Kongsberg Simrad CP200-5C, Руководство по установке, Страница: 50 / 110

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Cavitation
Cavitation is the formation of small air bubbles
close to the transducer face. The bubbles appear
because the local pressure becomes negative during
parts of the acoustic pressure cycles. The cavitation
threshold increases with the hydrostatic pressure.
The noise is made when the bubbles implode.
Cavitation noise may appear near extruding objects
at higher speeds, but more often it is caused by the
propellers. Propeller cavitation is a severe source
of noise. The cavitation starts when the water flows in the same direction as the propeller
blades. This is where the propeller blades move downwards.
In some cases a resonant phenomenon is set up in a hole near the hull. This sound will have
a discrete frequency, while all other flow noise will have a wide frequency spectrum.
(Image from U. S. Navy in the public domain.)
Flow noise
The upper water layers of the sea contain a myriad of small air bubbles created by the
breaking waves. When the hull moves through water it will cause a disturbance, and this
will generate friction. The friction zone is called the
flow boundary layer . The water flow in
this boundary layer may be laminar or turbulent .
• The laminar flow is a nicely ordered, parallel movement of the water.
• The turbulent flow is a disorderly flow pattern, full of eddies.
A Turbulent flow
B Laminar flow
C Air bubbles
Air bubbles absorb and reflect the sound energy, and they may in worst cases block the
sound transmission altogether.
Simrad CP200-5C Installation Manual