147 lines
7.6 KiB
Plaintext
147 lines
7.6 KiB
Plaintext
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| File Name : CAVITY1.ASC | Online Date : 01/01/95 |
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| Contributed by : Dale Pond | Dir Category : KEELY |
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| From : KeelyNet BBS | DataLine : (214) 324-3501 |
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| KeelyNet * PO BOX 870716 * Mesquite, Texas * USA * 75187 |
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| A FREE Alternative Sciences BBS sponsored by Vanguard Sciences |
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This file shared with KeelyNet courtesy of Dale Pond.
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CAVITATION
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There are four basic types of cavitation. Fundamentally cavitation results
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from a drop in pressure on a liquid creates pockets or bubbles in the liquid -
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an increase in pressure causes these bubbles to collapse resulting in a
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tremendous "local" force which can cause damage to metals, emulsification, de-
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gasification, sonoluminescence and many other strange and wonderous phenomena.
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CAVITATION: Cavitation is the formation and activity of bubbles (or cavities)
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in a liquid. Here the word 'formation' refers, in a general way, both to the
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creation of a new cavity or to the expansion of a pre-existing one to a size
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where macroscopic effects can be observed. These bubbles may be suspended in
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the liquid or may be trapped in tiny cracks either in the liquid's boundary
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surface or in solid particles suspended in the liquid.
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The expansion of the minute bubbles may effected by reducing the ambient
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pressure by static or dynamic means. The bubbles then become large enough to
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be visible to the unaided eye. The bubbles may contain gas or vapour or a
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mixture of both gas and vapour. If the bubbles contain gas, then the expansion
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may be by diffusion of dissolved gases from the liquid into the bubble, or by
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pressure reduction, or by temperature rise.
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If, however, the bubbles contain mainly vapour, reducing the ambient pressure
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sufficiently at essentially constant temperature causes an 'explosive'
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vaporization into the cavities which is the phenomenon that is called
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cavitation, whereas raising the temperature sufficiently causes the mainly
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vapour bubbles to grow continuously producing the effect known as boiling.
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This means that 'explosive' vaporization or boiling do not occur until a
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threshold is reached.
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There are thus four ways of inducing bubble growth:
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1. For a gas-filled bubble, by pressure reduction or increase in
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temperature. This is called gaseous cavitation.
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2. For a vapour-filled bubble, by pressure reduction. This is called
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vapourous cavitation.
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3. For a gas-filled bubble, by diffusion. This is called degassing as gas
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comes out of the liquid.
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4. For a vapour-filled bubble, by sufficient temperature rise. This is
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called boiling.
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The situation is complicated because the bubble usually contains a mixture of
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gas and vapour.
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Looking at it another way, we may distinguish between four different kinds of
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cavitation according to how it is produced:
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1. Hydrodynamic cavitation is produced by pressure variations in a flowing
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liquid due to the geometry of the system.
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2. Acoustic cavitation is produced by sound waves in a liquid due to
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pressure variations.
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3. Optic cavitation is produced by photons of high intensity (laser) light
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rupturing in a liquid.
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4. Particle cavitation is produced by any other type of elementary
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particles, e.g., a proton, rupturing a liquid, as in a bubble chamber.
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It has been pointed out that whereas hydrodynamic and acoustic cavitation are
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brought about by tension in the liquid, optic and particle cavitation are
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achieved by a local deposition of energy.
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HYDRODYNAMIC CAVITATION: In a flowing system, the liquid velocity varies
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locally and at the points of highest velocity, low pressures and cavities
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occur.
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INCIPIENT cavitation is the term used to describe the type and stage of
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cavitation that is just detectable as the cavitation appears.
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DESINENT cavitation is the term used to describe cavitation just before it
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disappears.
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The conditions which mark the boundary or threshold between no cavitation and
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detectable cavitation are not always identical. For example, the pressure of
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disappearance of cavitation has been generally found to be greater, and less
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variable, than the pressure of appearance.
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Three cases of flow cavitation arise:
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1. Travelling cavitation occurs when cavities or bubbles form in the
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liquid, and travel with the liquid as they expand and subsequently
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collapse.
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2. Fixed cavitation occurs when a cavity or pocket attached to the rigid
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boundary of an immersed body or a flow passage forms, and remains fixed
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in position in an unsteady state.
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3. Vortex cavitation occurs in the cores of vortices which form in regions
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of high shear, and often occurs on the blade tips of ship's propellers
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- hence the name "tip" cavitation.
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ACOUSTIC CAVITATION: In a non-flowing system the ambient pressure can be
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varied by sending sound waves through the liquid. If the amplitude of the
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pressure variation is great enough to bring the pressure locally down to, or
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below, the vapour pressure in the negative parts of the sound cycle traversing
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the liquid, any minute cavities or bubbles will grow.
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If the pressure amplitude is increased to produce zero, and then negative,
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pressures (i.e. tensions) locally in the liquid, the bubble growth is
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increased. The tiny bubble is thus set into motion, growing and contracting
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in the sound field. This motion may be of various kinds, usually complicated.
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Two distinct types of bubble motion are possible: in the first are stable
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cavities or bubbles that oscillate for many periods of the sound field,
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whereas in the second are transient cavities that exist for less than one
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cycle.
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Two important characteristics of acoustic cavitation should be mentioned here.
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The first is that generally it is a non-linear process in that the change in
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the radius of the bubble is not proportional to the sound pressure. The second
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is that the high compressibility of the gas bubbles means that much potential
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energy is obtained from the sound waves when the bubbles expand and that
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kinetic energy is concentrated when the bubbles collapse.
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In transient cavitation, this transformation of a low energy density sound
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wave into a high energy density collapsing bubble occurs because the motion is
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non-linear. Because it concentrates the energy into very small volumes it can
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produce very high pressures and temperatures which can erode solids, initiate
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chemical reactions and produce luminescence.
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OPTIC AND PARTICLE CAVITATION: Optic cavitation occurs when, say, large pulses
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of a Q-switched ruby laser are focused on a liquid. Breakdown of the liquid
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occurs and bubbles are formed. The bubbles can then be photographed by a high
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speed rotating mirror camera.
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Particle cavitation is based on the growth of bubbles in a superheated liquid.
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If a charged particle is sent through the liquid it leaves an ionization trail
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for a fraction of a second. Some of the energy from these ions goes into a few
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fast electrons, which can give up about 1000 electron volts of energy in a
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small volume to produce rapid local heating. If the liquid has been
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superheated by expansion, boiling will occur along the track which will appear
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as a line of tiny bubbles.
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