Simrad ST90 - Page 81

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Propellercavitationisaseveresourceofnoise."Singing"propellersmightbeasourceof
noise,whichinterferesatdiscretefrequencies.Insomecasesstaticdischargefromthe
rotatingpropellershaftmaybequitedisturbing.
Cavitation
Cavitationistheformationofsmallairbubbles
closetothetransducerface.Thebubblesappear
becausethelocalpressurebecomesnegative
duringpartsoftheacousticpressurecycles.
Thecavitationthresholdincreaseswiththe
hydrostaticpressure.Thenoiseismadewhen
thebubblesimplode.
Cavitationnoisemayappearnearextruding
objectsathigherspeeds,butmoreoftenitis
causedbythepropellers.Propellercavitationisaseveresourceofnoise.Thecavitation
startswhenthewaterowsinthesamedirectionasthepropellerblades.Thisiswhere
thepropellerbladesmovedownwards.
Insomecasesaresonantphenomenonissetupinaholenearthehull.Thissoundwill
haveadiscretefrequency,whileallotherownoisewillhaveawidefrequencyspectrum.
(ImagefromU.S.Navyinthepublicdomain.)
Flownoise
Theupperwaterlayersoftheseacontainamyriadofsmallairbubblescreatedbythe
breakingwaves.Whenthehullmovesthroughwateritwillcauseadisturbance,andthis
willgeneratefriction.Thefrictionzoneiscalledtheowboundarylayer.Theowin
thisboundarylayermaybelaminarorturbulent.
•Thelaminarowisanicelyordered,parallelmovementofthewater.
•Theturbulentowisadisorderlyowpattern,fullofeddies.
ATurbulentow
BLaminarow
CAirbubbles
Preparations