# UDF Data access problems

 Register Blogs Members List Search Today's Posts Mark Forums Read

 June 16, 2011, 18:45 UDF Data access problems #1 New Member   Aaron Mosher Join Date: Jun 2011 Location: St. Louis Posts: 5 Rep Power: 14 I'm fairly new to Fluent, and as part of teaching myself the UDF structure I made a small UDF to read area and flow at the boundary. However, when I printed the data to the screen it gave me nothing but nonsense values. Also, when I directly printed the "M_PI" function, the output was somewhere above 1.6 trillion. Has anyone encountered similar problems, or have any thoughts about what might be going wrong? For reference, here is the small UDF I wrote. I applied it as a pressure outlet boundary condition for laminar flow in an axi-symmetric pipe. /************************************************** ************************************** Test function to debug Fluent/UDF operations ************************************************** **************************************/ #include "udf.h" /*Test Axisymmetric Floow and Area*/ DEFINE_PROFILE( area_test, t, i ) { /*Variables*/ real r = 0; real ri[ND_ND]; real q = 0; face_t f; /*Loop Over Faces*/ begin_f_loop( f, t ) { q += 2*M_PI*F_FLUX( f, t ); F_AREA(ri,f,t); r += NV_MAG(ri); F_PROFILE(f,t,i) = 0; printf( "Inside Loop area: %d\n", F_AREA(ri,f,t) ); printf( "INside LOop flux: %d\n", F_FLUX(f,t) ); } end_f_loop( f, t ) /*Print out data*/ printf( "Outlet Area is: %d\n", r ); printf( "Outlet flow is: %d\n", q ); printf( "Fluent thinks pi is %d\n", M_PI); }

 June 17, 2011, 03:37 #2 Senior Member   Max Join Date: Mar 2009 Posts: 133 Rep Power: 17 printf("%d",x) outputs data as integer numbers. Your variables are defined as real = floating numbers! The correct format is %g. cheers

 June 17, 2011, 09:52 #3 New Member   John Palmore Join Date: Jun 2010 Posts: 8 Rep Power: 15 One thing to add. I have had problems with UDFs that use FLEUNT builtin functions. Many of the values are not defined until after you initialize your solution. Make sure you initialize your solution before hooking in your UDF. Not sure if this is your error, but if you do this it will help you out in the future.

 June 17, 2011, 11:35 Issue Resolved #4 New Member   Aaron Mosher Join Date: Jun 2011 Location: St. Louis Posts: 5 Rep Power: 14 Thanks everyone for the suggestions, switching the printf statements from %d to %g fixed the issue. I thought the output values looked like min or max integer values, and I was very confused. As a separate question, does anyone have any suggestions for tracking and updating a user defined variable between iterations? I'm trying to model an autoregulation response in the arteries and then adjust a pressure boundary condition accordingly. The closest thing I have found so far is the F_UDMI function. ~Aaron

June 17, 2011, 13:16
#5
Member

Pranab N Jha
Join Date: Nov 2009
Location: Houston, TX
Posts: 86
Rep Power: 16
Quote:
 Originally Posted by Mosheraa As a separate question, does anyone have any suggestions for tracking and updating a user defined variable between iterations? I'm trying to model an autoregulation response in the arteries and then adjust a pressure boundary condition accordingly. The closest thing I have found so far is the F_UDMI function. ~Aaron
Yes, you can use the F_UDMI macro. I have done it.
You can use the value stored at the memory location in the next time step (supposing you are doing this way) and then store the new value back in the memory.
This method worked for me. Earlier I used to start my job in batch mode right after reading in the UDFs and used to get memory errors.

 Tags data access, debugging, udf error