235
1 INTRODUCTION
Oneoftheenergysavingdevicesusedwidelyinships
is the wake equalizing duct (WED) (Schneekluth’s
duct).Itconsistsoftwoaerofoilssectionedhalf‐ring
ducts integrated to the hull in front of the upper
region of the propeller. Some important parameters
fortheeffectivenessoftheWEDaretheanglesofduct
axistoship’scenterlinepla
ne,longitudinalpositions,
inner diameters, profile section shapes, angles of
sectiontoductaxisandlengthsofthehalf‐ringducts.
ItisassumedthattheWEDacceleratestheinflowof
the upper region of the propeller where the flow is
slowrelativetothelowerregionofthepropeller;and
it improves the uniformity of the wake over the
propellerdisc,sothepropellerefficiencyisincreased.
In addition, a well‐designed WED reduces the
am
ountofflowseparationattheafterbody,generates
an additional thrust as in the accelerating type of
duct,reducesthepropeller excited vibra
tionsdueto
theuniformwake,andimprovesthesteeringqualities
becauseofthemorestraightenedflowcomingtothe
rudder. If the WED is installed to an existing ship,
constructionalchangesoradaptationofthepropeller
design are not needed. A WED can also be used in
combinat
ionwithotherenergysavingdevicessuchas
vanewheelandasymmetricstern(Schneekluth,1986).
Marinepropellers are themost common
propulsion systems owing to the high efficiency
supplied by them; nevertheless, it is possible to
improveitspropulsiveperformanceusingadditional
Cavitation of a Propeller and Influence of a Wake
Equalizing Duct
G.Martinas
M
aritimeUniversityofConstanta,Romania
ABSTRACT:Thewakeequalizingduct(WED)isoneofthemostcommonlyused energy saving devicesfor
improving the propulsion performance of a ship and reducing the propeller‐excited vibrations and viscous
resistance forces. During the last three decades considerable researchand development activities have been
donewithinthi
scontext.Mostofthesedevicesareusedtoimprovepropulsiveefficiency,butsomeofthemaim
toimproveotherperformancecharacteristics,suchascavitations,vibration,noise,maneuverability,etc.Marine
propellersarethe mostcommonpropulsionsystems;nevertheless,itis possibletoimprovetheirpropulsive
performance using additionalauxiliary propulsion devices(unconventional propulsors).Two versions of an
existingshipinnormalversionandf
ittedwithWEDwereanalyzedinordertodemonstratetheinfluenceon
theWEDonthepropellercavitations.Itwasdeterminedthatthevaluesforthepressurecoefficientare1.98for
thecasewithoutWEDand2.029forthecasewithWED.Thedifferenceisnotsosignificant
;thus,theconclusion
isthattheWED device did not influence thecavitations ofthe propeller. Moreover,the optimizationof the
dimensionandformofWEDdidnothelpinreducingnegativeeffectsofcavitations.Becausethispaperworkis
notastudy,inordertodecreasetheca
vitationswehaveotherchoicesincludingasounddesignofthepropeller
biasedtoimprovethepropellerbehaviorincavitations.WEDisclearlynotachoice.
http://www.transnav.eu
the International Journal
on Marine Navigation
and Safety of Sea Transportation
Volume 9
Number 2
June 2015
DOI:10.12716/1001.09.02.11
236
auxiliary propulsion devices (unconventional
propulsors). During the last three decades
considerable research and development activities
have been done within this context. Most of these
devicesareusedtoimprovepropulsiveefficiency,but
some of them aim to improve other performance
characteristics, such as cavitation, vibration, noise,
maneuverability, etc. There can
be found a lot of
reviewstudies aboutvariousunconventional
propulsors in Glover (1987), ITTC (1990), Blaurock
(1990),Patience (1991),Breslin andAndersen (1994),
andCarlton(1994).
Schneekluth(1986)reportsthattheeffectivenessof
aWEDismostevidentiftheshipspeedisbetween12
and18 knotsand
its block coefficient is higher than
0.6.
By now, most of the studies related to the
estimation of the effect of the WED on propulsion
characteristicsofashiphavebeencarried out based
on model tests. But it is difficult to extrapolate the
powering performance from model tests (especially
for
very large ships) due to the Reynolds number
effects(scaleeffects)statedinITTC(1999).Athigher
Reynolds numbers the scale effects occurs more
evidently, in such cases it is recommended that self
propulsiontestsshouldbeperformedtoreducethese
effects (ITTC, 1999). In addition, numerical flow
computationsas
analternativeofthemodeltestscan
alsobeusedtoestimatetheeffectivenessoftheWED.
