Cavitation

Cavitation is a general term used to describe the behavior of voids or bubbles in a liquid. Cavitation is usually divided into two classes of behavior: inertial (or transient) cavitation and non-inertial cavitation. Inertial cavitation is the process where a void or bubble in a liquid rapidly collapses, producing a shock wave. Such cavitation often occurs in pumps, propellers, impellers, and in the vascular tissues of plants. Non-inertial cavitation is the process where a bubble in a fluid is forced to oscillate in size or shape due to some form of energy input, such as an acoustic field. Such cavitation is often employed in ultrasonic cleaning baths and can also be observed in pumps, propellers etc.

Inertial cavitation
Inertial cavitation was first studied by Lord Rayleigh in the late 19th century when he considered the collapse of a spherical void within a liquid. When a volume of liquid is subjected to a sufficiently low pressure it may rupture and form a cavity. This phenomenon is termed cavitation inception and may occur behind the blade of a rapidly rotating propeller or on any surface vibrating underwater with sufficient amplitude and acceleration. Other ways of generating cavitation voids involve the local deposition of energy such as an intense focussed laser pulse (optic cavitation) or with an electrical discharge through a spark. Vapor gasses evaporate into the cavity from the surrounding medium, thus the cavity is not a perfect vacuum but has a relatively low gas pressure. Such a low pressure cavitation bubble in a liquid will begin to collapse due to the higher pressure of the surrounding medium. As the bubble collapses, the pressure and temperature of the vapor within will increase. The bubble will eventually collapse to a minute fraction of its original size, at which point the gas within dissipates into the surrounding liquid via a rather violent mechanism, which releases a significant amount of energy in the form of an acoustic shock-wave and as visible light. At the point of total collapse, the temperature of the vapor within the bubble may be several thousand kelvin, and the pressure several hundred atmospheres.

Inertial cavitation can also occur in the presence of an acoustic field. Microscopic gas bubbles which are generally present in a liquid will be forced to oscillate due to an applied acoustic field. If the acoustic intensity is sufficiently high, the bubbles will first grow in size, and then rapidly collapse. Hence, inertial cavitation can occur even if the rarefaction in the liquid is insufficient for a Rayleigh-like void to occur. High power ultrasonics usually utilize the inertial cavitation of microscopic vacuum bubbles for treatment of surfaces, liquids and slurries.

The physical process of cavitation inception is similar to boiling. The major difference between the two is the thermodynamic paths which precede the formation of the vapor. Boiling occurs when the local vapor pressure of the liquid rises above its local ambient pressure and sufficient energy is present to cause the phase change to a gas. Cavitation inception occurs when the local pressure falls sufficiently far below the saturated vapor pressure, a value given by the tensile strength of the liquid.

In order for cavitation inception to occur, the cavitation "bubbles" generally need a surface on which they can nucleate. This surface can be provided by the sides of a container or by impurities in the liquid or by small undissolved microbubble within the liquid. It is generally accepted that hydrophobic surfaces stabilize small bubbles. These pre-existing bubbles start to grow unbounded when they are exposed to a pressure below the threshold pressure, termed Blake's threshold.

Non-inertial cavitation
Non-inertial cavitation is the process where small bubbles in a liquid are forced to oscillate in the presence of an acoustic field, when the intensity of the acoustic field is insufficient to cause total bubble collapse. This form of cavitation causes significantly less erosion than inertial cavitation, and is often used for the cleaning of delicate materials, such as silicon wafers.

Problems
Cavitation is, in many cases, an undesirable occurrence. In devices such as propellers and pumps, cavitation causes a great deal of noise, damage to components, vibrations, and a loss of efficiency.

When the cavitation bubbles collapse, they force liquid energy to very small volumes, thereby creating spots of high temperature and emitting shock waves, the latter of which a source of noise. The noise created by cavitation is a particular problem for military submarines, as it increases the chances of being detected by sonar.

