I am trying to simulate the rise of a bubble due to buoyancy. I have run the simulation using VOF method and have obtained the desired result qualitatively. I am trying to find the center of mass of the bubble and the terminal velocity of the bubble. The formula for the above two require the data of the cell centers and the volume fraction of each cell in the domain. 

The data for these can be obtained using an UDF. I have two cell zones (fluid and bubble). When I am trying to obtain the volume fractions, the loop only goes to the fluid domain, but never the bubble domain. I need to make sure that I get the volume fractions of all the zones?

How do I do this?