SEDIMENT TRANSPORT AROUND THE BRIDGE PIER

SedimentTransportAroundtheBridgePier

Thesissubmitted inpartialfulfilmentoftherequirementsforawardofthedegreeof

BachelorofTechnology in CivilEngineering

Submittedby

AyushKumar(Enrol.No.171010012004)

HimeshPanchal(Enrol.No.171010011016)

PrashantGupta(Enrol.No.171010012009) SandeshKumarSingh(Enrol.No.171010012013)

Supervisors

Dr.ManojLanghi,AssistantProfessor

Dr.HimanshuSharma,AssistantProfessor

DepartmentofCivilEngineering

InstituteofInfrastructure,Technology,ResearchAndManagement (IITRAM),Ahmedabad-380026,Gujarat,India May2021

DepartmentofCivilEngineering InstituteofInfrastructure,Technology,ResearchAnd Management,Ahmedabad

DECLARATION

Wedeclarethatthiswrittensubmissionrepresentsourideasinourownwords.Whereothers’ ideas or words have been included, we have adequately cited and referenced the original sources.Wealsodeclarethatwehaveadheredtoalltheprinciplesofacademichonestyand integrity and have not misrepresented or fabricated or falsified any idea/data/fact/source in my submission. We understand that any violation of the above will cause for disciplinary actionbytheinstituteandcanalsoevokepenalactionfromthesourceswhichhavethusnot beenproperlycitedorfromwhomproperpermissionhasnotbeentakenwhenneeded.

AyushKumar (171010012004)

HimeshPanchal (171010011016)

PrashantGupta (171010012009)

Date: 15th June,2021

SandeshKumarSingh (171010012013)

DepartmentofCivilEngineering InstituteofInfrastructure,Technology,ResearchAnd Management,Ahmedabad

CERTIFICATE

This is to certify that the thesis entitled, “Sediment transport around the bridge pier” beingsubmittedbyMr.AyushKumar(Enrol.No.171010012004),Mr.HimeshPanchal (Enrol. No. 171010011016), Mr. Prashant Gupta (Enrol. No. 171010012009) and Mr. Sandesh Kumar Singh (Enrol. No. 171010012013) to the Institute of Infrastructure, Technology,ResearchandManagement(IITRAM),Ahmedabadfortheawardofthedegree of Bachelor of Technology in Civil Engineering is a bonafide record of research work carriedoutbyhimunderoursupervisionandguidance.Thethesiswork,inouropinion,has reached the requisite standard fulfilling the requirement for the degree of Bachelor of Technology. The results contained in this thesis have not been submitted, in part or full, to any otherUniversityorInstitutefortheawardofanydegreeordiploma.

Date: Place: Dr.ManojLanghi

AssistantProfessor, DepartmentofCivilEngineering,

Dr.HimanshuSharma AssistantProfessor, DepartmentofCivilEngineering, Dr.ManojLanghi Coordinator, DepartmentofCivilEngineering,

ABSTRACT

Thispaperpresentsthenumericalsimulationofflowaroundanobstructionandsediment transportin2-Dand3-Dwhichultimatelyleadsintotheprocessofscouring.Scouringisan effectofstrongturbulentfluctuationsandthedown-flowsaroundthebridgepier.Thestudy usesANSYSFLUENTCFDsoftwareinordertopredictthe3-Dimensionaland2-Dimensional flowfieldaroundthecircular.Themodelwasmadeusingstructuredwithoptimumorthogonal meshquality.Thepresentstudyusesk-ωandk-epsilonmodelwhilesimulatingthevarious 2Dand3Dmodelsrespectively.Initiallythestudyshowsthecharacteristicsofflowaroundan obstructionin2-Dimensionalmodelandtheresultsprovidedthenecessaryvisioninorderto moveaheadwiththe3-Dimensonalmodel.Insecondhalfofthepaperseveralcasesand conditionsaresimulatedinthe3-Dimensionalmodellikesedimenttransportmodel,free surfacemodeletc.,togetbetterunderstandingofthescourholedevelopment.Theresultsof theiso-surface,velocitystreamlinesandvelocityvectorsobtainedfromthisstudyprovidedthe greatinsightofallaboutscouring.Furthertheresultsofsedimenttransportpresentthegreat pictureofscouringallaroundthepier.

