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Turbulent flows are complex multi-scale and chaotic motions that need to be classified into more elementary components, referred to coherent turbulent structures. Such a structure must have temporal coherence, i.e. it must persist in its form for long enough periods that the methods of time-averaged statistics can be applied. Coherent structures are typically studied on very large scales, but can be broken down into more elementary structures with coherent properties of their own, such examples include hairpin vortices. Hairpins and coherent structures have been studied and noticed in data since the 1930s, and have been since cited in thousands of scientific papers and reviews.[1]
A depiction of an elementary substructure called Hairpin vortices. Based on the classical drawing by Theodorsen.[1]
Flow visualization experiments, using smoke and dye as tracers, have been historically used to simulate coherent structures and verify theories, but computer models are now the dominant tools widely used in the field to verify and understand the formation, evolution, and other properties of such structures. The kinematic properties of these motions include size, scale, shape, vorticity, energy, and the dynamic properties govern the way coherent structures grow, evolve, and decay. Most coherent structures are studied only within the confined forms of simple wall turbulence, which approximates the coherence to be steady, fully developed, incompressible, and with a zero pressure gradient in the boundary layer. Although such approximations depart from reality, they contain sufficient parameters needed to understand turbulent coherent structures in a highly conceptual degree.[2]
^ abGreen, Beverley (1995-03-31). Fluid Vortices - Vol. 30 of Fluid Mechanics and Its Applications. Dordrecht: Springer Science & Business Media. ISBN 978-0-7923-3376-0.
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Hussain, A. K. M. F. (1983-10-01). "Coherent structures—reality and myth". The Physics of Fluids. 26 (10). AIP Publishing: 2816–2850. doi:10.1063/1.864048. ISSN 0031-9171.
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