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OCR for page 262
MARINE PROPULSOR NOISE INVESTIGATIONS
IN THE HYDROACOUSTIC WATER TUNNEL
"G.T.H."
D. Frechou, C. Dugue, L. Briancon-Mariollet,
P. Fournier, M. Darquier, L Descotte, L. Merle
(Bassin d'Essais des Carenes, Chaussde du Vexin, 27100 Val de Reuil, France)
Smce its fi t use m 1988, the G md Turmel
Hyd odynamique (G T. H. ) hcs proved that it is c
mther mique experimentcl faility to conduct
i movative exp riments to improve the desig of
ship propulscrs speciclly wi6h regard to caviUtion
md hyd oaou tic performances This pcper
p~esents c ~eview of 6he experimentcl capabilities of
6he turmel md of the mecsurmg techmiques used,
wi6h emphasis on 6he sig fficmt cd ance in
propulsor noise mve tigations obtained fiom 6he
model tests pe fommed in 6his faility
2. INTRODUCIION
For seve~al years now, the noise reduction hcs been
c major gocl for the hyd odynamichyd oaoustic
st dies not only on Nc y ships, but clso on ships
like oceaog ahic or seismic research vessels md
cruise Imers From c general st mdpomt, th ee
domcms of mte~est [Aucher, 1996] m be
distinguished:
th flow noise which is th wall pressme
fluctuations induced either by tmbulence or
t~avellmg bubbles or b~eckmg of waves, which
demecses the pe fommance of c soncr sy tem
the ~adicted noise m 6he far fleld (~lOOm) 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 ),
c discrete flequency Imes noise et shaft rcte
harmonics might clso occurs ff the shaft line
p~esents c mech miccl problem (shaft
alig ment or torsion, g armg mcl-
perfommance ) md ff 6here me diffe~ences
betw en blades geomet y or bkdes pitch
setti g or bkdes eksticity,
c discrete fiequency Imes noise typ et
medi m fiequencies rmge which results
fiom 6he hull ~adiction excited by all the
mterm~l mahmery of 6he ship (motcr,
~eduction gear css mbly ),
c d6screte flequency lines noise type, et
medi m flequencies rmges, k ow es
"propeller si ging" which ~esults fiom 6he
fluid-structme interation et 6he hailmg edg
of the propeller bkdes [Bkke, 1977],
choadbad noise et low, medi m adhigh
fiequencies r mg which ~esults from the
fluctuctmg hyd odynamic forces on hull
mduced by 6he hull bo mdary Icyer
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turbulence and from the fluctuating forces on
blades induced by the inflow turbulence of
the wake (fJonson, 19951, PKirshner & al,
19931, tManoha, 199814. The broad band
noise is highly increased as soon as the
cavitation is appearing (high loading on
blade at maximum ship speed or under
trawling operations). As cavitation is
developing, the medium frequencies range
then the low frequencies range are concerned
and the blade rate frequency lines amplitude
level are also increased because the hull
excitation is increased LBaiter, 19921.
· the radiated noise inside the ship which more
concerns the passengers cabins of cruise liners
("Holland & Wong, 19951, fRaestad, 199614.
The generation of this noise is related to the
propeller induced hull excitation and the
response of the hull to this excitation (transfer
function of the hull).
\ propeller induced
_~! ~ _ hull vibrations
Force fluctuations
-. P ~ on shaft
. ~ ~ -~~ ~~llf~vtbratiorls~
~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ .
_ \ ~~ ; ~~~~~
~:~:>
sQIJrid~ra~ated
froln~pr~3~1er
(c~tati~,~sir~S~g)
~:~ ratliated~
rr~'n~h,'~ll
Sound power density Level
dB ref. 1,uPa & 1 Hz @ lm
lOdB:
. ~ ~~ ~1Q developped
~.~.~.~.~.~.~.~.~.~.~.~.~.~.~.~.~.~.~'~'~ ~~ ~c^~n
|| L ~ ~ developped
1 _ ~ ~ ~ ~ ~
I a d e s ~ ~ ~ a I /
swing / \
. .~.~.~.~,~,~,~,~,~,~,~,~,~,~,~,~,~,~,~,~
20 dB / decade
ma Leer- p~ropeHer~
ed~ propeller -1~11 induced
~ ~ ~ ~ ~ ~hro~ld~halli~iLl~1lr~i~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ hro~1 Harry oi ~~
1 000
cavitation
inception
/
1 0000
Freq. (Hz)
Figure 1: Sound pressure level radiated by ship
with cavitating and non cavitating propeller
With regard to the ship radiated noise in the far
field, it is necessary not only to investigate the
propeller radiated noise but also the hull radiated
noise. The investigation of the later one is generally
difficult to make using experiments on model of the
ship (hull & propeller) at reduced scale. The main
reason is that the hull model and shaft arrangement
are often difficult to manufacture with the same
mechanical structure as at full scale. Nevertheless, if
the model hull is stiff enough, it is possible to
investigate the propeller induced hull excitation, by
measuring the fluctuating forces induced on hull by
the propeller either directly or through the shaft
bearing. These hull fluctuating forces can be then
introduced as inlet data of computational vibro-
acoustics codes, for hull vibration and radiated
noise prediction. This means that from model scale
experiments in tunnel, we can only investigate
differences in radiated noise from different
propellers and that it is not possible to really
forecast the ship radiated noise. 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. This
means to have a facility with large test section in
which the complete or modified (dummy) hull
model can be used, and with high flow speed to
overcome the viscous scale effect between
model scale and full scale on the hull boundary
layer development,
with the same material (same mechanical
structure) as full scale for the propeller and with
a flow speed equal to the full scale ship speed,
with the same local pressure on blade (the flow
velocity is equal to the ship speed) and with
nuclei control for cavitation similarity,
- with a facility that allows a very low "minimum
measurable noise source level".
We should keep in mind that these similarity laws
are only for a ship in calm water. Additional
similarity laws are needed for ship in waves.
