Cavitation

Origin of cavitation

The depression can have two different origins:

it can be related to a flow of liquid at strong speed, for example in a venturi, or in the vicinity of a blade in a Pompe or on a Hélice of boat or Sous-marin. Indeed, a zone strong speed in a fluid corresponds to a low pressure (cf the Théorème of Bernoulli). One speaks about cavitation Hydrodynamique, discovered in 1917 by Lord Rayleigh. That results for example in a wake of bubbles, as by the appearance of a layer of vapor fixed on a blade (where the depression is strongest, i.e. towards the Trailing edge; attention however, the bubble often observed is generally only air pulled by a swirl, and not of cavitation).

it can be related to the variations of Densité of a liquid subjected to a acoustic Onde, in general of the Ultrason S of power. One speaks then about acoustic cavitation.

Effects

Whatever is the origin, cavitation has two different effects:

the vapor bubbles change the behavior of the liquid completely. Hydrodynamic cavitation destroys the output of the propeller or the pump. Energy is not any more transformed into movement (for example), but is it remains in the driving machine (which can not support to leave the normal conditions of work thus), that is to say it is diffused in an uncontrolled and thus probably harmful form.

generally the vapor bubble is transitory: its appearance eliminates instantaneously the conditions which gave him birth. It thus occurs an implosion of the bubble. The latter perhaps so violent that the Pressure and Température inside the bubble can take very high values (several thousands of bar, several thousands of Kelvin). While implosant, the bubble can emit a shock wave in the liquid, which makes it possible to break drops (emulsification), to disperse, désagglomérer or to break solid particles, or to clean or erode solid surfaces.

This is why the control of the phenomenon of cavitation is essential in hydrodynamics: it represents a limit, because of the loss of output even of the destruction (of the propellers and pumps) which it can cause. The impacts on those are always made at the highest points speed, interior of hydraulics vortex and outside of mono and multichannel hydraulics, as on the photograph).

But conversely, the good comprehension of the phenomenon can make it possible to exploit it. Applications for cleaning were proposed. Moreover, the density of energy thus reached in certain bubbles gives place to unusual chemical reactions, field called sonochimy, and even in certain cases with the emission of light, phenomenon called Sonoluminescence. Certain researchers think that it is possible to exploit these phenomena to carry out a nuclear Fusion. . When it closes again this enormous grip it results a very violent flow from it generating a hydrodynamic bubble of cavitation, which, while implosant, emits a Shock wave likely to strike, to even kill the surrounding plankton., it was believed a long time that the noise came from the grip itself, before a Dutch team shows the appearance of a bubble of cavitation using a fast camera. The same team showed besides that this bubble was luminescent.

References

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