experimental methods in marine dynamics

7/23/2019 Experimental Methods in Marine Dynamics

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FAKULTET FOR INGENIRVITENSKAP OG TEKNOLOGI NTNU TRONDHEIMNORGES TEKNISK-NATURVITENSKAPELIGE UNIVERSITET

FACULTY OF ENGINEERING SCIENCE AND TECHNOLOGY NTNU TRONDHEIMNORWEGIANUNIVERSITY OF SCIENCE AND TECHNOLOGY

_______________________________________________________________________________________________________________

TMR7

Experimental Methods in

Marine Hydrodynamics

Sverre Steen

Revised August 2014

MARINTEKNISK SENTER INSTITUTT FOR MARIN TEKNIKKMARINE TECHNOLOGY CENTRE DEPARTMENT OF MARINE TECHNOLOGYTRONDHEIM, NORWAY


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LecturenotesinExperimentalMethodsinMarineHydrodynamics,issuedAugust2014

Page1

CONTENTS

1 INTRODUCTION..................................................................................................31.1 Background...............................................................................................................................3

1.2 Whymodeltests.......................................................................................................................4

2 GENERALMODELLINGLAWS..............................................................................62.1 Geometricalsimilarity..............................................................................................................6

2.2 Kinematicsimilarity..................................................................................................................6

2.3 Dynamicsimilarity....................................................................................................................7

2.4 ScalingRatios..........................................................................................................................10

2.5 Hydroelasticity........................................................................................................................11

2.6 Cavitation................................................................................................................................13

3 EXPERIMENTALFACILITIES...............................................................................143.1 Introduction............................................................................................................................14

3.2 TowingTanks..........................................................................................................................14

3.3 CavitationTunnel....................................................................................................................18

3.4 OceanLaboratories.................................................................................................................20

3.5 Generationofenvironment....................................................................................................21

4 INSTRUMENTATION.........................................................................................284.1 Generaldescriptionofequipment.........................................................................................28

4.2 Strainanddisplacementmeasurements................................................................................28

4.3 Positionmeasurements..........................................................................................................32

4.4 Accelerations..........................................................................................................................34

4.5 PressureTransducers..............................................................................................................35

4.6 Velocities................................................................................................................................38

4.7 Forcemeasurements Dynamometers..................................................................................41

4.8 WaveMeasurements..............................................................................................................42

4.9 DataAcquisition......................................................................................................................44

4.10 SamplingFrequency...............................................................................................................48

4.11 LengthofRecords...................................................................................................................50

4.12 Calibration..............................................................................................................................53

4.13 Zeroing....................................................................................................................................54

5 PHYSICALMODELLING......................................................................................555.1 General...................................................................................................................................55

5.2 RigidModels...........................................................................................................................55

5.3 ElasticModels.........................................................................................................................57

6 CONVENTIONALSHIPTESTING.........................................................................616.1 General...................................................................................................................................61

6.2 Towingandpropulsiontestsintowingtank..........................................................................61

6.3 Cavitationtunneltests............................................................................................................65

6.4 Maneuveringtests..................................................................................................................67

7 SEAKEEPINGTESTING.......................................................................................717.1 General...................................................................................................................................71


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7.2 TestRequirement...................................................................................................................71

7.3 Testsetup..............................................................................................................................72

7.4 TestProcedure........................................................................................................................73

7.5 Tankwalleffects.....................................................................................................................75

8 OFFSHORESTRUCTURETESTING......................................................................77

8.1 General...................................................................................................................................77

8.2 TestRequirements..................................................................................................................77

8.3 Deepwaterstructuresrequirements.....................................................................................78

8.4 TestProcedure........................................................................................................................80

9 REALTIMEHYBRIDMODELTESTING................................................................829.1 Testingoffloatingoffshoreshipsandplatformswithmooringandflexiblerisersystems...83

9.2 Testingofhydrofoilships........................................................................................................83

9.3 Testingoffloatingoffshorewindturbines.............................................................................83

9.4 Challengesinhybridmodeltesting........................................................................................83

10 ANALYSISOFMEASUREDDATA........................................................................8510.1 General...................................................................................................................................85

10.2 Statictests..............................................................................................................................85

10.3 Decaytest...............................................................................................................................85

10.4 RegularWaveTest..................................................................................................................88

10.5 IrregularWaveTest................................................................................................................91

11 FULLSCALEMEASUREMENTS.........................................................................10311.1 Deliverytrials........................................................................................................................103

11.2 Shipmonitoringsystems......................................................................................................110

12 ERRORANALYSIS............................................................................................11312.1 Introduction..........................................................................................................................113

12.2 Uncertaintyanalysis.............................................................................................................113

12.3 DiscussionofErrorSources..................................................................................................119

13 MODELTESTSVSNUMERICALCALCULATIONS..............................................12713.1 General.................................................................................................................................127

13.2 ModeltestsforValidationofNumericalCalculations..........................................................128

14 REFERENCES...................................................................................................130

15 INDEX.............................................................................................................132

ANNEXA ExampleofReportingfromModelTest

ANNEXB ExampleofModelTestSpecification

ANNEXC ViscousSurgeDampingofFloatingProductionVesselMooredatSea

ANNEXD ErrorAnalysisofExperiments.LecturenotebyS.Ersdal

ANNEXE ITTCstandardforpoweringperformanceprediction


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1 INTRODUCTION

ThiscompendiumhasbeenpreparedforthecourseExperimentalMethodsinMarine

Hydrodynamics.Partsofthenotesarebasedonearlierlecturenoteswithinthisfield;seeHuse

(1999)andWalderhaug(1983).ExtensiverevisionsofthecompendiumwrittenbyAarsnesin2001

weremade

by

Steen

in

2004,

2005,

and

2006,

followed

by

smaller

revisions

in

2010

and

2012.

AlthoughthenameofthiscourseisExperimentalMethodsinMarineHydrodynamics,wewill

mainlybetalkingaboutmodeltesting,sincemostexperimentsinmarinehydrodynamicsaremade

inmodelscale.Also,modeltestinginvolvesmanyinterestingissues,likescalingandmodelling.Full

scaletestingishandledasaspecialcase,seechapter11.

Throughoutthetext,manyreferencesaregiventosupplementaryliterature,anditis

recommendedtoconsultthoseforamoreindepthtreatmentofspecialtopics.Agoodtextbook

thatcoversmostofthetopicsintheselecturenotesatanintroductorybutstillmorethoroughlevel

isthebookbyDunn(2005).

Figure 1.1Model tests in Peerlesspool in London in 1761

1.1 Background

Experimentalfacilities

for

model

testing

of

ships

have

along

tradition.

Improved

resistance

performanceoftheshipswasearlythemaindrivingforcebehindthedevelopmentofshipmodel


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testing.ItisknownthatLeonardodaVinci(aboutyear1500)carriedouttestswith3models

ofships,allwithequallength,butwithdifferentforeandaftshape.Basedonhisexperimentshe

wasabletogiverecommendationaboutwhichshapegivesthehighestspeed.LaterSamuelFortey

(16221651)alsodidtestswithshipmodelsandin1721EmanuelSwedenborggaveadetailed

proposalforshipmodeltestingintroducingtheprinciplewithfallingweightfortowingofthe

models.Inthiswayhewasabletoachieveaknownandconstanttowingforce. In1761thisprinciplewasusedinPeerlesspoolinLondonasshownonthepicturegiveninFigure1.1.Atthat

timenoscalinglawswereavailabletopredictfullscalebehaviourandonehadtoassumethatthe

winnerwasthebestalsoinfullscale.

WilliamFroude(18101879)isoftengiventhehonourforthemethodofreallyusingmodeltesting

forshipdesignbythedevelopmentofamethodforscalingfrommodelresistancetotheactualship

resistance.Thismayberight,butseveralotherworksfromthesametimealsocontribute

significantlytothisdevelopment.Theestablishingofthescalingmethodsshouldthereforemorebe

regardedasaresultoftheincreasinginterestandactivitieswithinthisfield.

FroudestowingtankwasbuiltinSouthEnglandinca1870andisregardedasthebeginningof

modernmodeltesting.ThemaindimensionofthetankwasLxBxd=85mx11mx3m.Itwas

equippedwitharailintheroof,whichcarriedthedynamometers.Maximumspeedwas5m/s.

Shortlyafterthistankwasestablished,severalothertankswerebuiltinEngland,Germanyand

elsewhere.ThetowingtankinTrondheimwascompletedin1939withdimensionLxBxd=170mx

10.5mx5m,whichwasanormaltanksizeatthattime.

Later,thedevelopmentwithinshiptechnologyhasinitiateddevelopmentandbuildingof

specialisedfacilitiesascavitationtunnels,manoeuvringandseakeepingbasins.Duringthelast20

25yearstheneedsfromtheoffshoreindustryhavepushedthisdevelopmentevenfurther,and

complexlaboratorieswiththepossibilityoftestingstructuresinrealisticconditionsincludingwind,

currentaswellasmultidirectionalwaves,havebeenbuilt.Anexampleofthistypeoflaboratoryis

theOceanBasinatMARINTEK.

Differenttypesoffacilitiesaredescribedinmoredetailsinchapter3.

Afurtherdescriptionandreviewofthehistoryanddevelopmentofshipmodeltestingcanbefound

inSNAME(1967)andinStoot(1959)

1.2 Whymodeltests

Hydrodynamicmodeltestingwillbasicallyhavethreedifferentaims:

1. Toachieverelevantdesigndatatoverifyperformanceofactualconceptsforshipsand

othermarinestructures

2. Verificationandcalibrationoftheoreticalmethodsandnumericalcodes

3. Toobtainabetterunderstandingofphysicalproblems.

Alltheaimscanbeassociatedtotheoftenverycomplicatednatureofproblemsconnectedtothe

interactionbetweenfixedandfloatingstructuresandthemarineenvironment.

Aim1isspeciallythecaseiftheanalysisisverycomplicatedforwhichverifiednumericaltoolsare

notavailable.Modeltestcanbeusedtoinvestigateeffectsofsimplificationsusedasbasisfor


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analyticalornumericalmodels.Inthiswaymodeltestresultscanbeusedtoassist

developmentofmorereliablenumericaltools.