OCR for page 264
Similarity
Geometrical similarity
N m dimensiomd parameter
geomet y,
P D for controllable pitch
propeller
mplications
he Occupy of the propeller geomeby is
peciwlly impo tat for the bade profile
Heading edge, nailing edge md 1 1a de tip)
A minimum model scale ratio is Then
requited
impossible to keep equal but necessary to
behigher thm 5 107
She complete hull model ( t le at the scaled
after body of She hull) is needed for c good
simul tion of She effective wake
Wake similarity (effective
wake)
(i e advance r tio similar ity)
V V,VtV,VrV
Re = ; K for She hull
Viscous flow on -'. propeller ~ D K f D
bkdessimibrity Re= ; ; St=
Similarity of prop Her
loading
Cuvit tion similarity
K T
p n D
~ Pa 07t Pv
p t33D3
+ m lei conte t
Pa = Pa
nm( p, ) = ~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)
Ma=
cw
D ; PI;
tructu e
same fluid (water)
~ same flow sped et mod I scale md full
scale
This is possible for She propeller but it is
very dffhcut to extend to the hull ad shaft
I'ne
his mainly concerns She characteristics of
She facility
Additiomd hequi ements
no had odynamicbicckage
effect
he field like acoustic
propagati m
he su ftce
Table 1: Summary of the simdlarity laws for hydroaeoustie tests on shop model
OCR for page 265
3. PRINCIPAL CHARACTERISTICS OF
THE G.T.H.
The GTH is a closed circuit tunnel of demineralized
and decarbonated water fully comparable to wind
tunnel. We recall hereby the principal
characteristics of the GTH which have been already
largely detailed (LLecoffre & al, 198714.
Figure 2: Overview of the test sections of the GTH
plexiglass
window
plexiglass
window
plexiglass
, window
1.14m \
/
2 m
\ _
plexiglass
window
SMALL TEST SECTION LARGE TEST SECTION AS CLOSED TEST SECTION
Figure 3: Cross section of the large test section and the small test section
.
Two test sections are available with large
Plexiglas windows (33 in the large tests section
and 21 in the small test section) that give a high
visibility on model. 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) is
more dedicated to studies at very high speed and
the large test section (dimensions: 2m x 1.35m
x tom) is more dedicated to propeller with
complete hull tests.
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Representative terms from entire chapter:
radiated noise
The water velocity and pressure are
continuously variable: tO
turn I is u oupled fiom the buildmg itself
by me ms of vibration absorbers Finally, the
vibration level of the laurel is less thm
Imm/s~
noise from the mom p mp: The 10 bled s
rotor axial p mp has been desig cd to be
free of cc itstion Ed to have c low shaft
revolution rate er en et high sp eds Ed low
ambient pressure conditions for both test
sections The water lubricated be mg has
also been desig cd to ensue kminar flow m
the chambers
noise from machine y: Pipmg Ed a diary
p mps have l en nut no Ilk isolated fiom
the tunnel They are gath red with She motor
Ed reduction gear of She p mp, in th
engine room with con Pete walls Ed ceding
that isolated She mom buildmg of 40dR from
airborne noise
noise fiom m lei pr do non: She m lei
production is generating noise at high
frequ ncies If: Itch ) This noise is not
crni cl for radiated noise mecsmements of c
cavitating propeller How ver, this noise is
trod lesome for radiated noise mecsmements
of c non cavitati g propeller in order to get
rid of this problem, th propeller noise
te tmg procedme includes et first c
determimttion of She cavitating domain (a,,.
Kt, P/D) of the propeller using the m lei
injection, Hen c noise meaner merit with
m lei injection for propeller operating
conditions wish cavitation, Ed finally c
noise mesmement witho t m lei injection
for propeller op rating conditions without
cavitation
4. MODEL EQUIPMENTS AND
INSTRUMENTATION FOR THE G.T.H.
4.1 Model equipment
The table m Appendi I s mmarizes the different
model test co figurations in bodh te t sections We
should mphcsie some of 6 is arr mgements:
The models are as much es possible supported
by She top cover of the te t sections, so that She
mo mtmg Ed She prelimmuy tests of the
in trumentation is done outside the test section
Specific top covers are as enable for smfae ship
Ed mde water vehicle (submarine, torpedo,
AW ) Any of She Pl:xighs ssmdosss c m be
replaced by acoustic wind ws (so Ed adsorbing
Imi g window or hyd ophone army window) or
c 6 components lorce balance for foils window
The shaft d ivmg >! tem for propu sors tested
with the ship hull propeller diameter of 20 mm
- 30 mm Ed hull oveeall lend h of 4m - m) is
done by m immersed AC elech ic motor (I OkW-
3200rpm) for She non Icon tic te Is Ed by
hyd mlic tab es (Type 1: 45kW-SOOOrpm;
Type 11: 50kW-2000 pm) for acoustic te Is As c
matter of fat, it is importmt not to lower the
hyd otcou tic quality of She tunnel by using
noi y shaft d ivmg sy em The use of hyd mlic
axial Ed muti-stage tab es provides several
tdh-mttges compared to mechmical Ed
electrical motor The frequ ncy line t pe noise
gearing noise, ball be tl3 noise,
electromagnetic noise) 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-2xSOOdaN-
2 25m daN) that c m be inclir.ed of +10°
For Icon tic studies of large scale propellers Ed
pumpjet, c silent shaft d ivmg ystem is used
with c high power hyd mlic tubme (530kW-
1500rpm-4000daN)
4.2 Instrumentation
~ Data acquisition systems
Eve y t pe of me decrements his its ow date
Requisition system: forces meaner merit on shaft
linckudmg th mgukr position of She shaft Ed She
revolution rate mecsmements), lorces measu merit
on cppendag s (includmg She angular position of
She tppendtgel. tatic pressures measurement m
model, Laser velocimeby measurement, aoustic-
v~brctiomflu lusting forces mecsmement Each dhh
acquisition sy tem aqui es She flow conditions in
She te t section (sped, pressme, temperatme, air
content level, nuclei injection settings). In addition,
an Ethernet network makes the communication
between data acquisition systems very easy.
