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Density user defined equation

Altair Forum User

I am learning CFD and I have a problem that has density as a function of Temperature and Pressure. To start, I wrote a user defined function (usrMyDensity.c) where density is only a function of temperature (linear equation). I was able to compile the function and call it in my input file, but when i run a simple analysis (internal flow with constant temperature at the walls), i am not able to plot density in FieldView. I'm assuming that i'm not using the correct problem set up.

I know that it is using the complied user function because initially it was giving me an error of negative density, which i fixed with an if statement in my user function.

Any advice will be greatly appreciated.

thank you for your time and patience

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acupro

In order to see contours of density, you'll need to request Derived Quantity Output, which is a separate command in the input file. The default setting is output_frequency = 0, meaning derived quantity output will not be written. You'll probably want to change that so the output_frequency is the same as you request for Nodal Output.

Altair Forum User

acupro,

thank you very much, that did it!!!

I have a followup question regarding compiling C files for a Acusolve user defined function. if you take a look at the attached doc on my original question (usrMyDensity.c), you'll see that it is a very simple linear equation. I kept it simple so I could get familiar with compiling user defined functions and calling them in the input file. However, I now want to create a user defined function that interpolates the density between temperature and pressure. I wrote a very crude C++ program in Visual Studio to open a .txt file, find pressure and temperatures, and interpolate the density. I run this file using Visual Studio and it works. in Visual Studio, to open and read a .txt file, i included fstream.h (#include <fstream>).

in Acusolve, I want to start by having the user-defined function open the .txt file and read one value (baby steps), but when i try to compile, it is giving me lots of errors. in the C code, how do I include the fstream.h needed to open and read text files?

thank you very much

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acupro

Probably better to contact support at hwsupport@altair.com for those details. By the way, if you're looking for ideal gas density - a function of absolute temperature and pressure - that is already supported in AcuSolve.