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2 GENERALMODELLINGLAWS

Physicalmodelsareintendedtorepresentthefullscalesystemascloseaspossibleata(much)

smallerscale.Tobeabletodeterminetheproperpropertiesofthemodelweneedmodellingor

scalinglawsthatensureasimilarbehaviourinmodelandfullscale.

Dimensionalanalysiscanbeusedtoderiveagroupofmeaningfuldimensionlessquantitiesfor

applicablevariables.Thisisparticularlyusefuliftheproblemiscomplex.Typicallyallthevarious

quantitiesassumedtobeofimportanceforacertainphenomenaislisted.Afunctionalrelationship

betweenthedifferentparametergroupsisthenestablishedforallflowgoverningquantities.The

scalinglawsareobtainedbytakingtheratioofthedifferentforces. Adetaileddescriptionof

dimensionalanalysiscanbefoundinTaylor(1974).Aderivationofthemostcommon

dimensionlessvariablesusedinfluiddynamics,usingBuckinghamsPitheoremisfoundinchapter5

ofWhite(2005).

Toachievesimilarityinforcesbetweenthemodelscaleandfullscalesituationthefollowing

conditionsmustbefulfilled:

Geometricalsimilarity

Kinematicsimilarity

Dynamicsimilarity

Inthefollowingtheserequirementswillbediscussed.Amorecomprehensivediscussionabout

modellawsisgivenbyChakrabarti(1998)

2.1 Geometricalsimilarity

Geometricalsimilarstructuresinmodelandfullscalehavethesameshape.Thismeansthata

constantlengthscalebetweenthemexist:

/F ML L

whereLMandLFareanydimensionsofthemodel/fullscalestructure.Therequirementtoequal

lengthratioforalldimensionsdoesnotapplyonlytothestructures,butalsotothesurrounding

environment.Atthefirstviewthisseemstobeaneasyrequirementtosatisfyforpracticaltesting.

Howeverthisneednotbetheactualsituation.Forexampletheactualsurfaceroughnessofaship

cannotbeaccuratelymodelled.Anotherexampleisthealmostunrestrictedextentofthe

surroundingwaterforasailingship(exceptforwaterdepthinsomecases).Thissituationisnot

possibletoreproduceinmodelscale,whichimpliesthatphysicalboundariesalwayspresentin

modeltestingcaninfluencethetestresults.

2.2 Kinematicsimilarity

Theratiosbetweenvelocitiesinmodelscalehavetobeequaltothecorrespondingratiosinfull

scale.Thisimpliesthatflowwillundergothegeometricalsimilarmotionsinbothcases.Asan

exampletheratiobetweentheforwardspeedofashipandtherotationalspeedofthepropeller

hastobethesame:


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(2 ) (2 )

F M

F F M M

V V

n R n R

or

F MF M

F F M M

V VJ J

n D n D

whereVistheshipspeed,nistherateofrevolutionofthepropeller,Risthepropellerradius,Dis

thepropellerdiameterandJistheadvancecoefficient.

2.3 Dynamicsimilarity

2.3.1 Forces

Dynamicsimilarityisachievedifwehavethesameratioatmodelscaleandfullscaleforthe

differentforcecontributionspresentintheproblem.Inprinciplethefollowingforcecontributions

willbeofimportance:

1. InertiaForces,Fi

2. Viscousforces,Fv

3. Gravitationalforces,Fg

4. Pressureforces, Fp

5. Elasticforcesinthefluid,Fe.

6. Surfaceforces,Fs.

Inaddition,forelasticmodelstheelasticrelativedeformationsmustbeidenticalinmodelandfull

scale.

Wewillusethefollowingdifferentphysicalquantitiestocharacterisethedifferentforce

contributions;physicallength;L,velocity;U,fluiddensity;,gravitationalacceleration;g,andthe

fluidviscositycoefficient;.ThefollowingdependenceofthephysicalparametersL,U,,gand

willexistforthedifferentforcecontributions:

InertiaForces:

2233

LULdt

dx

dx

dU

Ldt

dU

Fi

ViscousForces: ULLdx

dUFv

2

GravitationalForces:3

gLFg

PressureForces:2

pLFp

ElasticfluidForces:2LEF vve

SurfaceForces: LFs


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wherevistherelativeelongation(compression),Evisthevolumeelasticityandisthesurfacetension.

2.3.2 FroudeNumber

Thedynamicsimilarityrequirementappliedontheratiobetweeninertiaandgravityforcesgives

thefollowingrelation:

gL

U

gL

LU

F

F

g

i

2

3

22

Appliedonmodelandfullscalethisrequirementgives:

2 2

M F

M F

M FN

M F

U U

gL gL

U UF

gL gL

whereFNistheFroudenumber.Geometricalandkinematicsimilarity,andequalityinFroude

numberinmodelandfullscalewillthereforeensuresimilaritybetweeninertiaandgravityforces.

Sincesurfacewavesaregravitywaves,thisimpliesthatequalityinFroudenumbershouldgive

equalityinwaveresistancecoefficient.

2.3.3 ReynoldsNumber

Equalratiobetweeninertiaandviscousforceswillgive:

2 2

i

v

F U L UL UL ReF UL

whereReistheReynoldsnumberand=/ isthekinematicviscosity.EqualityinReynolds

numberbetweenfullscaleandmodelscalewillthereforeensurethattheviscousforcesare

correctlyscaled.

2.3.4 Machsnumber

Theelasticityofwaterwillinfluencethepressuretransmissioninwaterandwillthereforebe

importantforsometypeofmodeltesting.Equalratiobetweeninertiaandelasticforcesgives:

2

22

LE

LU

F

F

vve

i

Usingthegeometricalsimilarityrequirementthatvareequalinmodelandfullscalethis

requirementgives:

2 2 2 2

2 2

, ,

v v v vM F

M F

n

v M v F

U L U L

E L E L

U U

ME E


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whereMnistheMachnumberand vE isthespeedofsoundinwater.

2.3.5 Webersnumber

Theratiobetweeninertiaandsurfacetensionforcesisgivenfrom:

LU

L

LU

F

F

s

i222

Similarityrequirementforthisforceratioinmodelandfullscalewillnowgivethefollowing

requirement:

2 2

M F

U L U L

whichgives:

( ) ( )

M Fn

M F

M F

U UW

L L

whereWnistheWebersnumber


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2.4 ScalingRatios

Thefollowingdimensionlessquantitiesarecommonlyusedfortestingofshipandoffshore

structures:

Symbol DimensionlessNumber ForceRatio Definition

Re ReynoldsNumber Inertia/Viscous UL

FN FroudeNumber Inertia/GravityU

gL

Mn MachsNumber Inertia/ElasticityV

U

E

Wn WebersNumber Inertia/SurfacetensionU

L

St Strouhallnumber vf D

U

KC KeuleganCarpenterNumber Drag/Inertia AU T

D

TheStrouhalNumberisnotderivedfromaforceratio.fvisthevortexsheddingfrequencyandStis

thenondimensionalvortexsheddingfrequency,whichagaindeterminetheoscillationfrequency

ofthetransverseliftforcesactingonacylinderwithcrossdimensionD.

TheKeuleganCarpenterNumberisdeterminedfromforceratiobetweendragandinertiaforces

forthecasewithoscillatingflowpastacylinder.TistheperiodofoscillationandUAisthevelocity

amplitude.EqualKCinmodelandfullscaleisforexampleachievedifthesameratiobetweenwave

heightandcylinderdiameterisused.

Inpracticaltestingitwillnotbepossibletosatisfysimultaneouslythedifferentscalinglaws.For

exampleshipsandoffshorestructuresareformostpracticalsituationinfluencedbysurfacewave

effects,eitherfromincomingwavesorwavegeneratedbytheforwardspeedormotionsofthe

structure.Gravitationalforceswillgovernthesurfacewaveformation.Thisimpliesthatforthese

conditionsequalityinFroudenumberinmodelandfullscalemustbeachieved.Ifviscousforcesare

importantfortheactualsituation,therequirementofequalityinReynoldsnumbershouldin

principlealsobesatisfied.Thisisnotpossibletoachieve.Theviscousforceswillnotbecorrectly

scaledandinthescalingprocessfrommodeltofullscalethiseffecthastobeevaluated.

OtherpracticallimitationsforachievingequalityinRearemodelsizeandnecessarymodelspeed.

TherequirementofconstantUL(assumingconstant)willformostcasesbeimpossibletoachieve.

Inconventionalmodeltestingofshipsandoffshorestructures,physicalscalingandtestexecution

aremostcommonlycarriedoutbasedonFroudeScaling.TheeffectofdifferentReynoldsnumberis

accountedforbydifferentscalingprocedures.Atypicalexampleisshipresistancetests,where

scalingmethodsforcorrectingforeffectofdifferentReynoldsnumberiswellestablished.Forother

applicationsnoestablishedmethodexistsforaccountingfortheeffectofReynoldsnumber.Thiswillbediscussedinmoredetailsinchapter12.2.


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AssumingFroudescalingisappliedandgeometricalsimilaritywithscaleratio /F ML L ,

fromtheequalityinFroudenumberwehave:

M F

M F

FF M M

M

U U

gL gL

LU U U

L

Theotherphysicalparameterscannowbederivedfromthedimensionalanalysissfollows:

Structuralmass:3F

F M

M

M M

Force:

3F

F MM

F F

Moment: MM

F

F MM4

Acceleration: F Ma a

Time: F Mt t

Pressure: FF MM

p p

Theratio F M isincludedtoaccountforpossibledifferenceinfluiddensitybetweenfullscale

andmodelscale(usuallyseawaterinfullscalerelativetofreshwaterinthetesttank).

2.5 Hydroelasticity

Inhydroelasticproblemsthehydrodynamicforcesareinfluencedbytheelasticdeformationofthe

structure.Thisdeformationisgovernedbytheinertiaforcesandelasticforcesinthestructure.Themodellingoftheelasticpropertiesofstructureswillthereforegiveseveraladditionalproblems

comparedtothemodellingofwaveinduceddynamicresponseofrigidstructures.Exampleswhere

correctlyscaledelasticbehaviourofthemodelwillbeimportantisspringingandwhippingofships,

anddynamicbehaviourofmarinerisersandmooringlines.

