32 Turbulence Physical Picture

Since a precise definition of turbulence is difficult, pictures and other visualizations of turbulent flows may give some idea of the complex characteristics of turbulence. Several such visualizations are available, including the famous painting 'The Deluge' by Leonardo da Vinci. Banerjee (1992) included several such pictures in his excellent paper on turbulence structures. Van Dyke (1982) published an album of fluid motion which is a 'must see' for any turbulence researcher. Several websites hold treasures of visual information on turbulence (see, for example, links listed at sites such as www.cfd-online.com and www.efluids.com). Pictures included in these resources show various aspects of turbulent flows and may give some intuitive understanding of turbulence.

Turbulence is intrinsically unsteady, even when constant boundary conditions are imposed. Velocity and all other flow properties fluctuate in a random and chaotic way. Turbulent fluctuations always have a three-dimensional spatial character. There have been many attempts to analyze and to construct a physical picture of turbulence, following several different approaches. These different approaches, broadly classified into three categories, are discussed in this section.

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