[Anna Tian]

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]

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]

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.
'
.

[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.

[Anna Tian]

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'.