position declaration on a 3D curved surface
 

Hi all,
I am simulating a 3D model for heat transfer from a metallic tube to flowing liquid. While the centre of the tube is at the origin (000) with y axis verticle and tube length along the z axis. Since it is symmetry, I'm modeling a half tube, which looks like a horizontal semi cylinder. I wanna declare a heat flux profile around the tube by an UDF, where the heat flux will vary along the Y direction, and that will translate throughout the tube along the z axis. But I don't understand how I can declare the curved surface of the tube in the udf.
Could any of you please help me to get rid of this maze?
Regards
Murad