Additionalrequirementstotheelasticmodelcanbesummarisedasfollows:

Correctlyscaledglobalstructuralstiffness

Structuraldampingmustbesimilartofullscalevalues

Themassdistributionmustbesimilar.


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Geometricalsimilaritybetweenmodelandfullscaleforanelasticstructurewillrequirethat

theelasticdeformationsaresimilar.Toillustratethiswewillconsiderthedeflectionofacantilever

beamasanexample.Thedeflection,,isgivenfrom:

EI

FL3

whereEIistheflexuralrigidityandFisthehydrodynamicforcewhichcanbeexpressedas:

22LUCF

whereCisaforcecoefficientdependentonFN,Reetc.Therequirementofsimilarityindeformation

inmodelandfullscalegives:

F MF M

F ML L

Usingtheaboveequationsthisrequirementissatisfiediftheratio:

2 4C U L

EI

isequalinmodelandfullscale.Assumingequalforcecoefficientanddensityweobtainthe

followingrequirementtothestructuralrigidity:

2 4 2 4

5

F M

F M

U L U LEI EI

EI EI

Ifalldimensionsofthecrosssectionalshapeofthebeamarescaledgeometricalsimilar,the

momentofinertia,I,willsatisfytherelation:

4

F MI I

WearethanleftthefollowingrequirementtotheYoungsmodulus,E:

F ME E

ThisimpliesthattheYoungsmodulusforthemodelmustbe1/timesthevalueofthefullscale

structure.

Itshouldbenotedthatthetwolastequationsisnottoberegardedasrequirementstothemodel.

ThebendingstiffnessrequirementisgivenforEI.Inpracticalmodeltestingtherequirementgiven

toscalingofEIisoftensatisfiedbymanipulatingthedifferentparametersbyapplyingother

materials,otherwallthickness,orbymodifyingthestructuralbuildupofthebeam.Theouter

geometry,whichisexposedtothehydrodynamicforces,hastobemodeledgeometricallycorrect.

Alsotherequirementtocorrectmodelingofmassdistributionandstructuraldampinghastobe

satisfied.Thiswillbefurtherdiscussedaspartofthephysicalmodeling,seechapter5.3.

Similarresultswillbefoundfortheaxialandtorsionstiffnesses.Therequirementfortheaxial

stiffnesscaseis:


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3)()( MF EAEA

whereEAistheaxialstiffness.Thisrelationgivesequalstraininmodelandfullscale.Thecross

sectionalarea,A,willsatisfytherelation2

MF AA ,whichgivesthesamerequirementtothe

Youngsmodulusasshownabove.

2.6 Cavitation

Ifcavitationoccurs,dynamicsimilarityalsorequiresthatthelawofequalcavitationnumberis

accountedforintheexperiments.Therequirementisthatthecavitationnumbergivenas:

0

2

( )

1/ 2

vgh p p

U

hastobethesameforthemodelasinfullscale.p0istheatmosphericpressure,ghisthe

hydrostaticpressureandpvisthevapourpressure.Tosatisfythisrequirementacavitationtunnel,

withpossibilitytolowertheatmosphericpressurehastobeapplied.


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3 EXPERIMENTALFACILITIES

3.1 Introduction

Thedifferenttypeofexperimentalfacilitiesusedforshipsandoffshorestructurescanbe

categorizedasfollows:

Towingtanks,conventionalandfacilitiestailormadeforspecificpurposes.

Cavitationtunnels

Oceanbasins

Usuallywewillfindtwoormoredifferenttypeoftestsfacilitiesateachresearchandtesting

institution.ForexampleatMARINTEKthereisthreetowingtanks,acavitationtunnelandanocean

basin.InFigure3.1anoverviewofthetestfacilitiesatMARINTEKispresented.

Theexperimentalfacilitiesfortestingofshipandoffshorestructuresarenotonlythephysicaltank/basinwherethetestsareexecuted.Thetestingfacilitieshavealsotocoverdifferent

additionalfunctionsasworkshopsforconstructionandbuildingofmodels,instrumentation,

simulationofenvironmentandsoftwareandtoolstorecordandanalyzethemeasureddata.A

typicallayoutofatowingtank,includingutilityfunctions,isshowninFigure3.2. Fortestingof

realisticbehaviorofstructuresinaseaway,equipmentforgenerationofwind,wavesandcurrent

andefficientwaveabsorptionareofvitalimportance.

3.2 Towing

Tanks

Thefirsttowingtankswerebuiltforperformingtowingandpropulsiontests.Thelengthofthe

towingtankhastobelongenoughtogetasufficientlongtimewithsteadyflowconditionsfor

measurementsoftowingandpropulsionforces.Therequiredsizewillthereforebedependenton

typeofshipstobetested,scaleratioandforwardspeed. Todayalargenumberoftowingtanks

exist,morethan200areinregularuse. Thelengthofthetowingtanksisfrom20mtomorethan

1000m.

Thesmalltanksaretypicallyconnectedtoteachingandresearchinstitutions.Theverylongtanks

aremainlyconnectedtonavalactivities.AnexampleisthehighspeedtankatDavidTaylorNaval

ShipResearchandDevelopmentCentrewithdimensions900mx6.4mx3mwithamaximum

towingcarriagespeedofupto50m/s.Thistankwasbuiltespeciallyfortestingofhighspeedships.

Theconstructionofthistankwasadirectresultoftherequirements:Navyin50kn,definedin

about1960asatargetfortheUSNavy.AsimilarfacilityalsoexistsinSt.Petersburg.

AtypicalsizeforcommercialworkingtowingtanksisLxBxd=250mx10mx5m. Typicalshipmodel

lengthis58m.Thissizeoffacilitiesseemstorepresentareasonablecompromisebetweencostfor

tankconstruction,costformodelmanufactureandoperationalcosts(whichtogetherdetermine

thecostofmodeltesting)andtherequiredscaleratioandcorrespondingaccuracythatcanbe

achieved.ThesizeofthelargehydrodynamiclaboratoriesattheMarineTechnologyCentreis

showninFigure3.1,withmoredetailsaboutthetowingtanksgiveninFigure3.3.


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Almostalltowingtanksuseatowingcarriagetomovethemodeltroughthewater.A

typicaltowingcarriagedesignisshowninFigure3.3.Thetypicalmaxcarriagespeedis10m/s.

Duringcalmwatertowingandpropulsionthemodeliskeptfixedinsurgeswayandyaw,butfreeto

heaveandpitch.

Figure 3.1 Overview of test facilities at MARINTEK


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Figure 3.2 A typical towing carriage design.

Tobeabletoperformseakeepingtestsorothertypeoftestinginsurfacewaves,manytowing

tanksareequippedwithawavemakeratoneendofthetank.Generationofwavesandtypeofwavegeneratorsarediscussedinmoredetailsinchapter3.5.1.Inordertopreventreflectionsof

wavesfromtheoppositeside,awavebeach,whichisabsorbingthewaveenergy,hastobe

installedatthissideofthetowingtank.

Ithasalsobeenconstructedtowingtanksthatarehighlyspecialisedforagivenpurpose.An

exampleistheDutchandChinesevacuumtanks,wheretheentirespaceabovethetankis

evacuatedandairpressuredownto0.04barcanbeachieved.Thepurposeofthistypeoffacilityis

todopropulsiontestswithsurfaceeffectsandcompleteshipmodelpresent,atthelowpressure

requiredforequalityincavitationnumber.

AnotherexampleofaspecialisedtankistheicetanksinHamburgandHelsinki.Iceismodelledby

freezing,usinghighsalinitywaterandchemicalstocontrolthemechanicalprioritiesoftheice.

Thesetanksareusedfortestingoficebreakersandoffshorestructuresexposedtotheactionof

driftingice.

ThetowingtankatMARINTEKwascompletedin1939withdimensions170x10.5x5m.Later,in

1978,extendedto260mwherethedepthoftheextensionis10.0m.Thelayoutofthetowingtank

isshowninFigure3.4.Thetowingcarriageisofconventionaltype. Fortestingofhighspeed

vesselsthecarriageisequippedwithaFreetoSurgerig.The8mlongrigismountedinfrontof

thetowingcarriageasshowninFigure3.4.Usingthisrigthewinddisturbanceatthepositionofthe

modeliseliminatedandthemodelisallowedtofreelysurge,heaveandpitchduringwavetesting.


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Towingtankdata

TankI TankII TankIII TankI+III*

Length:

Width:

Depth:

175m

10.5m

5.6m

25m

2.8m

1.0m

85m

10.5m

10m

260m

10.5m

5.6/10.0m

Tot.weightcarriage:

Wheelbase:

Speedrange:

Max.acceleration:

20tons

11.04m

0.0210m/s

1m/s2

0.2ton

3m

0.051.75m/s

1m/s2

15tons

11.04m

00.9m/s

1m/s2

20/15tons

11.04m

0.0210m/s

1m/s2

Modelsizerange: 8m 1m 8m

Wavemaker:

Max.waveheight:

Waveperiodrange:

Max.wavesteepness:

Singleflap

Regularandirregularwaves

0.3m

0.253sec.

1:8

Doubleflap


0.9m

0.85sec.

1:10

Doubleflap


0.9m

0.85sec.

1:10

Wavespectra: Computergenerated

* Tank I and III can be used separately and also as one long tank (Tank I + III) by removing the gate (12)

and wave absorber (15). In Tank I + III either of the two carriages can be used.

Figure 3.3 Towing Tanks at MARINTEK.

11

4

10 9 8 7

136

5

3 2 1

1

5.

6

10.

0

28

39 85

260

13.

5

10.

5

Model store

Drawing office

Reception

TankII

Ship model manufacturing

shop

Trimming tank

NC milling machine for

model production

Instrumentation workshop

Carpenter workshop

Propeller model

manufacturing shop

Cavitation laboratory

Dock gate

Wave absorber, Tank I

and Tank I+III

Wavemaker, Tank III

and Tank I+III

Wave absorber, Tank III

12 15

1

2

3

4

5

6

7

8

9

10

11

12

13

14

15

TANKI TANKIII 1


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Page18

Figure 3.4 Free to surge rig in front of towing carriage.