· Cavitation images recording
Cavitation images on models are recorded with
standard video cameras and with stroboscopic
lights.
Video No 7 ~
Figure 5: Visualization and video recording
arrangement for cavitation on rotating propeller
The strobe lights are triggered with a shaft encoder
signal that allows selecting a given blade angular
position. The operating flow conditions (speed,
pressure, shaft revolution, blade angular position)
are directly fitted into the video images. Two views
are at least recorded: either two cameras looking at
the suction side of the blade from both side of the
test section, or one camera looking at the suction
side of the blade from one side of the test section
and a second camera looking at the pressure side of
the blade from the same side of the test section. The
radiated noise is recorded on the audio channel of
the video tape.
The cavitation images can be digitized and analyzed
through numerical image processing algorithms
tGodefroy & al, 19981 in order to obtain statistical
information on the 2D dimensions of the cavitation
and location on the blade surface at different
angular positions.
· Fluctuating forces on shaft measurement
- Mean thrust and torque measurements:
In house designed dynamometers are mounted on
propeller shaft for mean thrust and torque
measurements. These dynamometers use strain
gauges technology sensors and special design to
minimize cross-talk between torque and thrust
measurements sections. They are also able to work
at low pressure level (5kPa) and high shaft
revolution rate (n=5000rpm). However, this type of
dynamometer is not able to measure time-dependent
thrust and torque.
- Fluctuating thrust measurements:
For the fluctuating thrust measurement, an unsteady
thrust dynamometer (Figure 6) is integrated in the
shaft closed to the propeller hub. This dynamometer
is similar to the one developed at ARE Penn State
fJonson, 19951.
~ low compliance section
/ piezo-electric sensor
hemisphere
Figure 6: Thrust fluctuation measurements
The sensor is a piezoelectric crystal that provides a
high stiffness mounted on the shaft centerline with
steel hemisphere to be insensitive to side forces and
bending. The crystal is pre-loaded so that thrust
fluctuation in both axial directions can be measured.
Because of the high stiffness of the crystal, this
technique is able to measure very low thrust
fluctuations (~\T/T << 1%) . The shaft mass is at
least 10 times higher than the propeller mass in
order to obtain an impedance break. A pre-
amplification of the piezoelectric signal is included
in the shaft before the slip ring transmission to
increase the signal to noise ratio. Within the
frequency bandwidth obtained that goes up to lkHz,
there are inevitably resonant frequencies of the
whole shaft (between propeller and drive motor)
that can be considered as a multiple lumped-mass-
spring system. This is the reason why a force
calibration is made using a dynamic force shaker at
zero rpm of the shaft. From the acquisition of the
thrust triggered by the shaft encoder signal, a
synchronous analysis is made to sort the frequency
lines related to the shaft revolution rate and its
harmonics, and the propeller inflow spatial
periodicity. It is then possible to compare the effect
of different propellers geometry or different wakes
fields on the same propeller.
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.
· Fluctuating forces on hull measurement
Fluctuating forces on the hull induced by the blade
passage closed to the hull are measured by a
pressure transducers array. About 20 transducers
are flush mounted on the hull surface just above the
propeller plane. The afterbody of ship model is
stiffened using glass reinforced plastic in order to
measure only the hull excitation and not the
excitation with the response of the hull
, '::w \
/ shaft encoder
/ 1-
Figure 7: Hull pressure transducers arrangement
on stiffened hull with a cavitating propeller of an
oceanographic vessel
The pressure signals acquisition are triggered by a
2048 pulses shaft encoder. A spectrum analysis is
then process on the pressure signal in order to get
the pressure amplitude at the blade rate frequency
and its harmonics. As for the fluctuating thrust
measurements, the hull pressure transducers are of
piezoelectric type (equivalent to hydrophore
transducers). These transducers measure only the
unsteady part of the pressure but with a high signal
to noise ratio (>80dB) compared to classical
pressure transducers such as strain gauge type. This
is necessary when we want to look at the high
harmonics pressure amplitude without any
cavitation on the propeller or to compare the hull
excitation of two propellers geometry. 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) and the hydrophore .
Figure 8: Radiated sound measurements
(hydrophore plug and streamlined hydrophore
fairing)
The principal acoustical techniques (Figure 8) used
in the GTH were designed to overcome these
problems:
— hydrophore plugs for the radiated noise in the
cross section of the main flow. These
hydrophore plugs are flush mounted and are
made up of a standard hydrophore in a box
filled with polyurethane coating. The
polyurethane elastomer, the box dimensions
and the location of the hydrophore relative to
the internal wall of the tunnel were chosen in
order to provide both vibration isolation from
wall structure accelerations and attenuation of
turbulence near field wall pressures.
Concerning flow induced pressure
fluctuations, the hydrodynamic wave lengths
are so short that the hydrophore plug
dimensions is doing a spatial filtering of the
turbulent boundary layer pressure fluctuations.
As a matter of fact, the effective wave length
of these pressure, i.e.
(/ ~0.7V/f with V < 20m/s), are at least
100 time shorter than the acoustic wave length
from the source (/ ~ c/f with c=1450m/s).
— one streamlined hydrophore for measuring
noise from downstream the model. This
hydrophore is made up of a piezoelectric
sensing element inside a shell head
m mufatmed m polymer me cocti g he
nose geomet y of the heed shell, m some way
similar to sonar dome, is desig cd to develop c
table Seminar bonndsrv layer which is not
sensitive to charges m turbulence level or
direction of She up beam flow
Using different measurement techmiqu ., w should
keep m mind Nat closedjet te t sections me not
fiee-field en ir merit he large difference of
aoustic imped mce between water Ed the
ensemble Pimple ighs-st~inless feel of the tunnel
shuctme make different sound power propagation
dep ndmg on th fiequ y we w mt to look at in c
short cut, we c m say that for c noise son e located
on the te t section axis, She propagation is of plan
wave t pe et low flequ ies (wave length higher
f m She characteristic lend h scale of th test
section, which me propagatmg in She te t section
axis di ection) Ed of spherical wave t pe et high
fiequ ncies Iwese length low r f m She
characteristic le 3th scale of She test section) h
order to redu e She reverberation et high
fiequ ncies, two sound absorbing Immg windows
have been built in order to assess These aoustic
'blockage" effects, aoustic cclibmtion me made
using c k ow sound son e located on the test
section axis, Ed wish water turn I velocity equal to
zero he complex t msfer fu tion to apply to :.
