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March 21, 2014, 10:50 
CEL expression with cubic function

#1 
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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 

March 21, 2014, 13:14 

#2 
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Edmund Singer P.E.
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(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]


March 22, 2014, 14:52 

#3 
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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? 

March 23, 2014, 04:18 

#4 
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Glenn Horrocks
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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. 

March 25, 2014, 16:36 

#5  
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Bruno
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Quote:
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 => 19228E04 8.87642*10^7 => 8.87642E07 (...) 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 = 19228E04 [kg/(m^3 K^2)] D = 8.87642E7 [kg/(m^3 K^3)] TCelsius = T  273.15 [K] rho = A + B * TCelsius + C * TCelsius^2 + D * TCelsius^3 Hope it helps. 

Tags 
cel, cubic function, material properties 
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