Hidraulica

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Introduction to Turbulence in Fluid Mechanics

1.1 Is it possible to define turbulence?
Everyday life gives us an intuitive knowledge of turbulence in fluids: the smoke
of a cigarette or over a fire exhibits a disordered behaviour characteristic of
the motion of the air which transports it. The wind is subject to abrupt
changes in direction and velocity, which may have dramatic consequencesfor
the seafarer or the hang-glider. During air travel, one often hears the word
turbulence generally associated with the fastening of seat-belts. Turbulence is
also mentioned to describe the flow of a stream, and in a river it has important
consequences concerning the sediment transport and the motion of the bed.
The rapid flow of any fluid passing an obstacle or an airfoil creates turbulencein the boundary layers and develops a turbulent wake which will generally
increase the drag exerted by the flow on the obstacle (and measured by the
famous Cx coefficient): so turbulence has to be avoided in order to obtain better aerodynamic performance for cars or planes. The majority of atmospheric
or oceanic currents cannot be predicted accurately and fall into the category
of turbulentflows, even in the large planetary scales. Small-scale turbulence
in the atmosphere can be an obstacle towards the accuracy of astronomic observations, and observatory locations have to be chosen in consequence. The
atmospheres of planets such as Jupiter and Saturn, the solar atmosphere or
the Earth’s outer core are turbulent. Galaxies look strikingly like the eddies
which are observed in turbulentflows such as the mixing layer between two
flows of different velocity, and are, in a manner of speaking, the eddies of a
turbulent universe. Turbulence is also produced in the Earth’s outer magnetosphere, due to the development of instabilities caused by the interaction of
the solar wind with the magnetosphere. Numerous other examples of turbulent flows arise in aeronautics, hydraulics, nuclear andchemical engineering,
oceanography, meteorology, astrophysics and internal geophysics.

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1 Introduction to Turbulence in Fluid Mechanics

It can be said that a turbulent flow is a flow which is disordered in time
and space. But this, of course, is not a precise mathematical definition. The
flows one calls “turbulent” may possess fairly different dynamics, may be
three-dimensional orsometimes quasi two-dimensional, may exhibit well organized structures or otherwise. A common property which is required of
them is that they should be able to mix transported quantities much more
rapidly than if only molecular diffusion processes were involved. It is this latter property which is certainly the more important for people interested in
turbulence because of its practical applications:the engineer, for instance, is
mainly concerned with the knowledge of turbulent heat diffusion coefficients,
or the turbulent drag (depending on turbulent momentum diffusion in the
flow). The following definition of turbulence can thus be tentatively proposed
and may contribute to avoiding the somewhat semantic discussions on this
matter:
• Firstly, a turbulent flow must be unpredictable, in thesense that a small
uncertainty as to its knowledge at a given initial time will amplify so as to
render impossible a precise deterministic prediction of its evolution (a).
• Secondly, it has to satisfy the increased mixing property defined above (b).
• Thirdly, it must involve a wide range of spatial wave lengths (c).
Such a definition allows in particular an application of the term “turbulent”
tosome two-dimensional flows. It also implies that certain non-dimensional
parameters characteristic of the flow should be much greater than one: indeed,
let l be a characteristic length associated to the large energetic eddies of
turbulence, and v a characteristic fluctuating velocity; a very rough analogy
between the mixing processes due to turbulence and the incoherent random
walk allows...