Acknowledgement

WewouldliketoexpressourdeepestgratitudetoourguideandmotivatorDr.ManojLanghi andDr.HimanshuSharma,CivilDepartment,INSTITUTEOFINFRASTRUCTURE TECHNOLOGY,RESEARCHANDMANAGEMEMNT,Ahmedabadfortheirvaluable guidance,sympathyandcooperationforprovidingnecessaryguidanceandsourcesduringthe entireperiodofthisproject.WealsoliketothankourSeniorResearchScholarMr.Sanil, Mechanical Department, INSTITUTE OF INFRASTRUCTURE TECHNOLOGY, RESEARCHANDMANAGEMEMNT,Ahmedabadforhelpingusinvariousproblemswhile workingintheAnsyssoftware.

Chapter5

ResultsandDiscussion

5.12-DSinglePierModel:ResultsandDiscussion

5.23-DSinglePier-1Model:ResultsandDiscussion

5.3 ResultsforSedimentTransport

5.43-DFreeSurfacemodel:ResultsandDiscussion

5.53-DFreeSurfacePierModel:ResultsandDiscussion

Chapter6 Conclusions

ListofTables

TableNo. Title PageNo.

Table4.2.1 Dimensionof2-Dmodel 9

Table4.4.1 ReferencevaluesforSinglePier 14

Table4.4.2 SolutionmethodsforSinglepier 14

Table4.4.3 CalculationMethodsforSinglePier 15

Table4.6.1 3-DModelDescriptionSinglePier 16

Table4.8.1 SolutionMethodsforSingle3-Dpier-1 20

Table4.8.2 ReferenceValuesfor3-DSinglepier-1 20

Table4.8.3 Calculationfor3-DSinglePier-1 21

Table4.9.1 3-DModelDescriptionSinglePier-2 22

Table4.11.1 SolutionMethodsforSinglePier-2 24

Table4.11.2 ReferencesValuesforSinglepIer-2 24

Table4.11.3 CalculationforSinglePier2 24

Table4.12.1 3-DFreeSurfaceModel 25

Table4.13.1 SolutionMethodfor3-dFreeSurfaceModel 26

Table4.13.2 Calculationfor3-DfreeSurfaceModel 27

Table4.14.1 3-DFreeSurfacePierModelDescription 28

Table4.16.1 3-DFreeSurfacePierSolutionMethods 30

Table4.16.2 3-DFreeSurfacePierCalculation 31

ListofFigures

FigureNo. Title PageNo.

