Cavitation

Cavitation (pronounced kav-i-tey-shuhn)

(1) The formation of pits on a surface.

(2) In fluid dynamics, the rapid formation and collapse of vapor pockets in a flowing liquid in regions of very low pressure (associated especially with devices such as rotating marine propellers or the impellers used in pumps.

(3) Such a pocket formed in a flowing liquid; the formation of cavities in a structure.

(4) In biology, the formation of cavities in an organ (used originally to describe those appearing in lung tissue as a result of consumption (tuberculosis)).

1868: The construct was cavit(y) + -ation.  Cavity was a mid-sixteenth century borrowing from Middle French cavité or the Late Latin cavitās, from the Classical Latin cavus (hollow, excavated, concave), the construct being cav +‎-ity (the nominal suffix).  The suffix -ation was from the Middle English -acioun & -acion, from the Old French acion & -ation, from the Latin -ātiō, an alternative form of -tiō (thus the eventual English form -tion).  It was appended to words to indicate (1) an action or process, (2) the result of an action or process or (3) a state or quality.  Cavitation is a noun, cavitate, cavitated & cavitating are verbs and cavitatory & cavitatory are adjectives; the noun plural is cavitations.

The original use of cavitation dates from 1868 and appeared in the literature of human pathology, describing “the formation of cavities in the body”, especially those appearing in lung tissue as a result of consumption (tuberculosis).  The use in fluid dynamics (particularly pumps and marine engineering) emerged in circa 1895 although oral use may have predated this: the verb cavitate (to form cavities or bubbles (in a fluid)) documented since 1892 so it was either a back-formation from cavitation or the construct was cavit(y) + -ate.  The related verbs were cavitated & cavitating.  The suffix -ate was a word-forming element used in forming nouns from Latin words ending in -ātus, -āta, & -ātum (such as estate, primate & senate).  Those that came to English via French often began with -at, but an -e was added in the fifteenth century or later to indicate the long vowel.  It can also mark adjectives formed from Latin perfect passive participle suffixes of first conjugation verbs -ātus, -āta, & -ātum (such as desolate, moderate & separate).  Again, often they were adopted in Middle English with an –at suffix, the -e appended after circa 1400; a doublet of –ee.  The noun supercavitation was a creation of plasma physics and described an extreme form of cavitation in which a single bubble of gas forms around an object moving through a liquid, significantly reducing drag.  As observational technology & techniques improved, the form ultracavitation also appeared to describe instances where instances of the phenomenon meant drag tended as close to zero as was possible.

Cavitation is an interesting aspect of fluid dynamics but it’s studied because it’s something which can cause component failure in devices like the pumps used for liquid, fluid & gas which can have catastrophic consequences for both connected equipment and people in the vicinity and beyond.  Such components typically feature robust construction but cavitation is a function of sustained operation (often 24/7) at high speeds and some vulnerable parts may be heavy and the fragmentation at high velocity of a heavy, reciprocating mass is obviously a serious problem.  Technically, it’s the formation of vapour- or gas-filled cavities in a flowing liquid when tensile stress is superimposed on the ambient pressure and one novelty in the science of cavitation was in 2021 noted by researchers in an oncology laboratory.  Using a gassy, explosive bacteria to destroy cancer cells by bombardment, the strikes were observed to produce a brief sonoluminescence (in physics, the emission of short bursts of light from imploding bubbles in a liquid when excited by sound), the cavitation bubbles producing a brief flash of light as they collapsed.

In the specific case of “pump cavitation”, the problem typically occurs when a hydraulic pumps which pumps liquids suffers a partial pressure drop.  What the change in pressure can induce is the formation of air bubbles, leading to cavity creation.  Inside the pump, the pressure shift transforms the liquid into a vapor which is then converted back to liquid by the spinning impellers.  The air bubbles thus are constantly moving inside the housing and as they implode during pressure changes, the surfaces of the impeller are eroded and it’s the creation of these tiny cavities which can accumulate sufficiently to weaken the structure to the point of failure.  The issue particularly affects centrifugal pumps but can occur in submersible devices.

Lindsay Lohan enjoying the effects of fluid dynamics.

Although something identified by engineers in the nineteenth century, the exact nature of cavitation wasn’t fully understood until the application in the 1950s of high-speed photography and the mathematical models developed then were later confirmed as close to exactly correct by computer simulations later in the century.  What was found was two causes of cavitation : (1) Inertial Cavitation in which a shock wave is produced by the collapse of bubble or void present in a liquid and (2) Non-inertial Cavitation which is initiated when a bubble in a fluid undergoes shape alterations due to an acoustic field or some other form of energy input.  Also observed were two behaviors of cavitation: (1) Suction Cavitation induced by high vacuum or low-pressure conditions which reduce the flow of fluid, bubbles forming near the eye of an impeller eye; as these bubbles move towards the pump’s discharge end, they are compressed into liquid, and they will implode against the impeller’s edge and (2) Discharge Cavitation which occurs when the pump’s discharge pressure becomes abnormally high, altering the flow of fluid, leading to internal recirculation, the liquid becoming “stuck” in a pattern between the housing (and the impeller) thereby creating a vacuum which in turn creates the air bubbles which will collapse and cavitate the impeller.

Representation of fluid dynamics under specific resonant conditions.

In fluid dynamics, a flow becoming “stuck” is often something to avoid but an aspect of the behavior can be exploited and it was a specific instance of certain “resonant conditions” Chrysler’s engineers exploited in 1959 when designing their Sonoramic induction system.  The idea wasn’t new, the math explained as early as 1863 and in racing cars it had been used for years but what Chrysler did was make it a focal point.  Sonoramic was an implementation of Sir Isaac Newton's (1642–1727) first law of motion, more commonly known as the law of inertia: “An object at rest tends to stay at rest and an object in motion tends to stay in motion” and it’s the second part which was exploited.  During the intake cycle of an engine, the fuel-air mix flows through the intake manifold, past the intake valve, and into the cylinder, then the intake valve shuts.  At that point, the law of inertia comes into play: Because the air was in motion, it wants to stay in motion but can’t because the valve is shut so it piles up against the valve with something of a concertina effect.  With one piece of air piling up on the next, the air becomes compressed and this compressed air has to go somewhere so it turns around and flows back through the intake manifold in the form of a pressure wave.  This pressure wave bounces back and forth in the runner and if it arrives back at the intake valve when the valve opens, it’s drawn into the engine.  This bouncing pressure wave of air and the proper arrival time at the intake valve creates a low-pressure form of supercharging but for this to be achieved all variables have to be aligned so the pressure wave arrives at the intake valve at the right time.  This combination of synchronized events is known as the “resonant conditions”.

Representation of cavitation in mechanical gears.

The behavior in pumps is now well understood and both design parameters and maintenance schedules are usually cognizant of cavitation and its potential consequences.  However, instances remain not infrequent, especially when pumps are fitted into systems by non-specialists, the most common causes being (1) low fluid pressure, (2) insufficient internal diameter of suction pipes, (3) excessive distances between a fluid source and a pump’s impeller(s), (4) pumps being run at too high a speed (which may be within a manufacturer’s recommendations but inappropriate for the system in which it’s installed), (5) too many fittings added to a suction pipe and (6) debris intrusion (often a consequence of inadequate filter cleaning & maintenance).  Cavitation is a function of speed and in devices such as slow-speed propellers (such as those in many marine applications), cavitation is not an issue, thus the frequent use of light, efficient, thin blades.