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-   -   simple CEL expression debuging (http://www.cfd-online.com/Forums/cfx/120042-simple-cel-expression-debuging.html)

Anna Tian June 29, 2013 05:29

simple CEL expression debuging
 
Hi

I'd like to define a position dependent variable by:

'
length = 0.0033 [m]
heat transfer coefficient = 0.1-x*(0.1-0.07)/length [m s^-1]
'
.

But the error message is

'
Units ([m s^-1]) are associated with a term that is not dimensionless.
'

It is quite strange because the unit of 'x' is [m], right? I'm really confused. Could anyone help?

ghorrocks June 29, 2013 07:11

The error message is quite clear. You assigne dthe variable "length" units of [m], but then try to re-assign it units [m s^-1] in the next line. You need to fix this by putting the units against the constants in both equations, do not put units against variables.

Anna Tian June 29, 2013 08:45

Quote:

Originally Posted by ghorrocks (Post 436720)
The error message is quite clear. You assigne dthe variable "length" units of [m], but then try to re-assign it units [m s^-1] in the next line. You need to fix this by putting the units against the constants in both equations, do not put units against variables.

I think it means x/length is not dimensionless. x is the coordinate X. Why x/length is not dimensionless?

If it's written in this way:

'
length = 0.0033 [m]
ratio = x/length []
heat transfer coefficient = 0.1-(0.1-0.07)/ratio [m s^-1]
'
,
then the error message will be

'
Error at position 10. Units ([]) are associated with a term that is not dimensionless.
'
.

ghorrocks June 29, 2013 08:54

:)

You cannot just assign x/length (which is dimensionless) a dimension. Instead multiply it by 1[m/s] to convert it to a velocity. But then you will have to make the 0.1 a velocity as adding a dimensionless number with a velocity is not meaningful either. And finally you appear to be setting a variable which looks like a heat transfer coefficient with a velocity and that does not make sense either.

It's late. I'm going to bed.

Anna Tian June 29, 2013 08:58

Quote:

Originally Posted by ghorrocks (Post 436746)
:)

You cannot just assign x/length (which is dimensionless) a dimension. Instead multiply it by 1[m/s] to convert it to a velocity. But then you will have to make the 0.1 a velocity as adding a dimensionless number with a velocity is not meaningful either. And finally you appear to be setting a variable which looks like a heat transfer coefficient with a velocity and that does not make sense either.

It's late. I'm going to bed.

The problem is: x/length is not dimensionless which is quite strange.

If it's written in this way:

'
length = 0.0033 [m]
ratio = x/length []
heat transfer coefficient = 0.1-x*(0.1-0.07)/ratio [m s^-1]
'
,
then the error message will be

'
Error at position 10. Units ([]) are associated with a term that is not dimensionless.
'

Btw, sorry that it does not mean heat transfer coefficient. It is wall function transfer coefficient. I just name it as 'heat transfer coefficient'. You can ignore the name 'heat transfer coefficient'.


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