229
1 INTRODUCTION
Thedriftoftheliferaftiscausedbytheinfluenceof
environmental factors and leeway. The leeway
prediction of the floating pneumatic life raft
represents a difficult task for the search and rescue
(SAR) because the raft tent is deformed by wind
pressure making it difficult
to clearly identify the
raft’s reference position and search area [2]. Search
and rescue services use available computer
applications(e.g.,Sarmap)todefinetheareaofsearch
[10]. Unfortunately, these applications do not take
intoaccountshapevariationduringdrift,sotheyare
not precise for deformable bodies. On the basis
of
previousresearchbytheGdyniaMaritimeUniversity,
theauthorscreatedanumericalsimulationtakinginto
account the variability of the shape of the research
object. This paper presents the results of a leeway
simulation which take into account the variable
windageareaofarafttent.Theauthorsbelieve
thata
detailed study of leeway and it’s factors could
ultimately limit the search area and increase the
effectivenessofrescueoperation[3,4,5].
2 LEEWAYOFPNEUMATICLIFERAFT
Theleewayofaliferaftisthemovementofanobject
onthewaterduetotheactionofthewind
pressureon
the above‐water part of raft (F
x) and the force of
hydrodynamicdragactingontheunderwaterpartof
thedriftingobject(F
0)asshowninFig.1.
Considering that leeway is the movement of the
raftcausedbytheactionofwindontheabove‐water
partoftheliferaftandthehydrodynamicdragofthe
underwaterpartoftheraft,bothforcesmustbetaken
into account. Numerical simulations carried out
earlier allowed to determine: the force of wind
pressure(takingintoaccountthevariableshapeofthe
flexible structure of the raft) and the force of
Numerical Prediction of Pneumatic Life Raft
Performance
J
.Jachowski&E.Książkiewicz
GdyniaMaritimeUniversity,Gdynia,Poland
ABSTRACT:ThesuccessoftheSearchandRescue(SAR)operationdependsonthecorrectassignationofthe
search area for the drifting pneumatic life raft, with particular emphasis on leeway. The leeway is directly
dependentonthehydrodynamicdragandwindpressureforceactingonabove
‐waterandunderwaterpartsof
the life raft. The paper presents the numerical study on the pneumatic life raft performance including
hydrodynamic and aerodynamic characteristics in a wide range of operational conditions. The numerical
simulationresultswerecomparedwithmodeltestresultsobtainedintowingtankandwindtunnel.The
proper
predictionofliferaftperformanceisimportantfordeterminationofliferaftsafetyfunctiondependentonwind
velocityandoperationalcharacteristics.Theresultsofnumericalsimulationareinlinewithdataavailablein
literatureandobtainedfromempiricalinvestigations.
http://www.transnav.eu
the International Journal
on Marine Navigation
and Safety of Sea Transportation
Volume 18
Number 1
March 2024
DOI:10.12716/1001.18.01.24
230
hydrodynamic drag. Using the previous results, a
simulation of leeway of the pneumatic life raft was
performedasadeformableobject[11‐14].
Figure1.Forcescausingleeway(source:currentstudy,print
screenofsimulation)[9]
3 RESEARCHOFTHEFORCEOFWIND
PRESSUREONTHEABOVE‐WATERPARTOF
THEPNEUMATICLIFERAFT
Tunnelaerodynamictestofapneumaticliferaftwere
carriedoutin2000 attheT‐3LowVelocityTunnelf
theInstituteofAviationinWarsaw.Windtunneltests
ofthelife
raftprovidedknowledgeofthedeformation
levelofliferafttentwhichispresentedinFig.2.
Figure2. Life raft during wind test (research report,
2000)[15]
Theresultsofthetunneltestsshowthatthevalue
of forces is function of the shape and inflow wind.
This knowledge is essential for modelling
environmentalconditionsduringnumericalresearch.
The numerical computations were carried out
usingFLOW‐3D.Numericalsimulationbased onthe
results of tests carried out in
the wind tunnel. The
shape of the raft was created on the basis of
photographic documentation from wind tunnel
research.
Anexemplarygeometricmodelofliferaftusedin
CFDsimulationsispresentedinFig.3.
A comprehensive approach to model and
numerical tests provided information on the
distribution of the
wind profile affecting the above‐
waterpartoftheraft.Fig.4showsthedistributionof
thewindvelocityfieldaroundtheliferaft.
Figure3. Example of geometry of life raft used the
calculations(source:currentstudy)[9]
Figure4. Aerodynamic drag in CFD calculation (source:
currentstudy)[9]
Theconductedresearchallowedtodetermineand
comparetheaverageddragcoefficientoftheliferaft
during the experiment and CFD simulation. The
obtainedresultsarepresentedinFig.5.
Figure5.CFDresults(left)andexperimentalresults(right)‐
Drag aerodynamic coefficient (Cx) for various wind
velocitiesandwinddirection(source:currentstudy)[9]
Comparison of the results of laboratory tests
carriedoutattheInstituteofAviationinWarsawwith
numerical simulations confirmed their convergence
andcorrectnessofthecalculationsperformed.