3.3 CavitationTunnel

Cavitationtunnelsaredesignedtobeabletotestpropellersandotherliftingsurfacesata

sufficientlylowpressuretoachievecorrectcavitationnumber.Mostcommercialmodeltest

institutionshaveoneormorecavitationtunnels,andabout100tunnelsistodayinregularuse.

Awiderangeofdifferentsizecavitationtunnelsexist,fromsmallsizetunnelsforresearchand

education,with

test

section

area

of

typically

0.25

x0.25

m,

to

very

big

circulating

water

tanks,

with

testsectiondimensionupto3x6mandlengthof11m(theBerlintunnel).Atypicalsize

conventionaltunneliswithcirculartestsectionwithdiameterofabout1mMaximumflowspeed

atmeasuringsectionisusually1020m/s.Largetunnelsoftenhavetestsectionsallowingfor

mountingofacompleteshiphullmodel.

Fortunnelsthatcannotallowtestingofentireshiphullmodels,anafterbodymodeloftheshipis

oftenappliedtoproducecorrectinflowtothepropeller,andmeshscreensareusedtoproducethe

specifiedwakedistribution.Thebenefitofusingafterbodymodelsandmeshscreens,insteadofa

completemodel,isthepossibilityofhavingthemeshscreensimulatefullscalewake,notonly

modelscalewake.Whentestingtheentireshipmodel,onlymodelscalewakemightbetested.

Somecavitationtunnelsareofthefreesurfacetype.Suchtunnelscanbeusedfortestingofhigh

speedpropellersoperatinginfullorsubmergedcondition.Thistypeoftunnelsisespeciallywell

suitedforstudyingventilationproblemsforpropeller,waterjetsandfoilsections.Largetunnels

withfreesurfaceenabletestwithnormalshipmodels.

ThecavitationtunnelatMARINTEKisshowninFigure3.6.Thediameteroftheworkingsectionis

1.2mandthelengthofworkingsectionis2.08m.Maximumwatervelocityis18m/s.The

minimumworkingpressureis0.1atm.Afterbodymodelsandmeshscreensareapplied.


20/182


Page19

Figure 3.5 Cavitation Tunnel at MARINTEK.


21/182


Page20

3.4 OceanLaboratories

Theoceanlaboratoriesareingeneralconstructedfortestingofoffshorestructuresandfor

seakeepingandmanoeuvringtestingofships.

Incontrasttothetraditionaltowingtankstheoceanbasinsandseakeepinglaboratoriesmakeit

possibleto

carry

out

tests

with

any

wave

heading

(oblique

waves)

for

ships

with

forward

speed.

For

manoeuvringtestsitisrequiredwithatowingcarriagewithcontrolledmotionsinbothlongitudinal

andtransversedirection.Thisisachievedbyasubcarriage,whichisconnectedunderneaththe

mainlongitudinalmotioncarriage(seeFigure3.7,takenfromtheseakeepinglaboratoryatSSPA,

Sweden).Combinedwiththelargewidthofthesefacilities(typically3050m)thearbitrary

horizontalmotionrequirementtendstomakethecarriagesystemcomplexandheavy.

Basinspurposebuiltforoffshoretestingisformostcasesbuiltafter1980.Foroffshoretestinga

largecarriagesystemisnotrequired.Oceanlaboratoriesareusuallyequippedwithadvanced

systemsforgenerationofwaves,oftencapableofgenerationofbothlongcrestedand

multidirectional(or

short

crested)

waves

as

well

as

wind

and

current.

In

this

way

it

is

possible

to

givearealisticrepresentationofthemarineenvironmentalconditions.

Figure 3.6 Carriage system in Ocean Basin (from SSPA, Sweden).

Examplesofcommercialoceanlaboratoriesfortestingofcoastalandoffshorestructuresare:

MARINTEK,Trondheim; LxB=80mx50m,d=010m

MARIN,Netherlands; LxB=45mx36m,d=010.5m,pitincentrewithd=30m

HydralicLab,Ottawa,Canada: LxB=50mx30m,d=3m

OTRC,TexasA&M: LxB=45.7mx30.5m,d=5.8m,pitwithd=16.8m


22/182


Page21

Somebasinsareequippedwithafalsebottomthatcanbesettodifferentdepths.Inthis

waytheactualwaterdepthcanbecorrectlymodeledinthetestsetup.Thispropertyalsoenable

mountingofmodels,mooringsystemandothersubseaequipmentonadrybottom,whichafter

installationofthesystemtobetested,islowereddowntothewantedwaterdepth.Thislargely

simplifiesthepreparationworkforthetest.

TheoceanlaboratoryatMARINTEKisfittedwithtwosetsofwavemakers.Alongthe50msideof

thebasinthereisadoubleflapwavemakercapableofgeneratinglongcrestedwaves.Alongthe80

msidethereisamultiflapwavemakerconsistingof144individuallycontrolledflapsforgeneration

ofshortcrestedandlongcrestedwaves.Waveabsorptionbeachesareinstalledonthetwo

oppositesidestoreducetheproblemswithwavereflections.Currentcanbemodeledindirection

alongthebasin(inwavedirectionofthedoubleflapwavemaker).Thewaterdepthisadjustable

from0m(surfaceposition)to10mbymovingthefalsebottom.Amoredetaileddescriptionofthe

OceanLaboratoryatMARINTEKisgivenbyHuseandTrum (1981)andNaeser.(1981).

3.5 Generationofenvironment

Reliablemodeltestingrequirescontrolledgenerationofwind,wavesandcurrentinbothtimeand

spacetoachievearealisticandwelldefinedenvironment.Commonlyusedequipmentfor

environmentgenerationisdescribedinthefollowing.

3.5.1 Wavegenerationandabsorption

Therearetwomainclassesofwavegenerators,thehorizontaldrivenflaptypewavemakerandthe

verticaldrivenwedge(plunger)typewavemaker.Inmoderntestfacilitiesalmostonlytheflaptype

isused.TwoexamplesofflaptypewavemakersareshowninFig3.8.Thefirstoneisadoubleflap

wavemakerasinstalledinthetowingtankandintheoceanbasinatMARINTEK.Hydraulic

actuatorsareused.TheotheristhesingleflapwavemakerasinstalledinMarineCybernetic

Laboratory(MCLab)atMARINTEK.Thiswavemakeriselectricallydriven.Therearsideoftheflap

maybeeitherdryorwet. Thedoubleflaptypeisusuallyusedfordeeperwater.Bythepossibility

ofusingtheupper,thelowerorbothflapsincombinationsforthedoubleflaptype,itispossibleto

generatewaveswithaminimumofdistortionforlargerwavelengthrangethanwhatispossiblefor

asingleflapsolution.


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Page22

Figure 3.7 Examples of flap type wave maker; single flap and double flap.

Othertypesofwavemakersarepistontype(asmallsketchisshowninFigure3.9),andpneumatic

wavemakers.Pneumaticwavemakersusevariableairpressureinachamberabovethewaterat

theedgeofthebasintocreatewaves.DavidTaylorModelBasininWashingtonDChasapneumatic

wavemakerintheiroceanbasin,exceptforthattheprincipleislittleused,andisconsidered

inferiorrelativetoflaptypewavemakers.

Figure 3.8 Wave-maker theory, Wave height to stroke ratio as function of relative depth.


24/182


Page23

Thegenerationofwavesarecontrolledbythefrequencyandamplitudeoftheflap.In

Figure3.9therelationbetweenflapstroke,S,andwaveheightH,isshownforflaptypewave

maker.Theresultingwaveheightisshownasafunctionoftheparameterkhwhere 2k is

thewavenumber,isthewavelengthandhisthewaterdepth.Theresultsarebasedonwave

makertheoryseee.g.DeanandDalrymple(1984).Thisratiobetweenthemechanicaldisplacement

ofthe

flap

to

the

wave

amplitude

is

the

transfer

function

of

the

wave

maker.

Aregularwaveelevationcanbegeneratedusingthetransferfunctiontodeterminetherequired

controlsignaltothewavemaker.InFigure3.10themaximumwaveheightforregularwavesas

functionofwaveperiodisshownforthedoubleflapwavemakersinthetowingtankatMARINTEK.

Itisobservedthatincreasingwaveperiod(andhencewavelength),givesdecreasingmaximum

waveheight.

Figure 3.9 Maximum wave height as function of wave period, Double Flap wave maker at

MARINTEK

Thegenerationofirregularwavesiscontrolledbyaninputsignalbasedontheselectedwave

spectrum

combined

with

the

transfer

function

of

the

wave

maker.

The

commonly

used

assumption

thattheseasurfaceelevationisastationaryGaussianprocesswithzeromeanisapplied. The

surfaceelevationasfunctionoftime,(t),canthanberepresentedbyafinitenumberofFourier

components:

1

( ) cos( )N

n n n

n

t a t

wheren isthephaseangleofcomponentncreatedfromarandomphasegenerator.Random

phaseisnecessarytoeliminateanycoherentfeaturesdevelopinginthewavesignal.anisthe

FourieramplitudeofcomponentndeterminedfromtheinputwavespectrumdensityS()as:

2 ( )n na S


25/182


Page24

isthefrequencyintervalforeachcomponent.Itisimportantwithasufficientnumberof

componentsinthewavegenerationtoavoidrepetitionofthewavesignalandaliasing,see

Newland(1975)forfurtherdetails.Typically,2000componentsareused.

FormostcasesthewavespectraaregeneratedaccordingtotheJONSWAPformulation:

2020 2()(exp4

052 )(25.1exp)( gS

where:

07.0 for 0

09.0 for 0

0 isthespectralpeakfrequency

isthepeakednessparameter.

InFigure3.11atypicalexampleoftheoreticalJONSWAPspectrumandmeasuredwavespectruminthewavetankareshown.Theagreementinenergydistributionisseentobeverygood.

