# simple CEL expression debuging

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 June 29, 2013, 05:29 simple CEL expression debuging #1 Senior Member     Meimei Wang Join Date: Jul 2012 Posts: 494 Rep Power: 9 Sponsored Links 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? __________________ Best regards, Meimei

 June 29, 2013, 07:11 #2 Super Moderator   Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 13,737 Rep Power: 106 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.

June 29, 2013, 08:45
#3
Senior Member

Meimei Wang
Join Date: Jul 2012
Posts: 494
Rep Power: 9
Quote:
 Originally Posted by ghorrocks 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.
'
.
__________________
Best regards,
Meimei

Last edited by Anna Tian; June 29, 2013 at 11:08.

 June 29, 2013, 08:54 #4 Super Moderator   Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 13,737 Rep Power: 106 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.

June 29, 2013, 08:58
#5
Senior Member

Meimei Wang
Join Date: Jul 2012
Posts: 494
Rep Power: 9
Quote:
 Originally Posted by ghorrocks 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'.
__________________
Best regards,
Meimei

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