CFD Online Logo CFD Online URL
www.cfd-online.com
[Sponsors]
Home > Forums > CFX

Dimension issues using CEL

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

Reply
 
LinkBack Thread Tools Display Modes
Old   April 7, 2008, 12:51
Default Dimension issues using CEL
  #1
Andy
Guest
 
Posts: n/a
hi,

I'm trying to write an expression for the input flow where the velocity varies with height as u=0.49ln(3631.82h) when I try this in ANSYS CFX I get an error saying that it expects the argument of the ln function to be dimensionless but obviuosly it's in m.

any help would be appreciated

Andy
  Reply With Quote

Old   April 7, 2008, 20:51
Default Re: Dimension issues using CEL
  #2
Rogerio Fernandes Brito
Guest
 
Posts: n/a
make like this (where "Fluxrogerio" is the name of my variable in [W m^-2] and t is the time in [s] and more, A and B were created by myself just to become a dimensionless variable) :

Fluxrogerio (B*(19453.29701*(t/A)^3.07678)/(4.73947^3.07678+(t/A)^3.07678))

A 1.0 [s] B 1.0 [W m^-2]
  Reply With Quote

Old   April 8, 2008, 03:50
Default Re: Dimension issues using CEL
  #3
andy2O
Guest
 
Posts: n/a
Note that h / ( 1 [m] ) is dimensionless. Note that because the output of 0.49*ln(....) will be a dimensionless number, you will have to assign the correct units to the expression yourself to use it as a velocity in CFX. So... If h is measured in metres, and u in metres per second, then you should replace:

u=0.49ln(3631.82h)

with

0.49 * ln(3631.82 * h / (1 [m]) ) * 1 [m s^-1]

or, equivalently:

0.49 * ln(3631.82 [m^-1] * h) [m s^-1]

There are several ways of typing it - I think it's up to you to choose the one that makes sense to you.

( NB: If you use other units of distance or speed you must alter the choice of units or add in conversion factors otherwise the answer will be wrong! )

HTH

Andy2o

  Reply With Quote

Old   April 8, 2008, 06:45
Default Re: Dimension issues using CEL
  #4
Andy
Guest
 
Posts: n/a
great, thanks for your help
  Reply With Quote

Old   April 8, 2008, 06:49
Default Re: Dimension issues using CEL
  #5
Rogerio Fernandes Brito
Guest
 
Posts: n/a
You´re welcome!
  Reply With Quote

Reply

Thread Tools
Display Modes

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 Off
Trackbacks are On
Pingbacks are On
Refbacks are On


Similar Threads
Thread Thread Starter Forum Replies Last Post
Angle variable under CEL. Gloria Gaynor CFX 11 November 26, 2012 07:29
the problem about dimension bojiezhang OpenFOAM 2 October 16, 2011 00:22
junction box routine and CEL function bornspur CFX 2 February 3, 2009 03:24
Rotate node via cel Elian81 CFX 2 September 25, 2007 05:31
dimension lucy FLUENT 1 July 13, 2001 12:42


All times are GMT -4. The time now is 19:43.