Although the collapse of a cavity is a relatively low energy event, highly localized collapses can erode metals, such as steel, over time. The pitting caused by the collapse of cavities produces great wear on components and can dramatically shorten a propeller or pump's lifetime.

In industry, cavitation is often used to homogenize, or mix and break down suspended particles in a colloidal liquid compound, such as paint mixtures, or milk. Many industrial mixing machines are based upon this design principle. It is usually achieved through impeller design, or by forcing the mixture through an annular opening that has a narrow entrance orifice with a much larger exit orifice. In the latter case, the drastic decrease in pressure as the liquid accelerates into a larger volume induces cavitation. This method can be controlled with hydraulic devices that control inlet orifice size, allowing for dynamic adjustment during the process, or modification for different substances. The outer surface of this type of mixing valve, upon which the cavitation bubbles are driven against to cause their implosion, undergoes tremendous stress, and is often constructed of super-hard or tough materials such as stainless steel, Stellite, or even polycrystalline diamond (PCD).

Cavitating water purification devices have also been designed, in which the extreme conditions of cavitation can break down pollutants and organic molecules. Spectral analysis of light emitted in sonochemical reactions reveal chemical and plasma based mechanisms of energy transfer. The light emitted from cavitation bubbles is termed sonoluminesence.

Hydrophobic chemicals are attracted underwater by cavitation as the pressure difference between the bubbles and the liquid water forces them to join together. This effect may assist in protein folding.

Biomedical application
Cavitation plays an important role for the destruction of kidney stones in shock wave lithotripsy. Currently it is tested if cavitation can be used to transfer large molecules into biological cells (sonoporation). Nitrogen cavitation is a method used in research to lyse cell membranes while leaving organelles intact. Cavitation also probably plays a role in HIFU, a non-invasive treatment methodology for cancer.

Pumps and propellers
Major places where cavitation occurs are in pumps, on propellers, or at restrictions in a flowing liquid.

As an impeller's (in a pump), or propeller's (as in the case of a ship or submarine) blades move through a fluid, low pressure areas are formed as the fluid accelerates around and moves past the blades. The faster the blades move, the lower the pressure around it can become. As it reaches vapor pressure, the fluid vaporizes and forms small bubbles of gas. This is cavitation. When the bubbles collapse later, they typically cause very strong local shockwaves in the fluid, which may be audible and may even damage the blades.

Cavitation in pumps may occur in two different forms:

Suction cavitation
Suction cavitation occurs when the pump suction is under a low pressure/high vacuum condition where the liquid turns into a vapor at the eye of the pump impeller. This vapor is carried over to the discharge side of the pump where it no longer sees vacuum and is compressed back into a liquid by the discharge pressure. This imploding action occurs violently and attacks the face of the impeller. An impeller that has been operating under a suction cavitation condition can have large chunks of material removed from its face or very small bits of material removed causing the impeller to look sponge like. Both cases will cause premature failure of the pump often due to bearing failure. Suction cavitation is often identified by a sound like gravel or marbles in the pump casing.

Discharge cavitation
Discharge cavitation occurs when the pump discharge pressure is extremely high, normally occurring in a pump that is running at less than 10% of its best efficiency point. The high discharge pressure causes the majority of the fluid to circulate inside the pump instead of being allowed to flow out the discharge. As the liquid flows around the impeller it must pass through the small clearance between the impeller and the pump cutwater at extremely high velocity. This velocity causes a vacuum to develop at the cutwater (similar to what occurs in a venturi) which turns the liquid into a vapor. A pump that has been operating under these conditions shows premature wear of the impeller vane tips and the pump cutwater. In addition, due to the high pressure conditions, premature failure of the pump's mechanical seal and bearings can be expected. Under extreme conditions, this can break the impeller shaft.

Discharge cavitation is believed to be the cause of the cracking of joints.

