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|  File Name      : CAVITY1.ASC      |  Online Date     :  01/01/95          |
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                                  CAVITATION

There are four basic types of cavitation. Fundamentally cavitation results
from a drop in pressure on a liquid creates pockets or bubbles in the liquid -
an increase in pressure causes these bubbles to collapse resulting in a
tremendous "local" force which can cause damage to metals, emulsification, de-
gasification, sonoluminescence and many other strange and wonderous phenomena.

CAVITATION: Cavitation is the formation and activity of bubbles (or cavities)
in a liquid. Here the word 'formation' refers, in a general way, both to the
creation of a new cavity or to the expansion of a pre-existing one to a size
where macroscopic effects can be observed. These bubbles may be suspended in
the liquid or may be trapped in tiny cracks either in the liquid's boundary
surface or in solid particles suspended in the liquid.

The expansion of the minute bubbles may effected by reducing the ambient
pressure by static or dynamic means. The bubbles then become large enough to
be visible to the unaided eye. The bubbles may contain gas or vapour or a
mixture of both gas and vapour. If the bubbles contain gas, then the expansion
may be by diffusion of dissolved gases from the liquid into the bubble, or by
pressure reduction, or by temperature rise.

If, however, the bubbles contain mainly vapour, reducing the ambient pressure
sufficiently at essentially constant temperature causes an 'explosive'
vaporization into the cavities which is the phenomenon that is called
cavitation, whereas raising the temperature sufficiently causes the mainly
vapour bubbles to grow continuously producing the effect known as boiling.
This means that 'explosive' vaporization or boiling do not occur until a
threshold is reached.

There are thus four ways of inducing bubble growth:

    1. For a gas-filled bubble, by pressure reduction or increase in
       temperature. This is called gaseous cavitation.

    2. For a vapour-filled bubble, by pressure reduction. This is called
       vapourous cavitation.

    3. For a gas-filled bubble, by diffusion. This is called degassing as gas
       comes out of the liquid.

    4. For a vapour-filled bubble, by sufficient temperature rise.  This is
       called boiling.

The situation is complicated because the bubble usually contains a mixture of
gas and vapour.

Looking at it another way, we may distinguish between four different kinds of
cavitation according to how it is produced:

    1. Hydrodynamic cavitation is produced by pressure variations in a flowing
       liquid due to the geometry of the system.

    2. Acoustic cavitation is produced by sound waves in a liquid due to
       pressure variations.

    3. Optic cavitation is produced by photons of high intensity (laser) light
       rupturing in a liquid.

    4. Particle cavitation is produced by any other type of elementary
       particles, e.g., a proton, rupturing a liquid, as in a bubble chamber.

It has been pointed out that whereas hydrodynamic and acoustic cavitation are
brought about by tension in the liquid, optic and particle cavitation are
achieved by a local deposition of energy.

HYDRODYNAMIC CAVITATION: In a flowing system, the liquid velocity varies
locally and at the points of highest velocity, low pressures and cavities
occur.

INCIPIENT cavitation is the term used to describe the type and stage of
cavitation that is just detectable as the cavitation appears.

DESINENT cavitation is the term used to describe cavitation just before it
disappears.

The conditions which mark the boundary or threshold between no cavitation and
detectable cavitation are not always identical. For example, the pressure of
disappearance of cavitation has been generally found to be greater, and less
variable, than the pressure of appearance.

Three cases of flow cavitation arise:

    1. Travelling cavitation occurs when cavities or bubbles form in the
       liquid, and travel with the liquid as they expand and subsequently
       collapse.

    2. Fixed cavitation occurs when a cavity or pocket attached to the rigid
       boundary of an immersed body or a flow passage forms, and remains fixed
       in position in an unsteady state.

    3. Vortex cavitation occurs in the cores of vortices which form in regions
       of high shear, and often occurs on the blade tips of ship's propellers
       - hence the name "tip" cavitation.

ACOUSTIC CAVITATION: In a non-flowing system the ambient pressure can be
varied by sending sound waves through the liquid. If the amplitude of the
pressure variation is great enough to bring the pressure locally down to, or
below, the vapour pressure in the negative parts of the sound cycle traversing
the liquid, any minute cavities or bubbles will grow.

If the pressure amplitude is increased to produce zero, and then negative,
pressures (i.e. tensions) locally in the liquid, the bubble growth is
increased.  The tiny bubble is thus set into motion, growing and contracting
in the sound field.  This motion may be of various kinds, usually complicated.

Two distinct types of bubble motion are possible: in the first are stable
cavities or bubbles that oscillate for many periods of the sound field,
whereas in the second are transient cavities that exist for less than one
cycle.

Two important characteristics of acoustic cavitation should be mentioned here.
The first is that generally it is a non-linear process in that the change in
the radius of the bubble is not proportional to the sound pressure. The second
is that the high compressibility of the gas bubbles means that much potential
energy is obtained from the sound waves when the bubbles expand and that
kinetic energy is concentrated when the bubbles collapse.

In transient cavitation, this transformation of a low energy density sound
wave into a high energy density collapsing bubble occurs because the motion is
non-linear.  Because it concentrates the energy into very small volumes it can
produce very high pressures and temperatures which can erode solids, initiate
chemical reactions and produce luminescence.

OPTIC AND PARTICLE CAVITATION: Optic cavitation occurs when, say, large pulses
of a Q-switched ruby laser are focused on a liquid.  Breakdown of the liquid
occurs and bubbles are formed. The bubbles can then be photographed by a high
speed rotating mirror camera.

Particle cavitation is based on the growth of bubbles in a superheated liquid.
If a charged particle is sent through the liquid it leaves an ionization trail
for a fraction of a second. Some of the energy from these ions goes into a few
fast electrons, which can give up about 1000 electron volts of energy in a
small volume to produce rapid local heating. If the liquid has been
superheated by expansion, boiling will occur along the track which will appear
as a line of tiny bubbles.
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