Deceived aoustic signal et She measurement yst m
is Hen identified provided Nat She coherence
between She Deceived sinful Ed the son cc signal is
close to one
he aoustic dam Requisition sy em is able to
process 32 slog mput signals at c sampling rate of
2 MH, Ed wish c 14 by words analog to digital
converters ad ari-alissmg filters St hndsrd Ed
sp ciflc dab processing tools me avibble mch as:
auto- peer an Ed cross-spechum analysis, joint
time-fieqmency crurlysis, harmonic ably is,
fiequ y d modulation, frequ ncy Ime d tection
algorithm, coh rence analysis
~ Vibration measurement
For noise irsestigations rented to flow indu d
shuctmal vibrations, coherence crurlysis is
pe fommed fiom noise signals Ed v~brcti m signals
For specific tests with model m mufatmed (foil,
du ted propeller) Ed tested acordmg to the
hyd oekstic similarity (ie same flow sped es full
scale Ed same mechanical tructme betw en mod-l
Ed full scale 1, the model response to the flow
excitation is mecsmed using t mdard
a elerometers The main d awhsck using standard
acelerometers is Nat the volume needed for the
acelerometer location in the model c m locally
ch mge She tru the response of th model The use
of Laser vibrometer Polytec) ermines to get rid of
f is problem on mall scale models [Ser aider & al,
1994]
Futhemmme vibration mecsmements on She hull
model Ed on She shaft Ime elements m wan the
test operator cutout my doubles me noise remlting
from unexpected pe formances of Hose parts of the
test arrangement
5. H tT~RO.~ 01: SO C PERFORMANCES OF
THE GTH
5.1 Kinede performances of the n 0.
Owing to She connation ratio of the convergent,
Ed the honeycomb Ed flow shaighteners of the
te t sections, She boumduy Dyer me of 40mm et
the inlet of the test sections ad of 10 mm et th
outlet of th test sections The turbulence level
(ratio of RAdS velocity Ed me m velocity) is of
0 3% over She flow sped rage (0 - 20 m/s)
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) to m air content of 30% of She
sahlrahon et ctmo ph no pressure (~7mg iter)
is done withm 2 hou s it is Hen possible to
carry out She dearction process es soon es it is
necessuy, which is not She case of most of She
hyd odynamic turn is As c matter of fat,
degassmg c tunnel I kc th GTH without
microbubbles injection Ed without c bubbles
separating talk, c m take more f m 12 hou s
St hndsrd cl conte t for cavitation Ed acoustics
te Is is 7mg iter
.
Lmin imum G + LbackgrOund
(@ lm)
~ 20 logy ) + Lbackground
receiver / source
The background noise of the GTH (See Appendix II
and Figures 9 & 10) is mainly dependent on the
flow speed, provided that the air content is low
enough.
The background noise measured with the
streamlined hydrophore is lower in the low
frequencies range because of the laminar boundary
layer developed on the head form of the
hydrophore but it is also lower in the high
frequencies range because of the low background
noise of the hydrophore sensing element.
The nuclei content control is compulsory in
hydrodynamic tunnel dedicated to cavitation
studies Cavitation Committee Report of 20th
ITTC, 19931. The injection process (flow rate,
water gassing pressure, number of injectors in
use over the 121 available ones) is able to
control the nuclei content from 0.1 nuclei/cm3
up to few tens nuclei/cm3 with an average
diameter of 50,um. Several studies (for instance
tGindroz & Billet, 19931) have confirmed the
merit of the nuclei control of the GTH for
cavitation tests.
5.3 Hydroacoustic performances:
· Minimum measurable sound source level ~~ ~0
The minimum measurable sound source level in a
tunnel is defined LAbbot & al, 19931 as the
minimum level of an equivalent sound source to the
propulsor, which can be measured by the acoustic
receiver, in the same flow operating conditions as
with propulsor (i.e. noise source located at the same
location as the propulsor, same flow speed and
pressure and same air content and nuclei content).
The minimum measurable sound source level is
then related on one hand to the background noise of
the tunnel and/or to the background noise of the
acoustic receiver, and on the other hand to the
transfer function G between the source and the
receiver. For frequencies range in which the
propagation is a free field type (f > lkHz), the
transfer function gain was found to be close to a
spherical spreading loss . This leads to a minimum
measurable sound source level defined as:
i60
i40
i20
loo
80
60
40
140
120
00
80
60
40
Sound Power Density Level
dB ret I}'Pa & 1 Hz
Test section pressure:
Hydrophone plug P=1.6 bar
20
100 1 non
Flow speed
V=lO m/s
.~ V=8 m|s
V=6 m|s
V=4 m|s
V=2 m|s
4~
0000 100000
Freq. (Hz)
Figure 9: Background noise of the large test
section measured with the hydrophore plug
Sound Power Density Level
dB ret I}'Pa & 1 Hz
Streamlined hydrophore Test section pressure:
~ ~ 1
__
160
Flow speed
14~
LO ~1
20
0 100 1000 10000 100000
Freq. (Hz)
Figure 10: Background noise of the large test
section measured with the streamlined
~ V=lO m/s
....~.... 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), with a target sound pressure
level of the full scale ship noise. We took the
example of a target sound level of an oceanographic
research vessel at a speed of 12 knots. The graph of
figure 11 clearly points out the capabilities of the
GTH for the hydroacoustic studies of propulsors.
The limitation at frequency below 3Hz is not critical
because the propeller blade rate frequency line is
always higher and the propeller broad band
signature is in the frequency range where the margin
is more than 30 dB.
