Date: Mar 2009
Location: Sydney, Australia
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If you want to calculate the Reynolds number you are comparing against literature values, you need to use the same definition of Reynolds Number.
These are my guesses: Velocity scale = areaAve(Velocity)@inlet Length scale = the diameter of the cylinder or chord length or whatever the geometry is density scale = areaAve(Density)@inlet Viscosity scale = areaAve(Viscosity)@inlet Then you can define Re number using these numbers. 

November 9, 2013, 06:50 

#8 
New Member
shirazi
Join Date: Dec 2012
Posts: 2
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Dear Ashton,
I am aware that it is a long time since you have posted here,but I need a help .I cannot find Reynolds number in CFX outputs in CFD post. I normally calculate Reynolds number by hand. As you have mentioned here; there is way how to get Reynolds number from cfx itself. Could you give me some information in this field? 

November 9, 2013, 07:21 

#9 
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Glenn Horrocks
Join Date: Mar 2009
Location: Sydney, Australia
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Write a CEL expression which calculates it and send it to a monitor point. Easy.
But don't be fooled by the Reynolds Number reported in the output file. 

November 9, 2013, 08:31 

#10 
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shirazi
Join Date: Dec 2012
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Dear Glenn,
Thanks for your replay, The problem is that different part of a problem have different length scale. Are you suggesting that there is a builtin length scale function in CFX? 

November 10, 2013, 05:30 

#11 
Super Moderator
Glenn Horrocks
Join Date: Mar 2009
Location: Sydney, Australia
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There is some built in functions. The turbulence transition model has some functions in the Gammatheta model. There is the wall distance function for the SST turbulence model.
But usually you have to write your own function appropriate for your geometry. 

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