Page 218 - Corrosion Engineering Principles and Practice

192 C h a p t e r 6 R e c o g n i z i n g t h e F o r m s o f C o r r o s i o n 193

FIGURE 6.43
Cavitation damage
of a diesel engine
piston liner on the
return stroke.
(Courtesy of
Defence R&D
Canada-Atlantic)

plates, and all places where the static pressure varies very abruptly
following the Bernoulli principle Eq. (6.8):
r
P + V + r gh = Constant (6.8)
2
2
where P is the absolute static pressure
h is the elevation
g is the gravitational acceleration

Strictly speaking, the Bernoulli equation applies to flow along a
streamline, however, in turbulent flow the bulk flow velocity can be
used with little error. Thus the increase in the velocity as the liquid is
accelerated through an orifice or over an impeller can result in a drop
of the local static pressure. As the liquid slows down, after it passes a
vena contracta or approaches the volute in a pump, the pressure rises
again, leading to the collapse of the cavities formed by the previous
drop in pressure (Fig. 6.39). Five different types of cavitation can be
observed depending on the flow conditions and geometry [35,36].

• Traveling cavitation is a type of cavitation in which individual
transient cavities or bubbles form in the liquid and move with
it as they expand or shrink during their life cycles. To the naked
eye traveling cavitation may appear as sheet cavitation.
• Cloud cavitation is caused by vorticity shed into the flow
field. It causes strong vibration, noise and erosion. The
shedding of cloud cavitation is periodic and the reentrant jet
is the basic mechanism to generate cloud cavitation.