UDF to calculate mass flow rate
 

Can someone find mistake in given below udf. Value calculated by udf at different time steps & reported by printf function is always coming zero......Why???please help




DEFINE_EXECUTE_AT_END(mass_flow)
{
Domain *d;
real flow=0.0;
Thread *f_thread;
face_t f;
d=Get_Domain(1);
f_thread=Lookup_Thread(d, 13);
begin_f_loop(f,f_thread)
{
flow += F_FLUX(f,f_thread);
}
end_f_loop(f, f_thread)
printf("Mass flow rate is %g kg/s\t \n",flow);
flow_tot=flow;
}