Dimension issues using CEL
 

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

Re: Dimension issues using CEL
 

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]

Re: Dimension issues using CEL
 

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

Re: Dimension issues using CEL
 

great, thanks for your help

Re: Dimension issues using CEL
 

You´re welcome!