Oneofthe issues ofintensedebate is whetheror
not the WED device has any influence upon the
cavitationconditionsthatappearwhenthepropeller
ofamaritimeshipisrotating.
2 CAD
ANDFINITEVOLUMEANALISYS(FVA)
MODELOFTHESHIP
The goal of this paper is to calculate via software
Ansys13
TM
theinfluenceofplacementofaWEDtoan
existingshipoverthepropellerbehaviourintermsof
cavitation.
The model has as a starting point a real port
container as seen below, with the following
parameters:
LengthL‐[m]‐173
BreadthB‐[m]‐25
DraughtT‐[m]‐9.50
DiameterD‐[m]‐5
NumberofbladesZ‐6
PropellerRPM‐120
AverageSpeed‐16knots(7m/s)
Figure1.Port‐Container
Inordertohaveastartingpointforthesimulation,
first of all the afterbody was firstly CAD generated
withoutthe WEDdevice, and all the parameters for
fluidflowwerecalculatedaccordingly.Secondlythe
WEDdevicewasattachedtotheCADshipafterbody
and, using the same boundary parameters for
this
second simulation, made possible to compare the
results and draw the proper conclusions. The two
CADgeometriesareshownbelow:
a.
b.
Figure2.CADgeometriesa‐withoutWED,b‐withWED
Inordertoprovidemoredetailsonthegeometry
of WED device, the below figure is shown, with
dimensionsin[mm]:
237
Figure3.WEDdevicegeometry
Thefluidarea was divided intwo: the fluidarea
which surrounds the afterbody having the relative
velocity on Oz axis of 7 m/s and the Propeller fluid
area with CFX option of “frozen Rotor” where the
fluid is moving circularly around OZ axis with 120
RPM.Therewereestablishedinterfaces
betweenthese
twoareas.Theotherboundaryconditionswereinlet,
outletandopeningsasshownbelow:
Figure4.BoudaryConditions
In order to make clear some important surfaces,
threecontrolplanesweredefinedasfollows:
Control plane number 1 (P1) placed at 1200 mm
above the propeller axis and coplanar with the two
WEDdevicesaxis;
Control plane number 2 (P2) which includes the
propelleraxis;
Control plane number 3 (P3)
placed at 1500 mm
awayfromthepropellerdomain;
TargetPlanewhichis infactoneofthepropeller
interfacesasbelow:
Figure5.ControlPlanes
Cavitationisthephenomenonthatappearsinlow
pressure zones of a rotating propeller where fluid
vapoursarepronetodevelop.Cavitationisaharming
phenomenon tending to destroy the integrity of the
propeller surfaces by the implosion of the vapours
near the surface leading to the pitting of those
surfaces.
To simulate this phenomenon in FVA a
homogenous multiphase flow of the fluid will be
considered.For this the absolutesaturation pressure
is3574Pa.
3 FINITEVOLUMEANALYSIS(FVA)
SIMULATIONANDRESULTS
Afterreachingtheconvergence ofthe given models,
someimportantresultswerecalculated.Next,thetwo
models
arepresentedsimultaneouslyinordertoease
thecomparison.
PressuresincontrolplanesP1andP2
a.
b.
Figure6‐PressurefieldsforP1andP2
a‐without
Bycomparingtheabovefigures,themaximumof
pressuresforWEDfreeversionis2,72e5Pawhereas
theWEDretrofittedversionis1,077e5Pa.
At the same time the shape of pressure fields is
different for the two versions, the inner zone of the
WEDhasbiggerpressurefields.
238
FluidvelocitiesoncontrolplanesP1andP2
a.
b.
Figure7.VelocityfieldsforP1andP2
a‐withoutWED;b‐withWED
The maximum velocities are bigger for the
afterbodywithWED (33.56m/s). Near andafterthe
WED devices the fluid velocities are smaller
indicatinga“screeningeffect”.
VelocitiesforP3controlplane
a.
b.
Figure8. Velocity fields forP3, a‐without WED; b‐with
WED
ThisP3planeisneartotheTargetPlane(1200mm
away) so that the influence of the propeller rotation
motion is not so obvious here. As seen above the
maximumvelocityinboth casesis the same(6.8..6.9
m/s)butfielddistributionisaltered,theWEDdevices
concentratingthemass
fluxtowardtheircentresand,
implicitly, toward the Target plane. In plane words,
the WED is “stealing” streamlines of fluid from the
besides of the body and is concentrating them over
theTargetPlaneattheupperpartofthepropeller.
