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
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

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

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.

Have you tried areaAve(Velocity)@inlet ?

yup...Its showing 150.659 which is more higher

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

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.

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?

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.

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?

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.

What if the shape of geometry be so complicated, how can gain hydraulic diameter by CFD-post?

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