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-   -   Condensation as an additional variable (http://www.cfd-online.com/Forums/cfx/100111-condensation-additional-variable.html)

Clark Griswold April 20, 2012 10:42

Condensation as an additional variable
 
Hi all,

Could someone give me some pointers please and thank you...

I'd like to create an additional variable within a domain for condensation (as a predictor based solely on temperature) and be able to apply this either within a volume rendering or surface plot (contour) within post.

The idea being that the Relative Humidity within the domain remains constant at 95% throughout the simulation and the additional variable "Condensation" is plotted with a value of either 1 or 0 depending on whether the temperature in the cell or element is above or below the dewpoint. So it compares the temperature to the dewpoint and returns a 1 or a 0.

What I have created:

------------------
Additional Variable: Condensation
Variable Type: Volumetric
Units: []
Tensor Type: Scalar

------------------

I have created a user function called DewPointFunc:

Argument Units: [K]
Result Units: [K]

Coordinate Value
253.15 252.53
254.15 253.53
255.15 254.52
256.15 255.52

etc... to cover the range of expected temperatures within the domain

-----------------

Default Domian:

Condensation: Additional Variable Checked to enable
Option: Algebraic Equation
Add. Var. Value: This is where I am stuck......

I was trying to use something like this for Add. Var. Value: if(Temperature >=DewPointFunc(),1,0)

But I get the following error shown below - as I know its not correct. I think I need to create and apply some proper expressions but am unsure at the moment of what exactly to do... could someone help me out and explain how best to approach this. Or perhaps suggest a better way to accomplish the same thing. I'd like to learn a little here as well - so if you could provide some very detailed explanation like do this because.. and this expression is needed because etc... That would be totally awesome. And thank you again in advance for any time and effort put into this! It's most appreciated! :)

----------------------
Current error message:

Bad expression value 'if(Temperature >=DewPointFunc(),1,0)' detected in parameter 'Additional Variable Value' in object '/FLOW:Flow Analysis 1/DOMAIN:Default Domain/FLUID MODELS/ADDITIONAL VARIABLE:Condensation'.
CEL error:
In the expression assigned to 'Additional Variable Value', the function 'DewPointFunc' is called with the wrong number of arguments (0 - expected 1).

lffabiani April 20, 2012 14:43

Seems like the DewPointFunc function requires an argument (probably the temperature??)

Have you tried to use "DewPointFunc(Temperature)" ??

Best regards

ghorrocks April 21, 2012 07:20

Luis is right, the problem is pretty obvious. You need to give the DewPointFunc() an argument, probably temperature. The abbreviation for temperature is "T" - look in the CFX reference manual to confirm you are using the right variable names.


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