Figure 3.10 Typical example of theoretical JONSWAP spectrum and measured wave spectrum in

the wave tankMultidirectional(orshortcrested)wavescanbegeneratedusinganarrayofflapsalongonesideof

thebasin.Therearetypicallyaboutahundredindividualflaps.Themultiflapwavegeneratorscan


26/182


Page25

alsobeusedtogeneratelongcrestedwaveswithanarbitrarywaveheading. Togenerate

shortcrestedwavethedirectionalspreadingfunctionmustbespecifiedinadditiontotheenergy

densityspectrum.Thisgivesthefollowinggeneralisationforthesurfaceelevationasfunctionof

spaceandtime,(x,y,t):

nnnnnN

n

n tyxkatyx )sincos(cos),,( 1

whereannowistheFourieramplitudeofcomponentnincludingthespreadingfunction:

),()(2nnnn DSa

Toavoidreflectionandwavebuildingupinthebasin,anefficientwaveabsorptionsystemisalso

essential.Themostusedsystemforwaveabsorptioniswavebeaches.Anexampleisshownin

Figure3.12.Theshapeisparabolicwhichhasbeenfoundtobemoreefficientforalargewave

periodrangecomparedtoastraightbeach. Reflectedwaveheightoflessthan5%oftheincoming

waveheightwilltypicallybeachievedwiththisbeachdesign. Inoceanbasinswaveabsorbersareusuallymountedonthesideswithoutwavemakers,typicallywithtwosideswithwavemakersand

twosideswithwaveabsorbers(asinMARINTEKoceanbasin).

Intowingtanksthemainproblemwillbereflectionoftheshipgeneratedwavesystemfromthe

tankwalls.Itisnotpracticaltomountabeachalongsidethetankwallandtransversewaveswillbe

generated.Fortestswithforwardspeedthisisusuallynotaproblem,sincethereflectedwaveswill

hitthetestareaafterthemodelhasleft.Fortestswithzeroorverylowforwardspeed,the

problemofwavereflectionsmustbetakenveryseriously.Forlongtestruns,likeistypicalforatest

inirregularwaves,somekindofwaveabsorptionalongthetankwallisrequired.


27/182


Page26

Figure 3.11 Upper; Wave absorber, beach type. Lower; measured wave reflection from beach, in

% of incoming wave amplitude.

3.5.2 Windgeneration

Windgenerationismostconnectedtostationarymodeltestsinoceanbasins.Forfreelymoving

modelswind

is

not

easily

applied

in

practical

testing.

The

wind

in

the

basin

is

usually

generated

by

meansofabatteryofportableelectricalfans.Thefansareplacedsomedistancefromthetesting

areatoachieveahomogeneouswindspeeddistributionatthepositionofthetestmodel.Thewind

directioncanbechangedbymovingthepositionofthefans.

Twodifferentmethodsforcalibratingthewindspeedarecommonlyused:

1. Froudescalingofwindspeed,i.e: WindFWindM UU ,,

2. Usingprecalculatedwindforceactingonthemodelandtuningthemodelwindspeedto

thisforce

is

achieved.

Forthefirstcasethewindspeediscalibratedatthepositionofthemodel,butwithoutthemodel

present.Usingthisprocedurerequireaveryaccuratemodellingofthemodelsuperstructureto

obtainreliablewindforces. Thelastprocedurerequiresthatreliablewindforceestimatesare

availableonbeforehand.Ifthisisthecasethescaleeffectsonwindforcescanbeavoided.Usually

thisprocedurewillgiveabout20%higherwindspeedthanthespeedestablishedfromFroude

scalinglaw.

Theeffectofwindwillbeimportantforalmostalltypesofmooredstructures.Thewindspeedisin

general

non

steady

and

the

dynamic

effects

of

the

winds

can

be

an

important

exaction

source

for

resonancemotionsofmooredstructureandinspecialcasesforrollmotionsofships.Thedynamic


28/182


Page27

effectofwindcanbesimulatedinthetestbycontrollingthepowertothefans.A

frequentlyusedwindenergyspectrumforoffshoreapplicationsistheNPDspectrum.

Thefrequencyrangeofawindspectrumisquitebroadbanded,oftencoveringarangefrom0.005

to1Hz.

3.5.3

Current

Tosimulatecurrenttwodifferentapproachesarecommonlyused:

1. Directgenerationofcurrentinthebasinusingpumpingofwater

2. Towingofthemodelsetupwithspeedequaltothecurrentspeed.

Thefirstapproachrequireslargepumpswithrecirculationducts.Forbasinswithafalsebottom(as

atMARINTEK)thepumpscanrecirculatethewaterunderthefalsebottom. Alsoexternalpiping

(outsidethebasin,asusedinMCLabatMARINTEKandatMarinintheNetherlands)canbeusedfor

recirculationofwater.

Localcurrentcanbegeneratedbyplacingportablecurrentgeneratorsinfrontofthemodel(in

principleasforthewindgeneration).Howeverforthismethoditisdifficulttoachieveareasonable

stationarycurrentfieldatthepositionofthemodelduetolargeeddiesofbackflowingwater.The

effectofcurrentonwaveforceswillnotberealisticallyaccountedforbythisproceduredueto

largespacevariationincurrentfieldbetweenthewavemakerandmodel.

Theeffectofcurrentisespeciallyimportantformooredstructures,bothduetothedirectforces

duetothecurrentandduetotheinteractionbetweencurrentandwaves.Theinteractionbetween

currentandwaveseffectcanlargelyinfluencethewavedriftforcesandhenceinfluencemean

offsetandforcesaswellastheslowdriftmotions.Forthecasewithmooredstructuresthe

simulationofthecurrenteffectbytowingthemodelwithaspeedequaltothecurrentspeedisnot

apracticalsolution.Forthiscaseabasinwithdirectcurrentgenerationwillberequired.

ThegenerationofcurrentspeedisbasedontheFroudescalinglaw.Thisisnecessarytoproperly

representthewavecurrentinteractioneffects,butitmayintroducesomescaleeffectsforthe

currentforcesduetodifferenceinRenumber,resultinginpossiblydifferentflowregimesinmodel

andfullscale.

Currentcalibrationofspeedandprofileshouldbeperformedwithoutthemodelinthebasin.

Velocityfluctuationswillalwaysbepresentinbasingeneratedcurrent(inrealfullscalecurrent,

fluctuationswillalsobeobserved).Largefluctuationsincurrentmayrepresentanexcitationsource

forslowlyvaryingresonanceoscillationsandthemagnitudeshouldthereforebeaslowaspossible.

Astandarddeviationforthecurrentfluctuationofabout5%ofmeancurrentistypicallyachievedin

basinswithclosedrecirculationsolutions.Correctmodellingofthecurrentfluctuationsmightbe

importantforthedynamicsofdeepwatersystems,butwestillnotknowenoughaboutthis,andno

modeltestingfacilitiescurrentlyhavepossibilitiestocreatecontrolledcurrentfluctuations.


29/182


Page28

4 INSTRUMENTATION

4.1 Generaldescriptionofequipment

Alargerangeofdifferenttypeofmeasuringequipmentisusedfortestingofshipsandoffshore

structures.Usually,instrumentsaredesignedtogenerateananalogvoltageorcurrentsignalwhich

islinearlyproportionaltothemeasuredparameter.Nonlinearcharacteristicsoccurinrarecases.

Instrumentswithdigitaloutputareincreasinglyused,butanalogoutputisstillpreferred,inorder

toavoidthecomplexitiesofdealingwithdifferentdigitalsignalprotocols.

Thesystemrequiredforperformingmeasurementsincludesthefollowingcomponents:

Thetransducers

Amplifiers

Filters(analogand/ordigital)

ADconverter

Datastorageunit

Cablingbetweenthedifferentcomponents

AtypicalsetupisshowninFigure4.1.Itiscommonpracticetousetwoormoreindependent

computersystemsforoperatingthetankfacility.Onemachineisusedforrealtimegenerationof

controlsignalforthewavemakerandanadditionalmachineisusedforthedataacquisitionand

analysis. Additionalmachinesmightbeusedforcontrolofruddersorothercontroldevices,orfor

controlofthecarriage.

Figure 4.1 Schematic of typical set op of a data acquisition system for model testing

Afurtherdescriptionofinstrumentationandtransducersrelevantformodeltestingcanbefoundin

Olsen(1992).Adetaileddescriptionofmeasurementtechniquesforfluidmeasurementsisgivenby

Goldstein(1983)

4.2 Strainanddisplacementmeasurements

Themostusedmethodsforstrainanddisplacementmeasurementsinmodeltestingarebasedon

thefollowingprinciples:

Resistivetransducers, basedonchangeofresistanceduetostrain; straingauges

Inductivetransducers Capacitancetransducers.

TRANSDUCER

AMPLIFIER& SIGNALCONDITIONER

A/DCONVERTER

RESPONSE

C

OMPUTER

DATABUS


30/182


Page29

Inadditiontoadirectmeasureofstrainanddisplacements,thesetypeoftransducerare

alsocommonlyusedasthebasisforpressurecells,forcetransducers,velocitymeasurementsand

accelerometers.Theseapplicationswillbediscussedseparately.

4.2.1 Straingauges

Thestrain

gauge

measured

the

elongation

in

the

material

to

which

it

is

glued.

Strain

gauges

are

commonlyusedinanumberofdifferenttypesoftransducers.Examplesofstraingauge

constructionsareshowninFigure4.2.ThethreadsareusuallyCUNialloys.

Theuseofstraingaugesisbasedonthattheelongationofthestraingaugewillchangethe

resistance.Thegaugefactork,isdefinedas:

LL

RR

k

whereR

is

the

resistance

and

Lis

the

length

and

represent

the

change

of

length

or

resistance.

The

factork istypicallyabout2formetallicmaterials.Increasingthefactorkwillincreasethe

sensitivityofthestraingauge. kvaluesuptoabout100200canbeachievedusingpiezoresistive

materials.

Theelongationofthestraingaugesususuallymeasuredinmicrostrain,S=106S,where

LLS .

