Turbulence is a phenomenon that arises from the instability of laminar flow caused by the amplification of disturbances due to strongly non-linear inertial effects. The universally accepted theory is that developed by Kolmogorov, known as the energy cascade. According to this theory, a turbulent flow consists of vortices of various sizes, each associated with a different energy level. Figure 8.1 presents a schematic, referred to as the energy density spectrum, which illustrates the energy levels of turbulent vortices as a function of the inverse of their sizes.