CEL expression with cubic function
 

Hi,

I am looking to define a specific oil that I have used and need to input its properties into CFX. They density, Kinematic Viscosity and Heat capacity are defined by cubic functions which are all similar this

885.597 - (0.689367(T [C]))+(19228*10^-04*(T [C])^2)-(8.87642*10^-7*(T [C])^3)

However when I plot this, CFX says I have an error either side of the '-' sign at position 8. Correcting that to [C], the error moves along...Has anyone got a solution to this. I'm sure it's staring me in the face

Please & Cheers

(885.597 - (0.689367*(t/1 [k]))+(19228*10^-04*((t/1 [k]))^2)-(8.87642*10^-7*((t/1 [k]))^3))*1 [whatever units you need this in]

Doesn't seem to like this. The generated plot is increasing, but the function is decreasing, with increasing temperature.
I've also tried breaking it up, defining variables a, b & c, then adding/subracting them together, but no joy.
Any further ideas?

Some comment:
Are you sure the original function is correct?

It might be wise to not assume the order of operations is as you expect - for instance the section 8.87642*10^-7*(t/1[k]) could be interpreted as:
8.87642*10^(-7*(t/1[k]))
8.87642*(10^-7)*(t/1[k])
(8.87642*10)^(-7*(t/1[k]))
And these will all give very different answers.

So I would put lots of brackets in the expression to make sure there is no assumed order of operations anywhere.

There are lots of problems in your equation. First, I'm not sure what you're trying to do with this:

0.689367(T [C])

but my guess is you're expecting CFX to use temperature in Celcius, which it won't do unless you change it in the Options (it defaults to SI, so Kelvin). A better option would be to rewrite your equation considering the temperature will be in [K].

Second, try using proper scientific number format:
19228*10^-04 => 19228E-04
8.87642*10^-7 => 8.87642E-07
(...)

Last, use the appropriate units for each constant. So, if this is for density, use something like:

A = 885.597 [kg/m^3]
B = -0.689367 [kg/(m^3 K)]
C = 19228E-04 [kg/(m^3 K^2)]
D = -8.87642E-7 [kg/(m^3 K^3)]
TCelsius = T - 273.15 [K]

rho = A + B * TCelsius + C * TCelsius^2 + D * TCelsius^3

Hope it helps.