# Calculation of Reynolds Number In CFX

 Register Blogs Members List Search Today's Posts Mark Forums Read

 January 18, 2011, 03:38 Calculation of Reynolds Number In CFX #1 Senior Member   Ashkan Javadzadegan Join Date: Sep 2010 Posts: 234 Rep Power: 7 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

 January 18, 2011, 03:49 #2 Senior Member   Lance Join Date: Mar 2009 Posts: 522 Rep Power: 11 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

 January 25, 2011, 06:12 mean Velocity at Inlet #3 Member   Join Date: Jan 2011 Posts: 37 Rep Power: 6 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. Last edited by rskrishna87; January 25, 2011 at 06:37.

 January 25, 2011, 06:13 #4 Senior Member   Lance Join Date: Mar 2009 Posts: 522 Rep Power: 11 Have you tried areaAve(Velocity)@inlet ?

 January 25, 2011, 06:35 #5 Member   Join Date: Jan 2011 Posts: 37 Rep Power: 6 yup...Its showing 150.659 which is more higher

 January 25, 2011, 07:00 #6 Member   Join Date: Jan 2011 Posts: 37 Rep Power: 6 Does anyone know how to calculate Reynolds number manually in CFD-Post using an expression??

 January 25, 2011, 17:05 #7 Super Moderator   Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 10,814 Rep Power: 85 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. nimap likes this.

 November 9, 2013, 06:50 #8 New Member   shirazi Join Date: Dec 2012 Posts: 2 Rep Power: 0 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 Super Moderator   Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 10,814 Rep Power: 85 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 New Member   shirazi Join Date: Dec 2012 Posts: 2 Rep Power: 0 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?

 November 10, 2013, 05:30 #11 Super Moderator   Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 10,814 Rep Power: 85 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. mdshirazi likes this.

July 17, 2015, 03:56
#12
New Member

nima
Join Date: Jul 2014
Posts: 9
Rep Power: 3
Quote:
 Originally Posted by ghorrocks 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?

 July 17, 2015, 06:32 #13 Super Moderator   Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 10,814 Rep Power: 85 Any CFD textbook can define you how to get the hydraulic diameter. Then it is usually straight forward to implement it in CFD-Post.

 Thread Tools Display Modes Linear Mode

 Posting Rules You may not post new threads You may not post replies You may not post attachments You may not edit your posts BB code is On Smilies are On [IMG] code is On HTML code is OffTrackbacks are On Pingbacks are On Refbacks are On Forum Rules

 Similar Threads Thread Thread Starter Forum Replies Last Post vishwa OpenFOAM Native Meshers: snappyHexMesh and Others 23 August 6, 2014 03:50 xujjun CFX 9 June 9, 2009 07:59 Ogbeni CFX 2 June 2, 2005 23:34 tuks CFX 0 May 25, 2005 01:16 Athar Zaidi Main CFD Forum 0 October 31, 1999 14:59

All times are GMT -4. The time now is 22:11.