Core-Annular Flow (CAF) is a fluid dynamics regime where a highly viscous core fluid (such as heavy crude oil) flows through the center of a pipe while being lubricated and surrounded by a thin annular ring of a much less viscous fluid (such as water) along the pipe wall. The outer, low-viscosity fluid forms a ring at the high-shear region next to the pipe wall. Because water touches the wall instead of sticky oil, wall friction drops dramatically. Pumping requires pressures close to pumping water alone, reducing energy costs by factors matching the large oil-to-water viscosity ratio.