Fig2.3.1 ScourHole 4

Fig2.3.2 VortexFormation 4

Fig4.2.1 Sketchofsinglepiermodel 10

Fig4.2.2 Surfacecreatedfromsketch 10

Fig4.3.1 Meshingofmodel 12

Fig4.3.2 Closeviewaroundthepier 12

Fig4.3.3 Inflationaroundthepier 13

Fig4.4.1 Boundaryconditionforsinglepier 13

Fig4.4.2 InletandOutlet 15

Fig4.6.1 Modelwithoutsymmetrytool 17

Fig4.6.2 Geometry 17

Fig4.6.3 Modelafterusingsymmetrytool 17

Fig4.7.1 Modelwithstructuredmesh 18

Fig4.8.1 Boundaryconditions 19

Fig4.9.1 Model 22

Fig4.10.1 Modelwithstructuredmesh 23

Fig4.13.1 Boundaryconditions 26

Fig4.15.1 Meshing 29

Fig4.16.1 Boundaryconditions 30

Fig4.16.2 Patchingofsedimentatbottom 31

Fig5.1.1

Velocitycontouratt=0sec 32

Fig5.1.2 Velocitycontouratt=1sec 32

Fig5.1.3 Velocitycontouratt=2sec 32

Fig5.1.4 Velocitycontouratt=5sec 32

Fig5.1.5 Velocitycontouratt=15sec 33

Fig5.1.6 Velocitycontouratt=25sec 33

Fig5.1.7 Streamlinesatt=0sec 34

Fig5.1.8 Streamlinesatt=10sec 34

Fig5.1.9 Streamlinesatt=20sec 35

Fig5.1.10 Pressurecontouratt=0sec 35

Fig5.1.11 Pressurecontouratt=1sec 35

Fig5.1.12

Fig5.1.13

Fig5.1.14

Fig5.2.1

Fig5.2.2

Pressurecontouratt=2sec 36

Pressurecontouratt=5sec 36

Pressurecontouratt=10sec 36

Sedimenttransportatt=0sec 37

Sedimenttransportatt=0.5sec 37

Fig5.2.3 Sedimenttransportatt=1.5sec 38

Fig5.2.4 Sedimenttransportatt=2.5sec 38

Fig5.3.1 Iso-surfaceatt=2sec 39

Fig5.3.2 Iso-surfaceatt=4sec 40

Fig5.3.3 Iso-surfaceatt=6sec 40

Fig5.3.4 Iso-surfaceatt=10sec 41

Fig5.3.5 Iso-surfaceatt=15sec 41

Fig5.3.6 Iso-surfaceatt=30sec 42

Fig5.3.7

Iso-surfaceatvolumefraction0.3nearpier 43

Fig5.3.8 Iso-surfaceatvolumefraction0.3nearpier 43

Fig5.3.9

Fig5.3.10

Volumerenderingatt=8sec 44

Volumerenderingatt=12sec 45

Fig5.3.11 2Verticalplanes 46

Fig5.3.12 2Verticalplanes 46

Fig5.3.13

Velocitystreamlinesatt=0sec 47

Fig5.3.14 Velocitystreamlinesatt=5sec 47

Fig5.3.15 Velocitystreamlinesatt=10sec 47

Fig5.3.16 Velocitystreamlinesatt=20sec 47

Fig5.3.17 Velocitystreamlinesinthecanal 48

Fig5.3.18

Fig5.3.19

Fig5.3.20

Fig5.3.21

Fig5.3.22

Velocityvectorsatt=0sec 49

Velocityvectorsatt=2sec 49

Velocityvectorsatt=4sec 50

Velocityvectorsatt=8sec 50

Velocityvectorsatt=10sec 51

Fig5.3.23 Velocityvectorsatt=12sec 52

Fig5.3.24

Velocityvectorsatt=15sec 52

Fig5.3.25 Velocityvectorsatt=30sec 53

Fig5.4.1 Velocityprofile 54

ListofSymbols,AbbreviationsandNomenclature

mm Millimetre m meter GB K kg s $ ω

GigaByte Kelvin Kilogram Second Dollar Omega

CHAPTER1

INTRODUCTION

1.1:GENERAL

Theremovalofsedimentlikegravelandsandfromthesurroundingofanobstructionorpier in the channel is known as scouring. It can erode the channel beds, carry the sediment particles(heavierthanwater),anddeposittheparticlesatsomeotherlocationofthechannel. Local scour refers to the scour that occurs around the foundation of the pier. Contraction scourreferstoscourcausedbythecontractionofthewaterways.Thescourthatoccursasa resultofcontinuousflowingwateroverahugeperiodoftimeisknownasdegradationscour. Andsedimentscourhappensasthevolumeofsedimenttransportingoutofaregionexceeds thequantityofsedimententeringthearea.

