Twenty-Third Symposium on Naval Hydrodynamics (2001) / Chapter Skim
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Marine Propulsor Noise Investigations in the Hydroacoustic Water Tunnel 'G.T.H.'
Marine Propulsor Noise Investigations in the Hydroacoustic Water Tunnel 'G.T.H.'
Pages 262-283
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From page 262... ...
of c ship which is rekted to 6he fluctuating hyd odynamic forces on 6he rotati g blades of the prop ller md on the hull, es w 11 es to 6he fluctuating forces on hull mduced by 6he propeller These fluctuatmg fmces lecd to different typ s of noise es shown m Figme 1: c discrete frequency Imes noise t pe et low fieqmencies rage which cone pond to noise ~adiction from 6he propeller md 6he hull excited by the propeller either di ectly 6 ough th blade pcssage closed to the hull or th ough th shaft bearing The discrete fiequency lines conespond to the bkde ~evolution rcte harmonic k n Z) md th ir amplit des me d6rectly depende t on 6he hull wake m-h mogeneity md prop ller geomeby (m mber of bkdes, skew mgle )
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From page 263... ...
Indeed, the radiated noise inside the ship cannot be predicted from model tests alone. For propeller radiated noise investigations, the similarity conditions summarized in Table 1 compel to make propeller tests at model scale: with the right wake field as at full scale.
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From page 264... ...
= ~3 nFS(PC..t ) R y olds number his to be w high es possible be mse it is impossible to keep eqltl to full scale It ensues same average blade loading even ff Here are slight differences betw en model a d full scale pitches md hull wakes The pressure p is taken m the vertical plume of the propeller t 0 7R al ~ She shaft axis, m order to take into tccoumt the scale effect on She hyd o tatic pressure m She propeller phne if She Froude similarity is notkept Frond similarity F7 = AND This similarity is not compatible with the R y olds m mber similarity Model te t et higher flow peed thm the Frond speed generally prevails Acou tic & St u tmcl similar ity (structural vibration md mdu ed acoustic tdi tion)
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From page 265... ...
While one test section is in use, model preparation can be done on the second test section, as each test section can be isolated using closing doors located upstream plexiglass window 3.5 m and downstream the test sections. The small test section (dimensions: 1.14m x 1.14m x 6m)
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From page 266... ...
— flow noise in the circuit: The break in surface continuity of the internal walls of the test sections, convergents and diffuser are less than 3/10 mm. This prevents from local cavitation induced by boundary layer separation.
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From page 267... ...
is sig Tic mtly red s cd bythehighoumberof rages mdbkdesonrotor Ed starter, Ed She use of water lubricated bearing The broad b Ed noise is largely I wer thm She one of the propulsor mou ted bec Use the flow sped is low m the tmbme Ed She pet u i/ah m Ed the deaeration of the hyd mlic cir nit make the tubme fiee of cavitation Finally, du to the small size of th turbine, s Ed cbsorbmg limogs ar Ed She tu bine cone Mute to lower doss the broad b Ed radiated noise Tests of k g scale propellers Ed contra rotating propeller ~ > 4 mm) in open water or behind dummy hull model are done using c 2 coaxitl shafts d ivmg ystem wish m extermd elecnt 31 motor (250kW-SOOOrpm-2xSOOdaN2 25m daN)
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From page 268... ...
This fluctuating thrust measurements are the first step to investigate the differences that one could expect on sound pressure level at the blade rate frequency and its harmonics, and further more on broad band noise related to wake turbulence interaction with the propeller blades.
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From page 269... ...
From the spatial integration of the pressure amplitude, the resulting fluctuating forces and moments on the hull are calculated with reference to a given co-ordinate system. · Acoustic measurement For radiated noise measurements in closedjet type hydrodynamic tunnel, three major effects have to be taken into consideration: hydrophore support vibration isolation, turbulent flow noise isolation and acoustic impedance between the noise source (propeller)
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From page 270... ...
This tmbuence level is measured in She flequ ncy r mge of IHz - IkHz with c 2D Laser Doppler Velocimeter erJvmced in order to get c signal to noise ratio less thm 02% (forward scattermg mode used without th Bragg cell for flequ ncy ship ) The paticl dishnbution of the Icccl mem velocity m the cross section of th te t sections except She boumduy layer ah is not very large for the maximum discrepa y is less th m 0 2% Deaerction process from m cl content of 100% of the saturation et atmospheric pressme (~24mg liter)
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From page 271... ...
V=8 mls V=6 m|s V=4 m|s V=2 mls hydrophore Even if the sound pressure level of a propeller measured at model scale and extrapolated to full scale do not account for the hull amplification because the model hull is not in mechanical similarity to full scale, we can still compare after extrapolation (given a model scale 1/20 and a test flow speed Vmo~ el) a propeller radiated sound pressure level measured in the GTH that would be equivalent to the minimum measurable sound level (background noise)
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From page 272... ...
