What is the closed curved plane of the concept of fluid kinetic energy?

This is a very vague question. What, in particular, are you talking about? How it stratifies? Differences in surface tension? My father in law has proofs and equations still taught as standards in texts books regarding fluid dynamics. I’ll help if I can.

Does this help? http://hyperphysics.phy-astr.gsu.edu/hbase/pber.html

It is a vague question. I suspect it might refer to the conservation of angular momentum or vorticity as Fluid Mechanikers often call it.