Environmental Engineering Reference
In-Depth Information
revolutions per minute (rpm) spike and reducing the eficiency of the desired acceleration.
Cavitation also occurs when transducers such as sonars are applied to water in amplitudes
that exceed its ability to modulate at given frequencies.
Mantis shrimp (or ninja or pistol shrimp) [3] can strike its claws as fast as a speeding
bullet, leaving the barrel of a gun with an acceleration of 10,400 g (102,000 m/s
2
), and with
top velocities of 23 m/s in nearly 1/10,000 of a second. Instantaneous forces of >1500 Nm
are caused by the impact itself against the striking surface. Even more astonishing is that it
does not require physical contact with the target at all, but instead when it begins to strike,
it can produce a cavitational shockwave generating cavitation bubbles at the remote target,
where microbubbles appear, and it is during the collapse of these bubbles that measur-
able forces are produced on their intended targets; the resulting shockwave alone can be
enough to kill or stun much larger prey without actual contact.
Amazingly, the shockwave from the cavitation produced by the snapping gesture of
the mantis shrimp can also produce very small amounts of light and very high tempera-
tures within the collapsing bubbles, or sonoluminescence left in the wake of its strike, and
although both the light and the high temperatures are short lived, they pave the way for a
need to better understand the mechanisms at work in laboratory and reproducing these.
There are several ways that cavitation can manifest itself: on partially attached cavities,
on travelling bubble-type cavitation, or vortex cavitation and as shear cavitation. When
cavitation does inally occur, microbubbles may visibly form, and subsequently collapse.
When they do collapse, these microbubbles can go on to the extreme case of emitting elec-
tromagnetic radiation, and at incredibly high temperatures, but only for time scales lasting
in the range of nanoseconds or even picoseconds, making their observation quite dificult,
but capable of producing catastrophic macroscale effects such as the pitting or failure of
propellers on maritime vessels [4-6]. For the very brief periods of time during the minute
implosions, collapsing these bubbles temperatures are capable of even melting steel [7-9].
According to NanoSpire Co., “a laser, ultrasound or other energy source is used to cre-
ate small high-energy vapor bubbles through a phase transition.” The collapse of cavita-
tion bubbles in close proximity to a wall generates supersonic liquid microjets naturally
pointed toward a wall or other restriction to the collapse of the bubble. NanoSpire's pat-
ented methods build on, but go beyond, the energetic process seen in nature. The size,
strength, and direction of cavitation reentrant microjets can be controlled by our patented
methods to a very high degree of accuracy. Cavitation microjets can travel at up to Mach 4
and are capable of drilling a hole as small as a few nanometers in a diamond. Multiple
controlled bubbles are also possible, allowing lines to be machined as well as other form
factors and machining applications. Cavitation allows a high aspect ratio machining to be
obtained and is very repeatable” [10,11].
The development of cavitation in a liquid low is characterized by a phase change from
liquid to vapor at almost constant temperature. Water's own surface tension brings a delay
in the inception of cavitation from microbubbles carried by the liquid. It is a reasonable
approximation to assume that the critical pressure for the onset of cavitation is equal to
the vapor pressure. In the case of vortex cavitation, and due to centripetal forces, the pres-
sure within the vortex axis is lower than the pressure far away from it, so that a minimum
pressure is expected at the vortex center axis. It helps to think of it as a centrifuge where
lesser dense material is left at the center axis and denser material is thrown out to the edge
of the tornado.
The dynamics of the motion of the bubble is characterized to a irst approximation by the
Rayleigh-Plesset equation derived from the incompressible Navier-Stokes equations and
describes the motion of the radius of the bubble
R
as a function of time
t
. Here, μ is the
