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 Andy April 7, 2008 12:51

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

 Rogerio Fernandes Brito April 7, 2008 20:51

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]

 andy2O April 8, 2008 03:50

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

 Andy April 8, 2008 06:45

Re: Dimension issues using CEL