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!

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