Hi Swathi, I apologize for the delay. I changed my approach and I am using the UDF that is published at page 301 of the 2021/R2 UDF guide. The UDF calculates the mass flow through a boundary surface. In my case, I am trying to calcuate through the sphere that is suspened in the fluid domain. The meshed sphere is outlet that acts as sink for the sourced mass that is transported from the front face of the cubic fluid domain. When I compile the UDF, and then define a report that uses the UDF, in the report definition screen I do not get the option to select a surface (i.e., the sphere-outlet) as the applicable surface for the UDF. Again, the goal is to calculate the mass flow through the sphere-outlet.