UDF for VOF model
 

I am trying to use the VOF model to study the rise of bubbles in water. I am writing a UDF to give me the location of the bubble centroid after each time step (using DEFINE_EXECUTE_AT_END). I am trying to use the looping macros to loop over all the cells of the secondary phase (air). Unfortunately the udf gives me the centroid of the entire domain rather than the centroid of the bubble.

My first question (may be a bit silly) is 1. what is the difference between phase domain and phase thread? 2. I tried using the sub_domain_loop and the mp_thread_loop_c with the begin_c_loop_all but I get the same answer. How do I loop over cells of the secondary phase only?

Thanks for your help!