Kongsberg EM 2040 MKII, Installation Manual

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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.
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
thickness depends on ships speed and on the roughness of the hull. All objects sticking
out from the hull, or dents in the hull, will disturb the flow and will increase the thickness
of the boundary layer. When the flow speed is high, the turbulence can be violent enough
to destroy the integrity of the water. Small voids or cavities in the water will occur and
this is called cavitation.
KONGSBERG EM 2040 MKII Installation Manual