Simrad SU90 - Page 52

SimradSU90
Thisnoiseisusuallywater-borne.Insomecases,however,shaftvibrationsorvibrations
inthehullnearthepropellermaybestructure-bornetothetransducer.Ifapropeller
bladeisdamaged,thismayincreasethenoiseconsiderably.
Propellercavitationisaseveresourceofnoise."Singing"propellersmightbeasourceof
noise,whichinterferesatdiscretefrequencies.Insomecasesstaticdischargefromthe
rotatingpropellershaftmaybequitedisturbing.
Flownoise
Cavitation
Everyobjectthatmovesthroughwater
willdisturbthemedium,anditwillcause
frictioninthewater.Thefrictionzoneis
calledtheowboundarylayer.Theow
inthisboundarylayermaybelaminaror
turbulent.
•Thelaminarowisanicelyordered,
parallelmovementofthewater.
•Theturbulentowisadisorderlyow
pattern,fullofeddies.
Theboundarylayerincreasesinthickness
whenitbecomesturbulent.Theboundary
layeristhinintheforwardpartofthevesselhull,andincreasesasitmovesaft.The
thicknessdependsonshipsspeedandontheroughnessofthehull.Allobjectssticking
outfromthehull,ordentsinthehull,willdisturbtheowandwillincreasethethickness
oftheboundarylayer.Whentheowspeedishigh,theturbulencecanbeviolentenough
todestroytheintegrityofthewater.Smallvoidsorcavitiesinthewaterwilloccurand
thisiscalledcavitation.
Boundarylayersunderneathavessel
ATurbulentow
BLaminarow
CAirbubblesinthewater
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