Cavitation in engines
Some bigger diesel engines suffer from cavitation due to high compression and undersized cylinder walls. Vibrations of the cylinder wall induce alternating low and high pressure in the coolant against the cylinder wall. The result is pitting of the cylinder wall that will eventually let cooling fluid leak into the cylinder and combustion gases to leak into the coolant.

It is possible to prevent this from happening with chemical additives in the cooling fluid that form a protecting layer on the cylinder wall. This layer will be exposed to the same cavitation, but rebuilds itself.

Vascular plants
Cavitation occurs in the xylem of vascular plants when the water potential becomes so great that dissolved air within the water expands to fill the plant cell - either vessel elements or tracheids. Plants are generally able to repair cavitated xylem, for example with root pressure, but for others such as vines, cavitation often leads to mortality. In some trees, the sound of the cavitation is clearly audible. In the autumn the dropping temperature increases the formation of air bubbles in the tracheids of some plant species, causing them to drop their leaves.

Marine Life
Just as cavitation bubbles form on a fast spinning boat propeller, they may also form on the tails and fins of aquatic animals. The effects of cavitation are especially important near the surface of the ocean where the ambient water pressure is relatively low and cavitation is more likely to occur.

For powerful swimming animals like dolphins and tuna, cavitation may be detrimental because it limits their maximum swimming speed. Even if they have the power to swim faster, dolphins may have to restrict their speed because collapsing cavitation bubbles on their tail are too painful. Cavitation also slows tuna, but for a different reason. Unlike dolphins, these fish do not feel the painful bubbles because they have bony fins without nerve endings. Nevertheless they cannot swim faster because the cavitation bubbles create an air film around their fins that limits their speed. Lesions have been found on tuna that are consistent with cavitation damage.

Cavitation is not always a limitation for sea life. Some animals have found ways to use it to their advantage when hunting prey. The pistol shrimp snaps a specialized claw to create cavitation, which can kill small fish. The mantis shrimp (type smasher) uses cavitation as well in order to stun, smash open, or kill the shellfish that it feasts upon.

Coastal erosion
In the last half a decade, coastal erosion in the form of inertial cavitation has been generally accepted. Air pockets in an incoming wave are forced into cracks in the cliff being eroded, the force of the wave then compresses the air pockets until the bubble implodes, becoming liquid, giving off various forms of energy which blast apart the rock.

Canada

 * National Research Council - Institute for Ocean Technology Cavitation Tunnel,, St. Johns, Newfoundland, Canada

France

 * "Tunnel de Cavitation" Ecole Navale, Lanveoc
 * "Grand Tunnel Hydrodynamique" Bassin d'Essais des Carènes, Val de Reuil

Germany

 * Multiple cavitation tunnels at the Versuchsanstalt für Wasserbau und Schiffbau |, Berlin
 * Large Cavitation tunnel at Hamburg Ship Model Basin ,Hamburg

Iran

 * Applied Hydrodynamics Laboratory, Iran University of Science and Technology,, Narmak, Tehran, Iran.

Netherlands

 * Large Cavitation Tunnel and High Speed Cavitation Tunnel | at the Maritime Research Institute, Wageningen.

Norway

 * "Cavitation Lab" NTNU, The Norwegian University of Science and Technology, Trondheim

Poland

 * Ship Design and Research Centre (CTO S.A.) Centrum Techniki Okrętowej S.A.,, Gdansk, Poland.

South Korea

 * Samsung Ship Model Basin (SSMB), Samsung Heavy Industries,, Daejeon, South Korea.

Sweden

 * SSPA

Taiwan

 * The Large Cavitation Tunnel at National Taiwan Ocean University, Keelung, Taiwan

United Kingdom

 * "Emerson Cavitation Tunnel," University of Newcastle upon Tyne.

United States

 * The Garfield Thomas Water Tunnel The Pennsylvania State University, State College, PA
 * The William B. Morgan Large Cavitation Channel, Memphis, TN
 * MIT's variable pressure water tunnel