# Dimension issues using CEL

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 April 7, 2008, 12:51 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

 April 7, 2008, 20:51 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]

 April 8, 2008, 03:50 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

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

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

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