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-   -   Calculation of Reynolds Number In CFX (http://www.cfd-online.com/Forums/cfx/84014-calculation-reynolds-number-cfx.html)

 ashtonJ January 18, 2011 03:38

Calculation of Reynolds Number In CFX

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 = 5.4614E-02
Minimum Extent = 1.9994E-02
Maximum Extent = 5.4000E-01
Density = 1.0600E+03
Dynamic Viscosity = 3.7100E-03
Velocity = 2.0012E-02
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

 Lance January 18, 2011 03:49

CFX use Volume^(1/3) to calculate the length scale, that why you dont get the same Reynolds number.

((0.02^2*pi/4)*(5.4e-1-2e-2))^(1/3)*1060*2e-2/3.7e-3 = 313

 rskrishna87 January 25, 2011 06:12

mean Velocity at Inlet

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 CFD-Post.

 Lance January 25, 2011 06:13

Have you tried areaAve(Velocity)@inlet ?

 rskrishna87 January 25, 2011 06:35

yup...Its showing 150.659 which is more higher

 rskrishna87 January 25, 2011 07:00

Does anyone know how to calculate Reynolds number manually in CFD-Post using an expression??

 ghorrocks January 25, 2011 17:05

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.

 mdshirazi November 9, 2013 06:50

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?

 ghorrocks November 9, 2013 07:21

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.

 mdshirazi November 9, 2013 08:31

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 built-in length scale function in CFX?

 ghorrocks November 10, 2013 05:30

There is some built in functions. The turbulence transition model has some functions in the Gamma-theta 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.

 nimap July 17, 2015 03:56

Quote:
 Originally Posted by ghorrocks (Post 292154) 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.
What if the shape of geometry be so complicated, how can gain hydraulic diameter by CFD-post?

 ghorrocks July 17, 2015 06:32

Any CFD textbook can define you how to get the hydraulic diameter. Then it is usually straight forward to implement it in CFD-Post.

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