231
4 RESEARCHONTHEHYDRODYNAMIC
RESISTANCEOFTHEUNDERWATERPARTOF
THEPNEUMATICLIFERAFT
The life raft towing performance has been tested in
thetowingtankofShipDesignandResearchCentre
inGdansk.
Figure6.Modeltestoftheliferaftinthetowingtank[1]
The hydrodynamic drag of the life raft and the
drift were calculated separately under calm water
conditions.Totaltowageresistancewasthesumofthe
liferaftandthedrift.
Thetestspeedsoftheliferaftinthenumericaland
modelingtests were thesame.Thesimulationswere
carried out
in the established domain in calm water
conditions.
Figure7.ModelofliferaftusedinCFDsimulation[1]
The result of the simulation obtained after the
post‐processing comprises the general flow pattern,
the velocity and pressure fields ( Fig.8) which are
importantelementsoftheconductedstudy.
Figure 8. CFD simulation of hydrodynamic drag (source:
currentstudy)[16]
In the verification of themodelingof the flow of
the pneumatic life raft using CFD methods, the
followingfactorsweretakenintoaccount:prediction
of towing resistance and the behavior of the raft in
calm water during towing. The concordance of the
numerical calculations and the results of the
experimental
tests was high. The tank towing
experiment and CFD calculations were performed
usingafull‐scaleprototypetoeliminateanyerrorof
scale[1].
5 NUMERICALSTUDIESOFLEEWAY
In previous articles, numerical calculations of
hydrodynamic and aerodynamic resistance were
carriedout.Theobtainedresultswerecomparedwith
the results
of life raft model tests on a real scale to
confirm their correctness. The convergence of the
results was high, which allowed the continuation of
numericalcalculationsinthefieldofleewayofthelife
raft.CFDsimulationsofleewaywereperformedfor2
typesofliferafts.Thetypesof
liferaftsareshownin
Fig.9andtheirparametersareshownintheTab.1.
Figure9. Twotypes oflife rafts used for CFD calculations
(source:currentstudy)
Table1.Liferaftsdata(source:currentstudy)
________________________________________________
8‐persons 10‐persons
________________________________________________
Liferaftnetweight 95kg100kg
Maxpersonsweight 660kg 825kg
Totalweight755kg 925kg
________________________________________________
Figure9. Graph of undisturbed leeway (source: current
study)
Based on the theory of leeway, the numerical
calculations assumed equality of hydrodynamic and
aerodynamic resistance acting on the life raft.
232
Calculations were made for calm water conditions
(withouttakingintoaccountwavesandseacurrents)
to reproduce the earlier conditions of experimental
research.Theforecastofanundisturbedleewayofthe
pneumaticliferaftispresentedinFig.10.
Thegraphshowsthedependenceoftheleewayof
the life
raft on the wind speed affecting its above‐
water part. For example, when the wind speed is 9
[m/s],the resistance ofthe above‐water partandthe
underwaterliferaft(readfromthediagram)is70[N],
andthespeedofleewayis0.2[m/s].
Theresultsoftheexperiment
andCFDcalculations
aresummarizedinTab.2.
Table2.Comparisonofmeanaerodynamicand
hydrodynamicforcesobtainedfromCFDsimulationversus
windtunnelandtowingtankexperiment(source:current
study)
________________________________________________
Hydrodynamicdrag
________________________________________________
Water MeanResistance[N] Meanpercentageerror
speed[m/s] CFD towingtanktests CFD/experiment
________________________________________________
0.7 1892058%
1.5 9219574%
________________________________________________
Aerodynamicdrag
________________________________________________
Wind MeanResistance[N] Meanpercentageerror
speed[m/s] CFD windtunneltests CFD/experiment
________________________________________________
10 64 606%
20 2502327%
________________________________________________
Summarized results in a Tab.2 for selected flow
velocities.Theaveragepercentageerrorisintherange
of 4‐8%, which should be considered a good
complianceforaflexibleobjectsubjecttodeformation
duringtesting.
6 CONCLUSIONS
The success of the rescue operation depends on the
correctdeterminationofthe
searcharea,whichtakes
into account the leeway of the life raft. Previous
numerical simulations of the life raft’s drag and
aerodynamic characteristics allowed correct
predictionofliferaft’sleeway.Theknowledgeofthe
relationshipbetweenthewindspeedandtheleeway
speedoftheliferaftcandirectlyaffect
thespeedand
efficiency of rescue operations at sea. In conclusion,
the authors believe that numerical research of life
raft’s leeway and the results obtained may directly
affect the narrowing of the search area and increase
safetyatsea.
ACKNOWLEDGEMENTS
This research was financed by
Gdynia Maritime
UniversityGrantNo.WN/2023/PZ/03&WN/PI/2023/04.
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