Chapter 3 Fundamentals of Inviscid, incompressible Flow Theoretical fluid dynamics, being a difficult subject, is for convenience, commonly divided into two branches, one treating frictionless or perfect fluids, the other treating of viscous or imperfect fluids. The frictionles fluid has no existence in nature, but is hypothesized by mathematicians in order to facilitate the investigation of important laws and principles that may be approximately true of viscous or natural fluids