PressureFieldsovertheTargetPlane
The target
plane as mentioned is positioned
exactly on the entering zone of the propeller fluid
domain where no doubt, the influence of propeller
motionisthemostpregnant.Inordertoquantifythe
variationofpressureinducedbyWED,anewvariable
isdefined to calculate the average fluid pressure on
the
targetPlane:
239
AreaAve(Pressure)@Target
a.
b.
Figure9.PressurefieldsfortargetPlane
a‐withoutWED;b‐withWED
The average pressure calculated is 48,213 Pa for
the WED free version and 49,823 Pa for the WED
versionmeaningthatis103%.
VelocityFieldsovertheTargetPlane
For the velocity fields the situation is quite
reversed as compared to the above results.
Introducing again a new variable to enable
us to
calculatetheaveragevelocitiesforthetargetplane:
areaAve(Velocity)@Target
a.
b.
Figure10.VelocitiesfieldsfortargetPlane
a‐withoutWED;b‐withWED
TheWEDfreeversionisgivinganaverageof6.25
m/s whereas the WED version is giving 23.2 2m/s
average velocity, meaning that the WED version is
increasingwith363.2%themassfluxoverthetarget
plane.
240
Thevapourvolumefractionforpropellerblades
a.
b.
Figure11‐Thevolumeofvaporfractionoverthepropeller
a‐withoutWED;b‐withWED
Thevapourvolumefractionisthefirstandalmost
thebestindicatorofthe cavitation appearing in that
propeller zones. Whether the conditions for vapour
development are good, then the formation of those
vapours and their subsequent implosion is almost
certain.Byanalyzingtheabovefigures,itisbecoming
obvious that
on the back of the blades (the blade’s
sides toward the ship) the vapour fraction has a
maximum of 97.7 % for both casesand therefore, at
firstsight,thereisnopositiveinfluenceofWEDover
thecavitationconditionsofthepropeller.Inorderto
quantifythiswewill
needanewvariableasbelow.
The average pressure coefficient on the blade
surfaces
Tohaveacertainpictureovertheaveragepressure
coefficient causing the cavitation, a new variable is
introducedasfollows:
CoefPres=(Pressure‐51957[Pa])/(0.5*1002[kgm^‐
3]*16.91[ms^‐1]^2)
where“Pressure”isextractingthepressurecalculated
foreachandeverycellofthepropellerblade,5195Pa
istherelativepressure,1002[kgm^
‐3]istheseawater
densityand16.91[ms^‐1]^2istheaveragevelocityof
thepropeller.
Theaboveformulaisaspertheequation:
min
min
2
1
2
p
p
p
C
V
(1)
wherep
ministheminimumpressurebelongingtothe
propeller.
a.
b.
Figure12.Thepressurecoefficientoverthepropeller
a‐withoutWED;b‐withWED
Themaximumvaluesforthiscoefficientis1.98for
WED free version and 2.029 for WED retrofitted
version. The difference is so small that without
chances of being wrong, the obvious conclusion is
thatWEDdevicehasnoinfluenceoverthecavitation
of the propeller. To decrease the cavitation we have
other choices, including a sound design of the
241
propeller biased to improve the propeller behaviour
incavitations.WEDisclearlynotachoice.
4 CONCLUSIONS
Thewakeequalizingduct(WED) is oneofthe most
commonlyusedenergysavingdevicesforimproving
the propulsion performance of a ship and reducing
thepropeller‐excitedvibrationsandviscousresistance
forces.
Inthispaperworktwoversionsofanexistingship
in normal version and retrofitted with WED device
wereanalyzedinordertodemonstratetheinfluence
oftheWEDdeviceonthepropellercavitation(ifany).
Itwasdemonstratedthatthemaximumvaluesforthe
pressurecoefficientis1.98forWED
freeversionand
2.029forWEDretrofittedversion.Thedifferenceisso
small that without chances of being wrong, the
obvious conclusion is that WED device has no
influence over the cavitation of the propeller. To
decrease the cavitation we have other choices,
including a sound design of the propeller biased
to
improvethepropellerbehaviorincavitation.WEDis
clearlynotachoice.
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