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-   -   UDF-accessing fluent variables (https://www.cfd-online.com/Forums/fluent/46637-udf-accessing-fluent-variables.html)

 manu November 21, 2007 06:36

UDF-accessing fluent variables

I'm writing a UDF in which I need to access the average value of the pressure on a compressor exit area (that have been calculated with 3D simulation). This so-called "area mass average" value is define as: sum(p_i*A_i*_m_i)/sum(A_i*m_i) where p_i is the pressure, A_i the area and m_i the mass flow in each cell of the exit face. I found the function "F_P(f,t)" that records the pressure in each cell of the face but not the average value. Does anybody knows how to compute this average then? Thanks Manu

 mohan raj November 21, 2007 15:11

Re: UDF-accessing fluent variables

//Please check this udf after changing id to the outlet ID

#include "udf.h" DEFINE_ON_DEMAND(average_pressure) { Domain *d; real P_avg; real A[ND_ND]; Thread *t; real m_sum=0.0; real sum=0.0; int id="GIVE YOUR ZONE ID"; face_t f; d = Get_Domain(1); t= Lookup_Thread(d,id)

begin_f_loop(f,t) { sum=sum+(F_P(f,t)*NV_MAG(A)*F_FLUX(f,t)); m_sum=NV_MAG(A)*F_FLUX(f,t); } end_f_loop(f,t)

P_avg=sum/m_sum; Message("MY AVERAGE PRESSURE=%f\n",P_avg); }

 manu November 26, 2007 13:12

Re: UDF-accessing fluent variables

Thank you very much, it looks great!!! I didn't try it yet because I have one small problem accessing the area of each cell. I don't exactly understand the syntax of the function: F_AREA(A,f,t). Does it suffice to add: (...) real A[ND_ND]; F_AREA(A,f,t); (...) and then A is assigned with the area vector?? Is it right so?

 mohanraj November 27, 2007 23:52

Re: UDF-accessing fluent variables

F_AREA(A,f,t); is a function which returns the face normal w.r.t to the grobal axis. A--> has three components if its 3-D or two componnts if 2-D. f and t are the face id for the thread 't'. These things are discussed in fluent manual pertaining to UDF. You can find these things from any search engines Typically, http://venus.imp.mx/hilario/SuperCom...tml/node56.htm

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