Kongsberg Simrad ST90, Reference Manual

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• 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.
Rattle noise
Rattle noise may be caused by loose objects in the vicinity of the transducer, like fixing
bolts. The rattle may also come from loose objects inside the hull.
Interference
Interference from other hydroacoustic equipment on board the same vessel may be an
annoying source of disturbance. Unless the same frequency is used for more than one piece
of equipment only the transmitted pulse will contribute to the interference.
In physics, interference is the phenomenon in which two waves superpose each other
to form a resultant wave of greater or lower amplitude. Interference usually refers to
the interaction of waves that are correlated or coherent with each other, either because
they come from the same source or because they have the same or nearly the same
frequency. Interference effects can be observed with all types of waves, for example,
light, radio, acoustic, surface water waves or matter waves.
Concept descriptions