Extrapolated minimum equivalent free field sound Power Density Spectrum
based on noise measurements in the large test section
Sound Power Density level
dB ret Papa & 1 Hz @ lm + model test (scale 1/20) at V=6m/s
~ ship noise target level at full scale 12 kts
160
150
140
130
120
1 1 0
100
90
80
70
60
1 10
l _
;\_
1 00 1 000 1 0000
Freq. (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), on the
distance in between the source and the receiver, on
the scale ratio:
for a cavitating propeller
4>pp(f )
2 V4 ( ~ )( ~ )2
r2 ~
as a function of f = f
V
and for a non cavitating propeller
as a function of f = f
· Shaft driving motor noise
The minimum measurable sound level of the tunnel
should also take into account the background noise
of the shaft driving motor. Using hydraulic turbine
as shaft driving motor avoids degradation of the
hydroacoustic performances of the GTH.
Figure 12 shows that the background noise of a
shaft driving motor using hydraulic turbine is lower
than the background noise of the tunnel for a given
flow speed in the small test section. It then becomes
possible to measure the propeller sound radiated
without any cavitation, which is impossible with
standard electrical motors.
Sound Power Density level
dB ret IpPa & I Hz
100
Hydrophone plug
1 000
1 0000
Hydraulic turbine in use, zero flow speed and bare hub
V=Om/s -n= 1200 1pm - bare hub - without nuclei injection
· Stopped hydraulic turbine, flow speed and bare hub
V=6m/s - n=0 rpm - bare hub - without nuclei injection
100000
freq. (Hz)
Running propeller (D=300mm) without cavitation
V=6m/s - n=1200 1pm- non cavitating propeller - without nuclei injection
Running propeller (D=300mm) with cavitation
V=6m/s - n=1500 1pm - cavitating propeller - with nuclei injection
Figure 12: Sound power density level using
hydraulic turbine in the small test section of the
GTH
6. SOME RESULTS OF HYDROACOUSTIC
SURVEYS IN GTH
Several studies have been carried out in the GTH
that emphasize the hydroacoustic performances of
the GTH. We present hereafter some examples of
these studies: study on the flow noise of transient
and turbulent boundary layer, study on propeller
induced fluctuating forces on shaft, study of
cavitation effect on propeller induced fluctuating
pressure on hull, noise radiated on propeller.
· Flow noise of transient and turbulent
boundary layer
In order to investigate the instability of laminar
boundary layer on a sonar dome, an experiment
was carried out on a laminar boundary with a
pressure distribution with negative gradient. The
boundary layer development was made on a flat
plate on which flush mounted transducers were
installed and a second plate with an appropriate
geometry was used to force a negative pressure
gradient along the flow axis (Figure 13) on the first
plate. In total, 17 fluctuating pressure transducers
flush mounted with a pinhole of O.lmm diameter, 3
hot film sensors for wall shear stress measurements
were used and velocity profile was measured using
Laser Doppler velocimeter in forward scattering
mode. Thanks to the very low turbulence level and
the very low background noise of the tunnel, a
laminar boundary layer of 1.8m at flow speed of
lOm/s (i.e. to Reynolds number of 18 106), was
achieved in this experiment and the sensitivity of
different roughness heights on the boundary layer
stability has been studied LPerraud & al, 19951.
Figure 13: Test set-up for boundary layer
development with an adverse pressure gradient
· Propeller operating condition with and
without cavitation
Before investigating the radiated noise of a
propeller, it is important to know the domain of
operating conditions of the propeller with and
without cavitation. As a matter of fact, the presence
of cavitation even at the inception point induces
large increases of the radiated noise and the
fluctuating forces on hull and on the shaft. Model
tests are therefore performed to explore different
operating conditions of the propeller (i.e. to
different Kt, On, P/D). The nuclei content has then a
major effect on the determination of the cavitating
and non cavitating domains of the propeller
operating conditions tGindroz & Billet, 19931. The
figure 14 shows a comparison between model scale
and full scale of inception point of tip vortex
cavitation on a marine propeller.
Cavitating / non cavitating domain
of tip vortex cavitation on a marine propeller
~ i IFS _ ~ FS ~
(<5 i Am ~ Re,,, )
~ non cavitating
· Full scale data
Advance ratio
Figure 14: Comparison between full scale and
model scale of cavitation inception points
This comparison can only be done if the similarity
of the loading of the propeller blades are the same.
This means that not only the global loading should
be equivalent (Ktm = KtFS) but also the radial
distribution of blades loading should be equivalent.
The later requirement imposes to have a hull wake
field similarity between model and full scale.
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. The fluctuating
thrust modification, when changing of course, gives
a good approximation of radiated noise
modification at blade rate harmonics frequencies.
Figure 15 presents the results of fluctuating thrust
measurements at first and second blade rate
harmonics frequencies for a propeller of submarine
tested at two drift angle 0° & 10°, in the large test
section of the GTH. The measurements were done
using the set-up described in the instrumentation
paragraph. The drift angle largely increases the
fluctuating thrust and the residual fluctuating thrust
with a bare hub instead of the propeller is far lower.
Therefore, it is possible to predict that the radiated
noise at these frequencies will increase of the same
level.
~ ~ -
Drift angle 0°
Drift angle 10°
_ I,;
revolution, over 250 shaft revolutions for both full
scale and model scale measurements. The results at
model scale have been obtained at two flow speeds,
the lower one corresponding to Froude speed and
the higher one corresponding to the highest flow
speed that was possible to achieve.
Hull pressure
Amplitude
Full Scale GTH at V=10.8m/s
/ ..
~ ~ A
hi
i..................
.
