||August 31, 2011 17:38
Yeah, UDF's are made in C, so you'll need to read up on user defined functions in general. You'll probably want to use DEFINE_PROFILE, and there are some nice examples that make use of that command if you read the user defined function guide. You could easily modify their code to make the BC you want (most likely), but it is going to take a few hours of reading the udf manual to understand it.
(courtesy of Wichita State University, but syntax may have changed a bit if you're not using Fluent 6. Read section 8.2 for an example of a UDF boundary condition.)