The flow behavior of non-newtonian fluids. Many important flow problems of practical significance cannot be treated within the framework of classical fluid mechanics, which rests on the application of the Navier-Stokes equations, comprising the continuum equations of motion together with Newton's law of viscosity. Non-newtonian flow, by definition, is concerned with fluids which do not have a constant viscosity in accordance with Newton's law. Some other rheological equation is required which describes the behavior of the fluid, and this is invariably more complicated.