4. Turbulent flows

4.1 Definitions. General

Turbulence in a flow is essentially random and always three-dimensional. It develops at relatively high Reynolds numbers and is a property of the flow, not of the fluid as is viscosity, for example. Turbulence increases transport capacity within the fluid by increasing diffusion terms (momentum diffusion, heat diffusion, etc.) and also increases the dissipation of mechanical energy into thermal energy.

As mentioned at the beginning of this article, turbulent flow is unsteady flow in which, according to Reynolds' theory (also known as statistical theory), mean parameter values can be identified, with fluctuations superimposed on them. Since the scale of the fluctuations is large compared to the mean free path of the molecules, the...