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Why this udf is not working????

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Old   December 27, 2011, 01:38
Default Why this udf is not working????
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Hi all,
Plz help me
I have written this udf for density a function of pressure.i am woking on narrow diameter pipe and woking fluid is r134a single phase. since pressure is changing across the pipe so i want to change density with press. but fluent is showing follwing error
I am taking press inlet and press outlet

*UDF for liquid density*/

real density;
real pressure = C_P(cell,thread);
density =1e-10*pressure*pressure-0.000*pressure+1385;
return density;

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Old   December 27, 2011, 10:52
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The is no error in you UDF I checked it with fluent 6.3 as interpreted UDF in 64bit machine. Something else is causing the trouble.
why are you using 0.0*pressure it makes nosense, it could add some machine error (values like 1e-45) and this could cause problem in 32bit machine.
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Old   December 29, 2011, 19:28
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While you can define whatever density you want, C_P(cell,thread) won't return the absolute pressure, which I think you're looking for. Consequently, I think you're getting a density approaching zero, which is probably throwing the error. Additionally, the second term in your formula (0.000*pressure+1385) will *never* be anything but zero, given that you're multiplying by zero.

Verify that you're using the proper calculation for density (more specifically, for pressure), and then let us know.

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