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Jess July 26, 2013 05:41

UDF Help
Hey all,

Im currently writing a UDF.

I calculate the force in an UDF called DEFINE_CG_MOTION

The question is how do i use this force to trigger the inflow in another UDF called DEFINE_PROFILE

I would use

If Force > number



but im not sure how to recall the force, as i get the error message, Force is not defined


Hershey July 28, 2013 08:29

You could do this a variety of ways I imagine.

First of all though, does your DEFINE_CG_MOTION only provide a single value for the entire domain every time it runs? I never used this macro before, but anyway, you should look into both F_UDMI or C_UDMI.

For a DEFINE_PROFILE, the macro is passed the thread so you would loop through either the faces or the cells for the F_UDMI and C_UDMI respectively.

Are you familiar with how User-Defined Memory works? I can continue on if this seems like the path you want to go down.

I also use Scheme variables, but you may find it easier just to stick with the UDM.

Please post back if you have questions or would like further explanation.

Jess July 28, 2013 15:41

my cg_motion sums up all the forces around the faces of a object everytime it runs.

This is what i understand of the macros u mentioned.

i can use F_UMI in my DEFINE_CG_MOTION to store the forces calculated and use Execute_on_demand? to use it in Define_PROFILE? or do i use the same macro F_UMI to retrieve the data?

I will give this a try tomorrow morning.


Hershey July 28, 2013 19:06

The F_UDMI will store the value calculated from your UDF into each respective face on the thread. For instance an example from the manual:

/* Compute face temperature and store in user-defined memory */
temp = F_T(f,t);
F_UDMI(f,t,0) = (temp - tmin) / (tmax-tmin);

You can see that the temperature is being retrieved by F_T(f,t) and after some manipulation it is stored in the respective face/thread combination of the face loop. If for instance F_UDMI(f,t,0) was equal to a constant, then the constant would be saved over every face/thread combination.

Once you save the value of your forces in F_UDMI, you can then access it from any UDF, even the DEFINE_PROFILE.

You shouldn't need an Execute_on_Demand macro.

Jess July 30, 2013 06:54

Yes method works :D
Thank you, now im just working on my inlet vel udf

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