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. Current practice
PCarlton & Bantham 19971 gives the following
acceptable hull pressure amplitude at the first blade
rate frequency:
General ship type
Cruise liner
Ro/Ro Ferry
Container and fast _
Cargo ships
Slow bulk trade ships
_ Typical blade rate hull
surface pressure range
_ (freq. = n.Z)
1 - 2 kPa
2 - 4 kPa
~ - ~ kPa
4 - 7 kPa
A comparison between model and full scale results
is shown in figure 16, for a fixed pitch propeller of
a tanker. The instantaneous measured pressure
signal is averaged for every fraction of shaft
GTH at V=2.5 m/s
Fraction of shaft revolution
Figure 16: Time trace over I shaft revolution of
the hull pressure signal of a 5 blades propeller of
a tanker
The results show that the cavitation at low flow
speed, is very unstable, which is not the case at high
speed. This clearly demonstrates that keeping the
similarity of the classical dimensionless numbers
(Gn, Kt, P/D) is not enough for a good
representation of the full scale signature and that the
test should be performed at maximum flow speed,
i.e. to flow speed as close as possible to full scale
ship speed, in order to simulate the right wake and
to get a more stable cavitation pattern Wordier & al,
19951.
Full scale hull excitation at blade rate harmonics has
now become so low, specially on twin screw ships
with highly skewed propellers, that vibration
induced by the broadband background energy of the
hull excitation become questionable (tCarlton &
Holland, 1998-199914. This broadband energy is
related to cavitation collapses in the tip region of
the blades. As shown by the Figure 17 for a four
blades propeller of a cruise liner, as the cavitation is
developing (i.e. Cavitation number Gn decreasing),
the broadband level induced by the collapse of
cavities tends to merge towards the low frequencies
range and this increases significantly the high
harmonic levels although the 1 St blade rate harmonic
amplitude is not modified. Typical cavitation
pattern related to this phenomena and to highly
skewed propeller is a combined sheet and tip vortex Hu11 pressure fluctuation level
cavitation.
Spectrum analysis of a hull pressure signal
Kp 4 blades propeller of a twin screw cruise liner
sli].3~
0 5 10 15
fin
20
Figure 17: Spectrum analysis on hullpressure
fluctuations on a 4 blades propeller with and
without cavitation of a twin screw ship
As already mentioned, it should be recall that the
hull structure response to the hull excitation is an
important issue not to forget. As matter of fact,
results of measurements at full scale of pressure
fluctuations and vibration level at a same location
on the hull above the port side propeller of a twin
screw navy ship (Figures 18 & 19) clearly point out
the amplification of the hull in a broadband
frequency range.
20 dE,
1 10
Figure 18: Full scale hull pressure fluctuation
for different RPM of the port side propeller
(fixed pitch) of a twin screw navy ship
Hull vibration velocity fluctuation level
20(1Bt
0 1 oo 1 ooo 1 oooo
Freq. (Hz)
Figure 19: Full scale hull velocity fluctuation
for different RPM of the port side propeller
(fixed pitch) 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. This is shown by Figure 20 which
presents the pressure signature with and without
cavitation (on=1~7 & an=8~0) for a same loading of
the blade (same Kt) and with maximum differences
of pitch setting between blades of 0.5°. This also
rises the necessity to make the harmonic analysis of
the hull pressure signals rather on the shaft rate
component basis than on the blade rate component
basis. With no blades differences, the shaft rate
component should not exist.
Averaged pressure signal over one revolution
Kp
o
Kp
fraction of revolution
Harmonic decomposition of the pressure signal
11R
RR
( 6n=1.7- Kt=0.22)
· without cavitation ( 6n=8.0 - Kt=0.22)
Figure 20: Effect of blades differences on
pressure signature of non cavitating and
cavitating 4 blades propeller
These remarks point out the importance of the
accuracy of the pitch setting of the blades of
controllable pitch propellers: typically an accuracy
of less than 0.2° at model scale as well as at full
scale is needed. In the case of highly skewed
propeller with either fixed or controllable pitch, the
accuracy of the tip region geometry of the blades is
also of much concern. In this respect, the propeller
manufacturing tolerances, specially at model scale,
have to be better than the ISO class S tolerances.
Finally, it should be recall, that the full scale ship
trials and propeller operating conditions must be
known with a great confidence in order to improve
the prediction of the higher harmonic pressure
amplitude from model scale Cavitation Induced
Pressure Fluctuations Committee Report of the 22
ITTC, 19991.
· Propeller radiated noise:
Concerning the radiated noise at blade rate
frequency and its harmonics, similar conclusions as
the one presented on the hull pressure excitation can
~ blade No.1 balde No.2 blade No.3 blade No.4 ~
be made, specially on the cavitation effect on the
level increase of these frequency lines and on the
hull amplification effect.
The propeller broad band noise level is very
sensitive to cavitation. Figure 21 shows the broad
band signature of a navy ship type propeller
measured in GTH operating at the same shaft
revolution rate, same flow speed but at two different
flow pressures such that one case is with cavitation
(ov=1.5) and the other one is without cavitation
(ov=24. A reference curve of noise radiated in the
same operating conditions but with a bare hub
instead of the propeller is superimposed. The results
show an increase of 20 dB of the broad band
radiated noise once the propeller is cavitating
(~v=1.54.
Soulful power rlensity spectrum
L: ..
20 dB | ~sig=l,S ~ ~ l
~ sig=2
—Bare hub
Frequency
Figure 21: Sound Power density spectrum of a
propeller of a twin screw navy ship with and
without cavitation at model scale
The frequencies lines at medium frequencies range
that appear on the spectrum and which do not
depend on the propeller rpm, result from the so
called 'propeller singing" induced by the trailing
edge geometry. If this frequencies lines do appear at
model scale, they will or will not appear at full
scale. On the contrary, if they do not appear at
model scale, they will not appear at full scale. This
is due to the fact that the trailing edge vortices
activity is reduced as the Reynolds number
Increases.
Figure 22 presents a comparison between radiated
noise of propeller extrapolated from model test and
radiated noise measured at full scale on a submarine
propeller at operating conditions without cavitation.
The model test was made at flow speed equivalent
to full scale ship speed. The sound power density
spectrum at model scale is extrapolated using the
following formula:
( PS )
LFS =Lm +30 10g( PS )
which is based on the dimensionless formulas of
paragraph 5.3 where the reference length scale is
taken equal to the propeller diameter and VFS is
equal to Vm..
