I posted earlier, but the response didn't help. I have looked at the UDF manual from Fluent. I couldn't find the help I needed. What I need help at is how can I create a UDF where I want a mass flow rate at a internal hole bc to become a predetermined value. This mass flow rate is related to the pressure at the pressure outlet, defined by a equation. Since the internal bc has a specific zone ID and the outlet has a different ID. How can I distinguish them in my UDF, since all the UDF's I have seen only pertain to one bc zone ID and the domain. I understand that when you select the UDF in the boundary condition it relates that zone ID to the thread. I have found a similar UDF that was on Fluent's help page, but that was to adjust the Thrust to a predetermind value while changing the Temp. But in my case I am changing the Pressure at one face, while the mass flow rate at the internal hole face must become a predetermined constant. I hope this is understandable. Can someone help.

I have found the solution without using UDF. Within Fluent6.1 at the text menu interface and within the boundary condition text menu there is target mass flow rate settings. This is new within Fluent6.1. I was able to set the max and min pressures at the outlet and then I went to the bc at the outlet and set the mass flow rate there to achieve the desired mass flow rate at the internal hole.