Figure 4.2 Examples of strain gauges designs

Tomeasure

the

change

of

resistance

over

astrain

gauge,

a

Wheatstonebridgecircuitisused.AbasicWheatstone

bridgecircuitcontainsfourresistances,aconstantvoltage

input,andavoltagegage.ForagivenvoltageinputVin,the

currentsflowingthroughABCandADCdependonthe

resistances,i.e.,

4321 RRIRRI

VVV

ADCABC

ADCABCin

Thevoltage

drops

from

A

to

B

and

from

A

to

D

are

given

by:

Figure 4.3 Wheatstone bridge circuit


31/182


Page30

4

34

4

1

21

1

RRR

VRIV

RRR

VRIV

in

ADCAD

in

ABCAB

ThevoltagegagereadingVgcanthenbeobtainedfrom:

in

inin

ADABg

VRRRR

RRRR

RRR

VR

RR

VVVV

3421

4231

4

34

1

21

Nowsupposethatallresistancescanchangeduringthemeasurement.Thecorrespondingchange

involtagereadingwillbe:

ingg

VRRRRRRRRRRRRRRRRVV

33442211

44223311

Ifthebridgeisinitiallybalanced,theinitialvoltagereadingVgshouldbezero.Thisyieldsthe

followingrelationshipbetweenthefourresistances:

rR

R

R

RorRRRR

VRRRR

RRRRV ing

1

0

3

4

2

14231

3421

4231

Wecanusethisresulttosimplifythepreviousequationthatincludesthechangesinthe

resistances.DoingsoresultsinthesolutionforthechangeinVg:

ing VR

R

R

R

R

R

R

R

r

rV

1

1 4

4

3

3

2

2

1

1

2

whereisdefinedby:

3

3

2

2

4

4

1

1

1

1

1

R

R

R

Rr

R

R

R

R

r

Moreover,whentheresistancechangesaresmall(


32/182


Page31

Inpractice,oneoftenusesthesameresistancevalueforallfourresistors,R1=R2=R3=R4=R.

Notingthatr=1inthiscase,thechangeinvoltagecanbefurthersimplifiedto

ing VR

RRRRV

4

4321

DifferenttypesofWheatstonebridgecircuitsareusedtomeasurethechangeofresistanceoverastraingauge. Averysimpleexampleformeasurementofforceisshowninfigure4.4. Onestrain

gaugeismountedtoeachsideofabeam.Twodummyresistances(usuallyintegratedinthe

amplifier)areusedforbalancingthebridge. Thissetupiscalledahalfbridge.Aconstantvoltageis

usedasexcitation,Vin.Theforcegivesrisetoanelongationofstraingauge1andcompressionof

straingauge2.Thisintroducesanunbalanceinthebridgeandavoltagecanbemeasuredatthe

exitatVg.Inafullbridgecircuit,allfourbranchesofthebridgearestraingauges.Mountingtwo

straingaugesoneachsideofthebeaminFigure4.4inafullbridgearrangementwouldgivetwice

thesensitivityofthehalfbridgearrangement.

Figure 4.4 Examples of half-bridge circuit for measurements of change of resistance of strain

gauges.

4.2.2 Inductivetransducers

Theinductivetransducersarebasedonthevoltageinducedbyamovablecore.Anexampleofan

applicationisshowninFigure4.5.TheshownsystemiscalledLVDT(linearvariabledifferential

transformer).OnesetofthecoilisexcitedbyACvoltageandtheinducedvoltageismeasuredin

thesecondset.

Thistypeoftransducerisavailableinawiderangeofsizes,frequencyrangesandaccuracys.Itis

usedfordirectpositionmeasurements,butalsoasbasisforpressurecellsforcetransducers,

velocitymeasurementsandaccelerometers.

1 2

Side view Front view

Force K

Straingauges R

+R

R

R-

R

R

A B

B

C

VgG

Vin


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Page32

Figure 4.5 LVDT transducer for displacement measurements.

4.2.3 Capacitancetransducers

Thecapacitancetransducersconsistoftwocloselyspacedplatesoraplatesuspendedbetweena

pairofouterplatesasshowninFigure4.6.Theplatesareconductiveandrelativemovement

betweentheplatesintroduceavariationofthecapacitance

Figure 4.6 Capacitance transducers for displacement measurements.

Thistypeoftransducersrequireamuchsmallerdrivingforcecomparedtoinductivetransducers,

buthaveahighernoiselevelandarethereforelessfrequentlyusedinpracticalmodeltesting.

4.3 Positionmeasurements

Typicalpositionmeasurementsofinterestforfloatingstructureswillbethe6degreesoffreedoms

rigidbodymotionsofship/platformsandmotionsofmooringlinesandrisers. Otherexamplesare

measurementsofdeflectionsofelasticmodelsasforspringingandwhippingresponseofships.

4.3.1 OpticalandVideosystems

Forfreerunningmodelsandmooredstructurestheglobalmotionsaremeasuredbyopticalor

videobasedsystems.Foropticalsystemminimum3lightemittingdiodesarelocatedonthemodel.

F

F


34/182


Page33

Forvideosystemsballshapedreflectorsmountedonthemodelisused.Onshorecameras,

minimum2,areusedforreadingthepositionofeachdiode.Basedontheinstantaneousposition

(x,yandz)ofeachofthe3diodes,themotionsin6DoFaredetermined.

Theaccuracyofthemeasuredmotionswillforbothopticalandvideosystemsbeoftheorderof

+/ 1mmforposition(inmodelscale)and+/0.05degreesforroll,pitchandyaw.Formostcases

thisisanacceptableaccuracyfortherigidbodyvesselmotions.

4.3.2 Gyros

Therollandpitchmotionscanalsobemeasuredusinggyros. Theprinciplebehindthegyrois

showninFigure4.7.Arotatingmasskeepstheinnerpartofthegyroinacontinuoushorizontal

position.Thenextlinkcanbetiltedaboutoneaxisandtheangleismeasuredusinga

potentiometer. Theouterlinkcanbetiltedaboutanaxisperpendiculartothefirstaxisandthe

anglecanbemeasuredinthesameway. Fromthemeasuredanglesandtheknownsequenceof

theanglestherollandpitchmotionsareuniquelydetermined.

Thegyroisarobusttoolandiscommonlyusedbothinmodeltestingandininstrumentsappliedin

fullscale.However,sinceitinvolvescomplexmechanicalcomponents,itisfairlylargeandfairly

expensive.Thesizelimitstheuseinmodels,andthecostlimitstheusebothinmodelandinfull

scaleexperiments.

Figure 4.7 Principle of a gyro for measurement of roll and pitch.

4.3.3 Potentiometer

Lowfrictionpotentiometerscanbeusedformeasurementofmotioninonedirection.ANylonline

isconnectedtothemodel,thenpassedaroundthepulleyonthepotentiometerspindleand

tensionedbysprings.Themotionofthemodelwillthereforebedirectlytransducedintoavoltage

signalbythepotentiometer.

Acommonlyusedsetupformeasurementsofheaveandpitch(trim)fortowingtestsisshownin

Figure4.8. Thepotentiometersareusedformeasurementsofthemotionsbetweentheshipmodelandthetowingcarriage.Thesetupisusedbothformeasurementsofrunningheaveandtrimin

calmwatertestingandformeasurementsofwaveinducedmotionsinheadseawaves.


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Page34

Figure 4.8 Example of using potentiometers for measurements of heave and pitch motions in

towing tests.

4.3.4 Positionbasedonmeasuredforceandacceleration.

Measurementofpositioncaninprinciplebeobtainedfrommeasuredacceleration(seebelow)bya

doubleintegrationofthemeasuredaccelerationsignal:

BAtdtdttatx )()( Ascanbeseen,positionbasedonintegratedaccelerationcannotgiveinformationaboutmean

levelorpossibleconstantdriftoftheposition.Increasingperiodofoscillationwillgivereduced

accuracyofthederivedposition.Forpracticalapplicationsthismethodarethereforeusuallylimited

tomeasurementsofthewavefrequencypartofthemotions.

Anotherindirectwaytoestablishthepositionisusingmeasuredforceincombinationwithalinear

spring:

k

tF

tx

)(

)(

Themethodrequiresaspringconnectionbetweenthemodelandafixedpoint(e.gtowingcarriage,

seabedetc.).Itisnecessarythatthespringstiffnessissufficientlowtoavoidanyinfluenceonthe

dynamicbehaviorofthemodel.

4.4 Accelerations

Measurementsmadebyaccelerometersarebasedontheratiobetweenforce,massand

acceleration:


36/182


Page35

m

tFta

)()(

Amasscanbeconnectedtoabeam.Whenexposedtoaccelerationthebeamwillbedeflectedby

theinertiaforces.Thedeflectionofthebeamisproportionaltotheacceleration.Straingaugescan

beusedformeasuringthedeflectionofthebeamandhencetheaccelerationisobtained.

Anothertypeofaccelerometersisbasedonthepiezoelectricaleffect.Apiezoelectricalmaterialis

amaterialwhichwhendeformedproducesanelectricalfield.Thevoltagegeneratedisproportional

tothesurfacepressureapplied.Combinedwithamassthisgivesasignalproportionaltothe

acceleration.TheprincipleisshowninFigure4.9. Thechargeistransferredtovoltageinacharge

amplifier,butsomeofthechargeleaksout.Thisgivestheaccelerometeralowerlimitforwhich

frequenciesthatcanbecovered.Thistypeofaccelerometerscanthereforeonlybeusedfor

dynamicmeasurements.

Figure 4.9 Piezo electric material exposed to surface pressure.

Theresonanceofthemassspringsystemmayinfluencethemeasurementofaccelerations.Forfrequencieswellbelowtheresonancefrequencythemasswillfollowthemotionsofthehousing

andwehavealinerrelationbetweentheaccelerationandthesignalout.Forfrequenciesinthe

resonanceregionthemasswillbeexitedandthesignaloutwillbefrequencydependent.The

dynamicamplificationswillbedependentofthedampingofthesystem,butingeneral

accelerometersshouldonlybeusedformeasurementsofresponseswithoscillationfrequencies

wellbelowthenaturalfrequencyoftheaccelerometer. AccelerometersbasedonPiezoelectricity

canbemadeverystiffwithresonancefrequencyhigherthan500KHz.Thismakesthemusefulfor

applicationsofmeasurementsofresponseduetoimpactloads. Toincreasethesensitivityofthe

accelerometerthemassmustbeincreasedorthestiffnessreduced.Accelerometerswithlow

naturalfrequencywillthereforebemoresensitivethantheaccelerometerswithhighresonancefrequency.

Theaccelerometersaddweighttothestructureanditisthereforeimportanttoensurethatthat

weightissufficientlylowtoavoidanyinfluenceonthedynamicbehaviour.Theweightofthe

accelerometercanbemadeverysmall,typicallydowntoafewgrams.

