419050/B
79
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 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.
The boundary layer increases in thickness when it becomes turbulent. The boundary
layer is thin in the forward part of the vessel hull, and increases as it moves aft. The
Summary of Contents for SC90
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Page 16: ...14 419050 B Simrad SC90 ...
Page 219: ...419050 B 217 Øverland UPC 3005 Dimensions Cable layout and interconnections ...
Page 223: ...419050 B 221 Øverland UPC 5000P Dimensions Cable layout and interconnections ...
Page 438: ...436 419050 B 427177 Transducer dock dimensions page 494 Simrad SC90 Installation Manual ...
Page 441: ...419050 B 439 Drawing file ...
Page 442: ...440 419050 B Simrad SC90 Installation Manual ...
Page 445: ...419050 B 443 Drawing file ...
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Page 455: ...419050 B 453 Drawing file ...
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Page 463: ...419050 B 461 Drawing file ...
Page 468: ...466 419050 B Simrad SC90 Installation Manual ...
Page 469: ...419050 B 467 Related topics Installing the optional gate valve DN350 page 84 Drawing file ...
Page 471: ...419050 B 469 Drawing file ...
Page 473: ...419050 B 471 214043 Gate valve installation DN350 Drawing file ...
Page 475: ...419050 B 473 422915 Gate valve installation DN350 Drawing file ...
Page 477: ...419050 B 475 083045 Gate valve installation DN500 Drawing file ...
Page 479: ...419050 B 477 33414 Gate valve dimensions DN350 Drawing file ...
Page 480: ...478 419050 B Simrad SC90 Installation Manual ...
Page 481: ...419050 B 479 Related topics Installing the optional gate valve DN350 page 84 Drawing file ...
Page 482: ...480 419050 B 33473 Gate valve dimensions DN350 Simrad SC90 Installation Manual ...
Page 483: ...419050 B 481 Related topics Installing the optional gate valve DN350 page 84 Drawing file ...
Page 484: ...482 419050 B 33498 Gate valve dimensions DN500 Simrad SC90 Installation Manual ...
Page 485: ...419050 B 483 Drawing file ...
Page 487: ...419050 B 485 37357 Gate valve dimensions DN500 Drawing file ...
Page 488: ...486 419050 B Simrad SC90 Installation Manual ...
Page 489: ...419050 B 487 Related topics Installing the optional gate valve DN500 page 103 Drawing file ...
Page 491: ...419050 B 489 Drawing file ...
Page 494: ...492 419050 B Simrad SC90 Installation Manual ...
Page 495: ...419050 B 493 Related topics Installing the optional gate valve DN350 page 84 Drawing file ...
Page 497: ...419050 B 495 Drawing file ...
Page 542: ... 2018Kongsberg Maritime ISBN xxx ...
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Page 544: ...Simrad SC90 Fish finding sonar Installation Manual ...