Sound power density spectrum
20 dB;
~"~
ml `,.~ ~ ~
—Full scale
~ . GTH
Figure 22: Extrapolated model scale and full
scale sound power density spectra of a
submarine propeller (without cavitation)
The full scale measurement was done by an
hydrophore towed by the submarine and located not
far away and downstream the propeller plane. The
results in Figure 21 show a very good agreement as
difference are less than 5dB at medium and high
frequency ranges. Furthermore, we have here a nice
case of propeller radiated noise comparison
between model scale and full scale because the
radiated noise measured at full scale is
predominantly the propeller noise and not the
radiated noise due to the hull and internal
machinery. We can then conclude that, from model
test in GTH, we can predict with a good confidence
the full scale propeller noise specially without
cavitation.
Figure 22 presents a comparison between radiated
noise of propeller extrapolated from model test and
radiated noise measured at full scale of a twin screw
navy surface ship. In this case, the full scale trials
involve an array of hydrophores hung vertically in
deep water at fixed location fUrick, 19751. The
vessel is arranged to run at constant speed and
course so to pass at the measurement hydrophores
at a known distance. Two ship speeds were
performed with no cavitation on the controllable
pitch propellers. The same laws as described before
is used for the extrapolation of the propellers noise.
Sound Power density spectrum
~ GTH- lOkts
+ Full scale - lOkts
GTH- 15kts
Full scale - 15kts
I ~ ^~a ~ ~~
. a... ~
. .~
Frequency
Figure 23: Radiated noise of propeller
extrapolated from model test and ship radiated
(twin screw navy surface ship)
The results show that the contribution of the
propeller on the ship noise is not predominant at
low speed. From this example, we can conclude that
the propeller noise is one noise source of the ship
but it is not the only one that matters.
7. CONCLUSIONS
From model test performed in accordance with the
hydro-acoustics similarities laws requirements, it is
possible to predict
- the cavitating and non cavitating operating
conditions of a propeller
the radiated noise of a propeller: broadband and
tonal noise
- the propeller induced hull and shaft excitation
- then to evaluate the contribution of the propeller
on the ship noise.
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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Nomenclature ~
,p water density
,Ps hull/propeller material
density
.\ scale ratio
(ship dimension divided
. by model dimension)
r
n
.p
..................................................................................................................................................................................
'c 'sound speed
.........................................................................................................................................................................................................................................................
'g 'gravity acceleration
.................................................................................................................................................................................
'f 'frequency
.........................................................................................................................................................................................................................................................
k 'roughness coefficient of '
the solid material
(propeller / hull)
...............................................................................................................................................................................
'elastic modulus of the
solid material (propeller
'/ hull? .............................................................................................................................
Poisson ratio
......................................................................................................
'damping constant of the
material
.................................................................................................................................................................................
SRk = (k . f/) k th shaft rate harmonic
.........................................................................................................................................................................................................................................................
'BRk 'k th blade rate harmonic
.........................................................................................................................................................................................................................................................
'Mpp 'Power Density
'Spectrum of the sound
'pressure
...........................................................................................................................................................................
'distance between source
(propeller / ship) and
r,eceiver (hydrophore)
Nuclei concentration
reference hydrostatic
'pressure (propeller shaft
axis or propeller shaft
'axis + SIR)
.........................................................................................................................................................................................................................................................
Pcrit 'Nuclei critical pressure
........................................................................................................................................................................................................................................................
,Pv .vapor pressure
.................................................................................................................................................................................
,p, ptt) fluctuating pressure
Amplitude on the hull,
hull pressure signal
.........................................................................................................................................................................................................................................................
flow speed in the test
Section or ship full scale
Speed
............................................................................................................
Wake deficit
1 ~ laxer)
.w= ~ d!
R A) V
prop.disk
...................................................................................................................................................................................................................................... ........... .........
L reference length
........................................................................................................................................................................................................................................................
,t,k,,,g,/,,m3j
· (kg/m3)
............................................................................
(m/s)
............................................................................
(m/s2)
........................................
(Hz)
............................................................................
(m)
........................................
, (Mpa)
........................................
........................................
............................................................................
............................................................................
(RPa2 '
Hz)
.....................................
(m)
~ by
(Pa)
..........................................................................
(Pa)
..........................................................................
(Pa)
.....................................
(Pa)
......................
(m/s)
......................................
..........................................................................
(m)
..........................................................................
sound pressure level (dB'
= to logoff pp ) ret IpPa
(Power density spectrum
level)
, ............................................................................................................................................
diameter (radius) of the ' (m)'
propeller
.........................................................................................................................................
pitch (ratio of pitch at
0.7R and propeller
diameter?
number of blades
shaft revolution rate
propeller Mast
propeller torque
'D ~ R
'P/D
'Z
'n
T
'Q........
Re=
n
........................................................................
pnD2
Re =
n
pnD
Fr =
A?
T
r n2D4
Kq = ~ Dim
s _ P-Pv
~ rn2D2
.........................................................................
_ P—Pv
.sv- 1
-rV2
2
Kp= P
,Si
Subscripts
.........................................................................
FS
........................................................................
m
.......................................................................... .
Reynolds number
propeller Reynolds
number
propeller Froude
number
propeller thrust
coefficient
propeller torque
coefficient
propeller cavitation
number based on shaft
revolution speed
where P is the
hydrostatic pressure at
+0.7R of the shaft axis
.............................................................................................................................................
propeller cavitation
number based on flow
speed
where P is the
hydrostatic pressure at
shaft axis
.............................................................................................................................................
hull fluctuating pressure,
amplitude coefficient
.................................................................................................................................................................................................................................................
propeller cavitation
number at inception
point
...........................................................................................................................................
, (rev/s),
. (,N?,
, (N.m),
full scale
..............................................................................................................................................
model scale
APPENDIX I: Tests & equipment in the GTH
Application
Submarine, torpedo
. _ ~ _
.[
Hi=
ma;
Single propeller and contra-
rotating propeller survey at large
Hi
~ _
C' :'
-
Single propeller and pumpjet
survey at large scale
. . . ~
... .,. ..~
Specific Propulsors survey (Pods)
Test section configuration
- closed test section
- complete hull
D < 250mm
V < lOm/s
- with ou without drift
angle
- closed test section
- complete hull
D 2 250mm
V 2 7mls
- specific operating
conditions (deceleration,
maneuvering & astern
pert...)