4.5 PressureTransducers

Pressuremeasurementsaremostlyperformedusingpressurecells.Threetypesofpressurecellsarecommonlyused:


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Page36

1. basedonpiezoelectricity

2. basedoninductivetransducers

3. basedonstraingauges.

Pressure

cells

are

basically

force

measurements

over

a

small

area.

Typical

dimensions

of

presser

cellsusedformodeltestingisD=210mm.Thedifferenttypesofpressuremeasuringdevicesare

illustratedinFigure4.10.

Figure4.10 Schematicrepresentationofthemostcommontypeofpressuretransducers.

(a):Capacitancetransducer, (b):Piezoelectricand(c):Straingauge

Pressurecellsbehaveinmanywayssimilarasanaccelerometerandtheresonanceofthemass

springsystemmayinfluencethemeasurements.Thedynamicamplificationswillbedependentof

thedampingofthesystem,butingeneralpressurecellsshouldonlybeusedformeasurementsof

responseswithfrequencieswellbelowthenaturalfrequencyofthecell. Straingaugetypecells

respondstodisplacementsfromdcto5kHz.Itisthereforewellsuitedformostpracticalmodel

testing.Pressurecellsbasedonpiezoelectricitycanbemadeverystiffwithresonancefrequency

uptomorethan500kHz. AnexampleofthistypeofpressurecellisshowninFigure4.11.This

transduceris

therefore

well

suited

to

measurements

of

pressure

behavior

with

very

low

rise

time

as

willbethecaseforslammingpressuremeasurements.Anexampleofmeasuredslammingpressure

fortheimpactofaflat,elasticplatetowardsawavecrestisshowninFigure4.12.Itisobserved

thatclosetothecenteroftheplatebottomwherethewavecresthit,therisetimeislessthan

0.0001sandthedurationofthepeakextremelyshort.Consequentlyapressurecellwithveryhigh

resonanceperiodwillberequiredtoaccuratelyreproducethispressurebehavior.


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Page37

Figure 4.10 Piezoelectric pressure transducer for pressure measurements.

Figure 4.11

Example of measured slamming pressure. Impact of a horizontal circular cylinder

towards calm water surface.

4.5.1 Measurementofpressuredistribution

Inthe

later

years,

pressure

sensing

film

has

been

developed

by

several

different

companies.

The

filmisbasicallyamatrixofsmallpressurecellsintegratedintoaflexibleplasticfilmthatcanbe

appliedtocurvedsurfaces,andwilleffectivelyreturnthepressuredistributionoverthesurface.

Thepressurecellsaremadeofalayerofsemiconductingmaterialwherethedegreeof

conductivitydependsonthepressureappliedtothematerial.Twocompaniesthatdevelop

pressuresensingfilmareTekscanhttp://www.tekscan.com/andPressureProfileSystems

http://www.pressureprofile.com/.Thistechnologyhasmainlybeendevelopedfordry

applications,liketestinganddevelopmentofcarseats,sportsequipment,andsimilar.Thus,itisnot

straightforwardtoapplyittomarinehydrodynamicsproblems,butthepossibilityofeasily

measuringthepressuredistribution,notonlypointpressures,meansthatthistechnologyis

probablygoingtobeappliedalsotomarinehydrodynamicsinthefuture.


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Page38

Aslightlysimilartechnologyispressuresensitivepaint(PSP).Thecolorofthepaintis

changingwiththepressure,whenaspeciallightsourceisused.Thisisalsoarecentmeasurement

technique,developedintheformerSovietUnionandknownintheWestsinceanadvertisementin

AviationWeeklyin1990.Itisprimarilyusedinwindtunneltestingofaircraft.Thepressure

sensitivepaintissensitivenotonlytopressure,buttotemperatureandoxygencontentintheflow,

somethingthatcomplicatestheapplication.

4.6 Velocities

Thevelocity inapointcanbeobtainedbyastraightforwardintegrationofmeasuredacceleration

orbyaderivationofmeasuredposition.Bothmethodsarecommonlyusedforvelocity

measurementsofstructuralcomponents.

Formeasurementoffluidvelocitydifferentprinciplesarepossible:

Basedonmeasurementsofpressure,e.g.pitottubes LDV

Ultrasonictransducers

Bymeasuringrateofrevolutionofasmallimpeller.

Thetwofirstmethodsarediscussedinthefollowing.

4.6.1 Pitottubes

Thepitottubesensoriscommonlyusedformeasurementofthewakesurveys,flowthroughwater

jetsetc.ThePrandtlpitottubeisshowninFigure4.13.Thepressuredifferencebetweenthetotal

pressureheadatthefront(inpos.Ainthefigure)andthestaticpressureattheside(atpositionB)

ismeasuredbyadifferentialpressurecell.Basicallythepitottubeisapressuredifferencemeasure,

butthevelocityisobtainedfromthewellknownrelation:

21 2p U

Toimproveaccuracythistheoreticalrelationisnotused,insteadthecalibratedrelationbetween

pressureandvelocitywillbeapplied.


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Page39

Figure 4.12 Prandtl pitot tube.

Tocovermorepositionsinthesamerun,severalpitottubescanbemountedtogetherasshownin

Figure4.14.

Usingpitottubeswithfiveholeslocatedindifferentangularpositiononasphericalhead,the

velocityinallthreedirectionscanbemeasured,incaseofwakesurveys;axial,tangentialandradial

velocity.

Figure 4.13 Pitot tube arrangement for measurement of velocity in several positions.


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Page40

4.6.2 LaserDopplerVelocimetry LDV

LaserDopplerVelocimetry(LDV)hasbeenusedformorethan20yearsformeasurementsofflow

aroundshipsandpropellers.Theusehasbeenmainlyforvalidationofpredictiontoolsforvelocity

distributionaroundliftingsurfaces,withinboundarylayersandforwakeflow.GenerallyLDV

measurementsaretimeconsumingandexpensivetoperformandtheuseforcommercialmodel

testinghave

been

very

limited.

See

ITTC,

(1996)

for

afurther

discussion

for

application

of

LDV

in

modeltesting.

LDVusestheDopplershiftinthereflectedlightfrequency(color)todeterminethevelocityand

directionoftheflow.Byusingtwobeamsfromdifferentdirection,the3Dvelocityvectorina

singlepointcanbemeasured.Tomeasurevelocityindifferentpositions,thetransmittingand

receivingopticsmustbemoved(seeFigure4.14).Thisiscommonlydoneusinganautomatic

traversingsystem.

OneofthemainbenefitsofLDVisthatitisanonintrusivemeasurementtechnique,whichmeans

thatone

does

not

have

to

put

any

sensors

into

the

area

where

one

wants

to

measure,

thus

one

is

notinterferingwiththeflowfieldofinterest.LDVdoalsogiveveryquickresponse,whichmeans

thatitissuitableformeasurementofturbulenceandsimilarlyrapidlychangingflowphenomena.

Formeasurementofaveragevelocityinonedirection,itisgenerallyrecommendedtousepitot

tubesorsimilartechniques.

ForLDVtoworkthereneedstobelightreflectingparticlesdissolvedinthewaterinorderto

providelightscattering.Ifthereisnoparticlesinthewaterthelightfromthelaserwillnotbe

reflected.Infact,oneisnotmeasuringthewatervelocity,butrathertheparticlevelocity.There

mightbesufficientdirtinthewaterfromthestart,butitiscommontohavetoapplyparticlesfor

thepurpose

of

LDV

measurement.

This

process

is

called

seeding.

Proper

seeding

is

one

of

the

keys

tosuccessfulapplicationofLDV.Whentestinginlargefacilities,likealargetowingtank,seeding

mightbeoneofthemainchallenges.Seedingtheentiretankisdifficultandexpensive.Seeding

locallyintheareaofmeasurementmightdisturbtheflow,anditmightbedifficulttoobtaina

reasonablyhomogeneousdistributionofparticles.

Figure 4.14 Principles of a LDV measurement system

Laser

Signal

processing

Transmitting

optics

Receiving opt ics

with detector

Signal

conditioner

Flow

HeNeAr-Ion

Nd:Yag

Diode

Beamsplitter(Freq. Shift)

Achrom. Lens

GasLiquid

Particle

Achrom. LensSpatial Filter

Photomultiplier

Photodiode

Spectrum analyser

Correlator

Counter, Tracker

Amplifier

Filter

PC


42/182


Page41

4.6.3 ParticleImageVelocimetry PIV

ParticleImageVelocimetry,orPIV,isanonintrusive,opticaltechniqueformeasurementofvelocity

vectors.AsforLDVlaserlightisused,andsimilartoLDVthewaterneedsparticlesseedingto

providelightscattering.WhileLDVmeasuresvelocityinasinglepointatatime,PIVmeasuresthe

velocityinanarea,soitisconsideredafieldmeasurementtechnique.PIVusesalaserlightsheet,

createdby

putting

aspreading

lens

in

front

of

the

laser

beam.

By

taking

two

stereo

photographs

(or

onedoubleexposurestereophotograph)withveryshorttimeintervalthevelocityoftheparticles

canbedeterminedfromhowfartheyhavemoved.Byusingstereophotographs,themovementin

space,notonlyinasingleplane,canbedetermined.PIVhasbeeninuseforalongtime,butitis

theadvanceindigitalhighspeedvideoandimageanalysisthatquiterecentlyhasmadePIV

interestingtoapplyinregularhydrodynamicsresearch.Inthetimeoneusedfilmbasedpictures

andmoreorlessmanualanalysis,PIVwasextremelytediousandtimeconsuming.Now,semi

automatinganalysisofthedigitalvideoimagesmeansthatlargeamountsofmeasurementdatacan

begeneratedfairlyquickly.Also,theaccuracyhasbeengreatlyincreased.PIVisincreasinglyused

insteadofLDV,sinceitismuchfastertomapavelocitydistributionactuallythedistributioninan

areaiscapturedatoneinstant,andnotoversometime,aswithLDV.

