The turbulence indicates the state of a Fluide, liquid or gas, in which speed presents in any point a swirling character: swirls whose size, localization and orientation vary constantly. The turbulent flows are thus characterized by a very disordered appearance, a not easily foreseeable behavior and the existence of many space and temporal scales. Such flows appear when the kinetic source of energy which puts the fluid moving is relatively intense in front of the forces of Viscosité that the fluid opposes to move.

Contrary, one calls Laminaire the character of a regular flow.

To translate the fact that, in a flow, the inertias override the forces of viscosity, a Reynolds number suitably selected must be higher than a certain threshold. For the study of turbulences in natural environment, it is preferable to use the Nombre of Richardson rather than the Reynolds number, because this last regards the density of the fluid as constant, which is not true in the general case.

The complex behavior of the turbulent flows is approached most of the time by the statistical way. One can consider that the study of turbulence belongs to the Physique statistics.

In the field of the Meteorology, turbulence explains the variations of the marine currents and the atmospheric winds. She is also studied in Aéronautique (, combustion chamber jet washes, wake of the paddles and compressors, etc), in the Chemical industry (considerable effectiveness of the process of turbulent mixture), like in Acoustique, Géophysique, etc

A property classically put in front of a turbulent flow lies in a process called cascades of energy: the division of the large swirls in smaller swirls allows a transfer of energy of the large scales towards the small scales. This process is limited by the effect of the molecular Dissipation, which prevents the too important variations speed. In practice, this transfer of energy is not with one way, the phenomenon of swirling pairing (in English backscatter ) allowing the specific transfer of swirling small structures (which amalgamate) towards one or of largest.

The physics of turbulences is into full rise, thanks to the generalization of the measuring instruments (like the Free Falling Profilers or the probes with Effect Doppler for the study of turbulences in aquatic environments), and with the progressive reduction of the costs of those. Since the years 1970, the numerical mechanics of the fluids makes it possible to the researchers studied turbulence, mainly by using an approach of Direct Digital simulation.

Kolmogorov, in 1941, put forth the assumption that this cascade was selfsimilar: the swirls divide all in the same way whatever their scale, as long as it is neither too small (if not it is necessary to take account of viscosity) nor too large (the large swirls depend on the geometry of the flow). It is what is called the inertial zone, and by arguments of dimensional Analyze, it expressed a law (law in -5/3) which characterizes the car-similarity of turbulence (a little like a curve fractale, when one " zoome" on a turbulence, one cannot know on which scale one is).

Turbulence increases the Traînée of the objects moving for high Reynolds numbers. For Reynolds numbers low but always turbulent, the provocation of turbulence will delay the separation of the Boundary layer and this fact of decreasing the coefficient of Traînée (ex: combinations of the swimmers (and swimmers) provided with scales to cause turbulence on surface and to decrease the force of trail which is opposed to their displacement).

See too

• Laminar

Bonds

• Wavelets & Turbulence: Use of the representation in ondelettes for the study of turbulence