- shaft inclination 0° et
10°
- closed test section
- complete hull
D 2 250mm
V 2 7mls
- single screw and twin
screw
- closed test section
- complete hull
D < 250mm
V < lOm /s
- wake generator of shaft
and brackets
- closed test section
- V max
- specific operating
conditions (deceleration,
manoeuvring & astern
pert...)
- shaft inclination and
drift angle 0° < ot° < 180°
Measurements
- cavitation (on,Kt)
- self propulsion with and
without cavitation Kt,Kq,ll
- nominal - effective wake
- acoustics
- hull fluctuating pressure
- fluctuating thrust
- velocity down and up-
stream the propulsor
- cavitation (on,Kt)
- self propulsion with and
without cavitation Kt,Kq,ll
- velocity down and up-
stream the propulsor
- cavitation (on,Kt)
- self propulsion with and
without cavitation Kt,Kq,ll
- velocity down and up-
stream the propulsor
- acoustic meas.
- cavitation (on,Kt)
- self propulsion with and
without cavitation Kt,Kq,ll
- nominal / effective wake
- acoustics
- hull fluctuating pressure
- fluctuating thrust
- blade forces
- velocity down and up-
stream the propulsor
- cavitation (on,Kt)
- self propulsion with and
without cavitation Kt ,Kq, 11
- acoustics
- velocity down and up-
stream 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
APPENDIX II: Background noise of the GTH
· Background noise of the small test section at high pressure and at different flow speeds.
The levels presented on the graphs hereafter are lower in the high frequencies than the one presented in the
ASME paper LBoissinot & al, 19901, because the bearing of the rotor of the pump has been slightly modified.
The levels on the graphs hereafter are lower than the one presented in the ASME paper LBoissinot & al, 19901,
because the skimmer downstream the large test section (LLecoffre & al, 19871) has been removed.
Sound power density Ieve' Small test section - P=3bar - Hydrophone plug
Hen
Sound Power density Ieve' Small test section - P=3bar - Streamlined hydrophore
V-18 m/s
V=16 m/s
V=14 m/s
V=12 m/s
V=10 m/s
V=8 m|s
V=6 mls
V=4 m/s
V=2 m|s
.,
~ ~ ..
::
~ .. .
:':
1
Background noise of the large test section at high pressure and at different flow speeds.
| Sound Power density Ieve'
| dB ret 1uPa & 1 Hz
.[ ~
1 10 100 1000 10000 100000 1 10 100 1000 10000 100000
I I Freq. (Hz)
| Freq. (Hz) 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. (Hz)
streamlined hydrophore
+ P=0,5bar - without nuclei inj.
P=lbar - without nuclei inj.
P=0,5bar - with nuclei inj.
P=lbar - with nucle
DISCUSSION
R. Arndt, University of Minesota, USA
How do you scale nuclei content between a model
propeller and a full scale propeller operating in a
ship's wake ?
AUTHOR'S REPLY
The measurement of nuclei is done using a centre-
body venturi. This means that the results is not a
size distribution but a critical pressure distribution.
This nuclei measurement technique has been used
at sea in the Atlantic Ocean not far away of the
French Brittany coast. The measurements were
done from a ship at rest, therefore not in a ship's
wake. The results show discrepancies according to
the sea state, depth and certainly temperature. For
sea state O. we have the critical pressure distribution
given on the following figure.
~.~.,
Nuclei measurement at sea
(French Brittany west coast)
A similar measurement in the large test section of
GTH for different tuning of the nuclei injection
gives the result presented on the following figure.
Nuclei / cm3
1n
1
0.1
0.01
0.001
~^
1 ~
Pvap-pcrit (mbar)
0.0001 .
1 00 1 000 1 0000
Nuclei measurement
in the large test section of GTH
Theoretically, we should have the following scale
effect between the critical pressure distribution:
P crit.m P crit. FS
m /(Pcrit. ~ = /3
n FS ~ Pcrit. ~
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). The first reason is
that, up to now, there is no other nuclei generation
process than the one on the GTH that can produce
such an amount of nuclei with a stable
concentration and distribution of sizes in large
cavitation facility. The second reason is that we do
not really need such a scaling amount for the lowest
critical pressure because we already get a cavitation
nuclei saturation from the nuclei with critical
pressure closed to vapor pressure.
DISCUSSION
G. Chahine, Dynaflow Inc., USA
Since you control the nuclei distribution in the GTH,
do you find a correlation between nuclei size
distribution and cavitation inception ?
Do you measure that distribution of nuclei as a
function of time ? Is the number of nuclei the only
important parameter as you said in your talk ? When
you showed the cavitation noise scaling between
the full scale and the model you used a correction of
30 dS. What does this correspond to in terms of
power of the ratio Dfu~ scale/ Dmo`3e~ ?
AUTHOR'S REPLY
Yes we do find a correlation between nuclei
distribution and cavitation inception. This
correlation has largely been discussed in the
paragraph 8 of the Cavitation Committee Final
Report of the 21St ITTC.
We do not measure the distribution of nuclei as a
function of time. Because the deaeration process is
very fast, we can adjust precisely the air content
every 4 hours, so that the air content do not change
within 10% of the value set for the test. Moreover,
because of the downstream tank, there is no
recirculation of the nuclei in the tunnel. A1SO7 the
nuclei generation process is monitored as a function
of flow speed and pressure in the tunnel in order to
keep the same nuclei distribution. Those are the
reasons why the nuclei content is stable during the
tests. This has been checked few times and that is
the reason why we do not have a continuous
measurement of the nuclei.
The scaling of 30. log; FS jiS related to power of
3 of the ratio D, scale/ Dmo,3e~ . This comes from the
1
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 )
Do re/ Dmo~er
2