(Hz) Figure 11: Extrapolated minimum equivalentfree field sound power density spectrum for a ship with a full scale speed of 12 knots, a model scale of 1120 and a model test at flow speed of 6m/s The extrapolation law applied on the minimum measurable sound pressure level is using the assumption that the measured sound power density level is only dependent on speed (flow speed at model scale and ship speed at full scale)
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From page 273... ...
Cordier & al t19951 showed that the similarity of the wake field is rather well predicted if model tests are run at flow speed equal to ship speed rather than if model tests are run at Froude speed. · Fluctuating forces on shaft: fluctuating thrust on a submarine propeller The wake field is largely modified when a ship is maneuvering and so it is for the radiated noise at blade rate harmonics frequencies.
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From page 274... ...
. Fluctuating thrust Amplitude BR 1~___~ drift angle 10° 111 BR2 ~1;~' ,:, ~ Figure 15: Fluctuating thrust on propeller shaft for 2 drift angles of a submarine Propeller induced fluctuating pressure on hull: cavitation effect Hull pressure fluctuations is not only used to determine the force excitation of the hull but also as a criterion for an acceptable propeller at the design stage for civilian shipyards.
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From page 275... ...
of a twin screw navy ship Another point to clear up in the analysis of the correlation between full scale and model scale for highly skewed propellers, is the effect of the differences between blades geometry and pitch setting on the hull excitation signature. Differences between blades hull pressure signature are largely increased due to the non linear effect of the cavitation.
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From page 276... ...
The model test was made at flow speed equivalent to full scale ship speed. The sound power density
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From page 277... ...
This has become possible because of the hydroacoustic performances of the GTH, its equipment and instrumentation, and specially: - the large test sections of the facility - the control of both nuclei content and air content the low background noise of the facility - the low background noise of the hydraulic motor - the high dynamical sensitivity of the transducers - and model test performed at flow speed equal to ship speed.
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From page 278... ...
& Trlher J., "L G md Tum I Hyd odynamiqu (GTH ", ASME Wmter A mu~l M etmg, Bost m, USA, December 1987 Manoha E., "Broadb md noise fi om z propeller in tmbulent fl w", ASME Symposium on Flow noise Mod Img, Measu ement md Control, Anaheim, USA, 1998 Perraud J., Arnal D., Arehambaud J.P., Perelman O., Judienne A., "Etude expGriment~1e de k t msition de couche limite sous g zdient de p~ession ndgatif et il g md nomb~e de R y olds", Sieme Jou nes de IHyd odynamiqu, Rou, France, March 1995 Serander A., Rittemard P., Deeroek H., "Vibmtion zrudysis of model hull zppendages by z s ummgLzser ibrometer yst m",loternatiomd Co ference m V~bration M zsmements by Laser techmiqu s: Ad ances md zpplications, Ancowg Itsly, October 1994 Raestad A.E., "Tip vortex ind x md engmeering zpproach to propeller noise p~ediction", The Naval A chitec, pp 11 -16, July 1996 Uriek R.J., "Prmciples of umderwater soumd", McG'aw-HillBookCompmy,2~ edition,l975 Wdls C.B., "Development of z compsrative acou tic te tmg procedme for model propellers", Tr msactiom of 6he Royal institution of Nzval A chitects, l989
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From page 279... ...
propeller cavitation number based on flow speed where P is the hydrostatic pressure at shaft axis ............................................................................................................................................. hull fluctuating pressure, amplitude coefficient .................................................................................................................................................................................................................................................
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From page 280... ...
- self propulsion with and without cavitation Kt ,Kq, 11 - acoustics - velocity down and upstream the propulsor Equipment - silent motorization - boundary layer blowing - dynamometry - hydrophone plug and/or streamlined hydrophore and anechoical window - 3D LDV - contra-rotating carter. - electric motorization - dynamometer - 3D LDV - silent motorization - dynamometers on duct and rotor - 3D LDV - hydrophone plug and/or streamlined hydrophore and anechoical window - specific top cover - silent motorization - boundary layer blowing - dynamometer - hydrophone plug - 3D LDV - silent motorization - dynamometer - 6 components force transducer - 3D LDV - hydrophone plug
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From page 281... ...
j i.! Large test section - P=1.6bar - hydrophone plug J ,>~ AL Sound Power density Level dB ret 1uPa & 1 Hz 180 160 140 ~ 120 :t 1 100 80 60 40 2n L Large test section - P=1.6bar - streamlined hydrophore 1 · Background noise of the large test section with and without nuclei injection BUred 1pPa & 1 HzY v Background noise - nuclei injection on/off - V=6m/s 180 160 140 120 100 80 60 40 20 '1 1: In Inn Freq.
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From page 282... ...
~ The concentration ratio is kept between full scale and model scale (scale~l/20) , for critical pressure close to the vapor pressure, but not for the lowest critical pressure (Pcri~.< -500mbar)
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From page 283... ...
fact Gnat w me comparing noise sp ctmm in terms of Pow r Density Spectmm if w do the noise scclmg m temms of Pow r Spectrum the scclmg wouldbe 20 log DP5 I ted to pow r of 2 of ( m )
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