Figure 4.15 Principles of a PIV measurement system

4.7 Forcemeasurements Dynamometers

Theloadcellsusedinmodeltestingareoftendesignedtofitaspecificpurposeandtocoverthe

expectedrangeofloadsduringthetests.Forexamplefortowingtests,veryhighaccuracyin

measuredforcewillberequiredandthetransducerwillbetailormadetomeasureforceinone

directionwithahighaspossibleresolution.Thisiscalledaresistancedynamometerandisstandard

equipmentinatowingtank.InFigure4.15adynamometerformeasurementsofpropellerthrust

andtorquedirectlyonthepropellerhubisshown. Thethrustismeasuredbyaninductiveposition

transducerandthetorquebystraingaugeswhichmeasuresheardeformationonahollowpartof

thepropellershaft.


43/182


Page42

Figure 4.16 Dynamometer for measurements of propeller thrust and torque.

Transducersformeasurementsofforcesinoneormoredegreesoffreedomareoftenproducedas

apurposemachinedpiecewithstraingaugesgluedtothebasematerial.Atypicaldesignfor

measurementofforcesin3d.o.fs(axialforceandshearforces)isshowninFigure4.17.

Figure 4.17 Example of force transducer based on strain gauges. Axial forces and shear forces

are measured.

Themachiningofthematerialgivesareaswithhighshearstresseswherethestraingaugesare

mounted.Bycarefuldesignofthetransducerthecrosstalkcanbekeptataminimum.Cross

talkmeanscouplingeffectsbetweenthedifferentdegreesoffreedom.Forexampleforthe

transducerinFigure4.17,ifapuretensioninxdirectionisapplied,themeasuredresponsefrom

thetransducerinyandzdirectionisaresultofcrosstalk.Fortransducerdesignitisimportant

withminimumcrosstalk.

Using3transducerofthistypemountedbetweentwoplatesa6d.o.ftransducerisobtained.

4.8 WaveMeasurements

Waveelevationismostcommonlymeasuredbymeansofwaveprobesoftheconductive(or

resistance)type. Avoltageisappliedontoparallelrods.Theresistanceisdeterminedbythelength


44/182


Page43

oftherodswhichiswetted(orsubmerged).Bymeasuringthecurrentduetotheapplied

voltageacrosstherodsthewettedpartandhencesurfaceelevation,isdirectlyachieved. Arraysof

threeormoreofthistypeofsensorscanbeusedtomeasurethedirectionaldistributioninshort

crestedwaves.

Waveprobesarealsousedformeasurementofrelativemotionsbetweenthestructureandthe

watersurface.Thewavegaugemaythenbemountedonthestructureandtherelativemotionin

thepointisdirectlyobtained.ExamplesoftransducersolutionsforthispurposeareshowninFigure

4.18. Bothrodsandconductivetapegluedtothemodelareusedforthispurpose.Rodsare

preferableatzerospeedduetoeasiermountingandcalibration,whileconductivetapeisoften

requiredformodelswithforwardspeed,toavoidresistanceandwatersprayfromtherods.

Figure 4.18 Example of wave transducers for measurements of relative motions.

Formeasurementofwavesathighforwardspeeds,thewirebasedprobesdontworkwell,dueto

thesprayandwavemakingofthesurfacepiercingwires.Runupinfrontofthewiresand

ventilationbehindthewiresleadstolargeerrorsinthemeasurements.Forforwardspeedsofmore

than2m/s,thewirebasedprobesshouldnotbeused.Alternativesaremainly:

Ultrasoundwaveprobes

Servoneedlewaveprobes

Ultrasoundwaveprobesworksbysendingoutahighfrequencysoundpulse,andmeasuringthe

timeittakesbeforethereflectedsoundwavereachestheprobe.Thetechniquehasbeenusedfor

alongtimeforlevelmeasurementintanksanddams,butitisnotuntilfairlyrecentlythat

instrumentswithsufficientaccuracyfortowingtankwavemeasurementhasbeenavailable.The

UltraLabsystems

from

General

Acoustics

is

in

practical

use

in

the

hydrodynamic

laboratories

at

the

MarineTechnologyCentre.Someofthesesystemshavelimitationswithrespecttoforwardspeed


45/182


Page44

andwavesteepness,whilethemostadvancedsystemscoverallconditionsofpractical

interestfortowingtankwork.

Servoneedlewaveprobesconsistofasensingneedlemountedonaservomechanism.Thesensing

needlemeasuresthedegreeofcontactwiththewater,andtheservomechanismmakessurethe

needlehasafairlyconstantsubmergence.Then,theactualwaveheightisfoundbymeasuringthe

positionoftheneedle.Thisisafairlycomplexandfragileinstrument,andtheonlyreasontouse

suchaninstrumentisthecapabilityofwavemeasurementathighforwardspeed.

4.9 DataAcquisition

Thedifferentinstrumentsappliedinthetestsetupareconnectedbycablestoadataacquisition

systemtorecordthemeasureddata.AschematicofadataacquisitionsystemisshowninFig4.1.

4.9.1 Amplifiers

Atypicalanalogtransducersignaliswithoutputinmicrovolts.Itisthereforeamplifiedbyan

amplifier,usuallyto+/10Voltrange.

Dependingontypeoftransducerdifferenttypeofamplifiersisused.Amplifiersforanalogsignals

fromstraingaugeandsimilartypeoftransducerscanhavebridgeexcitationandbridgebalancing

builtin.Thismeansthattheamplifierwillprovidethecurrentandvoltagerequiredforthe

measurementbridge(seesection4.2.1Straingauges),inadditiontoamplificationoftheoutput

signal. Piezoelectricaltransducersrequireachargeamplifier. Inductiontypetransducersrequire

ACexcitation,whichmeansthatthedrivingcurrentofthetransducerisACratherthanDC.Strain

gaugetransducerscanuseeitherACorDCexcitation.

4.9.2

Filters

Filterscanbeeitheranalogordigitalfilters.Analogfiltersareappliedbeforethesignalsare

convertedtodigitalunitsbytheADconverterandmaybebuiltinasanintegratedpartofthe

amplifier.ThedigitalfiltersareappliedaftertheADconversionandcanbeimplementedeitherasa

digitalcircuitorassoftwareinthecomputerusedforanalysis.

Thefiltersremovesignalsatcertainfrequencybands.Dependingonwhichfrequencybandis

removedtheycanbesplitintothreeclasses:

Lowpass,i.e.forremovingofhighfrequencycomponents

Highpass, i.e.forremovingoflowfrequencycomponents

Bandpass

thedifferenttypesareillustratedinFigure4.19.

Analoglowpassfilteriscommonlyusedforremovingofnoiseasthenoiseisusuallyappearingata

significantlyhigherfrequencythanthephysicalmeasuringsignal. ToavoidNyquistphenomena(se

discussionbelow)thecutofffrequencyoftheanaloglowpassfilteredshouldbesettobelower

thanhalfthesamplingfrequencyfS..Thegeneralrecommendationistosetthecutofffrequency

muchlowerthanhalfthesamplingfrequency1/10ofthesamplingfrequencyisthepreferredvalue,butitmightmeanthathighsamplingfrequenciesarerequired,andthereforecompromises

areoftenmade.


46/182


Page45

Figure 4.19 Illustration of different type of filters.

Lowpassfiltersappliedinrealtimecausesatimedelay.Thismightbeillustratedwithanexampleofaverysimplelowpassfilter,whichiscreatedbycreatingthelowpassfilteredsignalfroman

averageoftheunfilteredsignaloveracertainperiodoftime.Intheexample,showninFigure4.20,

asinusoidalsignalwithperiodof30secondshashadanothersinusoidalsignalwithperiod1.2

secondssuperimposed.Thefilterisimplementedasarunningaverageover2.4seconds,seenin

Figure4.20asthetimeittakesbeforethefilteredsignalappears.Itisclearlyseenfromthefigure

thatthefilteredsignallagsbehind,withhalftheaveragingtime.Thelongertheaverageperiod,the

lowerthefilterfrequency,andthelongerthetimedelay.Iftheaveragingwindowcouldhavebeen

symmetricallyplacedrelativetothetimeofthefilteredvaluethedelaywouldbeavoided.Thiscan

beachievedwhenfilteringanexistingdataset,butnotwhenfilteringinrealtime,sinceitwould

requiretheabilitytolookintothefuture.

Itshouldbenotedthatfilteringisinpracticenotperformedbysimpleaveraging.Thereisalarge

varietyofmethods,whichisnotcoveredhere.SeeforinstanceDunn(2005)foradiscussionof

filteralgorithms.

Amplitude

Frequency

Ideal characteristic

Real characteristic

Low pass filter

High pass filter

Band pass filter


47/182


Page46

Figure 4.20Illustration of time-delay due to filtering

4.9.3 AnalogtoDigital(AD)Conversion

Mosttransducertypes,includingthelargegroupofstraingaugebasedtransducers,areanalog

transducers,meaningthattheyproduceananalogoutputsignal.Torepresentthissignalina

computerit

has

to

be

digitized.

The

process

of

digitalization

is

performed

in

the

AD

converter.

TheADconverterisusuallyaphysicallyseparateunit;aboxoranextracardforthecomputer.The

lineinpartofacomputersoundcardisanADconvertercustommadeforsound,butreallyquite

similartotheADconvertersusedforothermeasurements.TheADconverterunitwilloftenalso

haveapossibilityfordigitaltoanalogconversionDAsimilartothelineoutofacomputersound

card.TheDAconvertermightforinstancebeusedtoproduceasignalforacontrolsystem.TheAD

convertermightbeintegratedwiththemeasurementamplifier.ThisisthecasefortheHottinger

MGC+digitalmeasurementamplifiersinuseinthehydrodynamiclaboratoriesattheMarine

TechnologyCentre.

TheADconverterunitwill,dependingontheactualmodel,haveacertainnumberofchannels,

whichmeanshowmanysignalscanbeconvertedsimultaneously.LimitationsintheADconverter

willdeterminehowfastthedatacanbesampledthesamplingfrequency.Itmeansthatfor

experimentsrequiringveryfastsampling,specialADconverterequipmentmighthavetobe

acquired.

OneshouldalsobeawarethatsomeADconverterssampleallchannelsatexactlythesameinstant,

whileotherswillsamplethechannelssequentiallyduringthesamplinginterval.Ifforinstance10

channelsaresampledat10Hz,thesequentialADconverterwillsampleasinglechannelevery

1/100second,

so

that

each

channel

is

sampled

every

1/10

second,

but

not

at

the

same

time.

This

is

usuallynotaproblem,sincet

experimental methods in marine dynamics

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