In hydraulic engineering, scouring is one of the major important issue. Sediment transportationaroundpiersisthebiggestdangerforthebridge’sstabilityaswellasthesafety ofpeoples.Thefailingofthesebridgescausesnotonlyeconomy-basedlossesbutalsocauses lossesofculturalheritage.Theenormouslossofhumanlifeandlargemonetarycosthighlight theneedforimprovedscourpreventionsystems.IntheUnitedStates,500bridgeshavefailed since1950,withthemajorityofthecollisioncausedbyscourandhydraulicissues.Between 1985and1987,around90bridgeswerefellinNewYork,Pennsylvania,duetobridgepier failure.In1995itwasapproximatedthat84percentofthe575,000bridgesintheNational Bridge Inventory were constructed over waterways (Richardson et al. 1995). Of these bridges, approximately 121,000 are examined to scour susceptible and of those 121,000, approximately13,000areconsideredtoscourcritical(Jones1993).

Scouringatthefoundationofthebridgepierisverycommonandeachyearahugeamount isspenttorepairtheseissues. IntheunitedstatesofAmerica,thecostofthefailureofbridges and highway was calculated as $100 million per event (Brice 1978). In New Zealand, the costestimatedforthedamagesduetoscouringwas36millionNZ$/year(Macky1990).

It was noticed that major floods are also responsible for the scouring process around the bridgepier.Therisinglimboffloodcauseserosionaroundthefoundationofthebridgepier.

Therecessionlimboffloodtriestorefilltheholeswhichwerecreatedaroundthepier,also knownasscourholes.Instudies,itwasfoundthattheredepositsedimentwhichispresentin thesesourholesismoreeasilyerodedbythefuturefloods.Inmostofthecases,thefailure ofthebridgeistheendresultofnumerousmajorfloods.

Estimatingscouraroundapierisamaintechniqueinhydraulicengineeringsinceitisused todesignthepier'sfoundation.Underestimationofscourholemayresultinbridgecollapse andhumanliveloss.Thus,anunderstandingofthevariousflowfieldsandsedimenttransport aroundthepierisveryimportanttopreventthestabilityandsafetyofthebridge.

1.2:OBJECTIVE

The main objective of the present study is to estimate the sediment transport around the bridgepier.Inordertoachievethemainobjectivefollowingsub-objectivesareperformedin duecourseofpresentstudy.

Estimationoftheturbulentcharacteristicsaroundthe2Dbridgepiermodel.

Estimationofscouringaroundthe3Dbridgepiermodel.

Estimation of the effect of free surface wave modeling on scouring characteristics aroundthe3Dbridgepiermodel

CHAPTER2

SCOURING

2.1:TYPES

Scourscanbeclassifiedintothreemaincategoriesaroundthefoundationofthebridgepier

1.Degradationscours-Thistypeofscouringoccursduetothelongchangesinstreambed elevationbecauseofnaturalorsomehumaninterferenceintheriverbed,thatmayaffectthe reachoftheriverclosetothebridge.

2.Contractionscours-Inthisscouringthevelocityoftheflowincreasesandcauseserosion duetocontractionofstreamflowareaatabridgelocation,resultinginthelossofsediment fromthesidesandbottomoftheriver.

3. Local scour-This typeofscouring createsdue to theeffectof vortices around the pier. Theturbulentflowofwateraroundthepiercausevorticesandvorticesextractthesediment aroundthefoundationofthebridgepierandalso createascourholearoundthepierwhich disturbsthebridgestability.

Total scour- It is the total results of all the processes of scouring taking place at a given location.

2.2:CAUSES

Causesofscouringrelyonthedifferenttypesofscouring:

Forthecontractiontypeofscouringthemaincauseisthedegreeofconstrictionwithwhich thechannelorwaterwaybecomesnarrower.Andvegetationpresentinthecatchmentasthe debrisalsonarrowthechannelbyblockingit.

Inthelocalscourthemaincausesaretheshapeandsizeofthebridgepierandtheposition ofthepierinthenaturalflowofwater.Thearmoreffecthasaneffectonthelocalscouring aswell.Whenthereiserosioninfinerbedparticles,thecoarserparticlesformanarmorlayer.

The key cause involved in general scour are natural and human actions. Natural causes include catchment area geomorphology and natural disturbance like an earthquake which uplift the riverbed. And Human actions include mining of riverbed and building of the hydraulicstructure.

Scourisalsoheavilyinfluencedbybedmaterialsproperties.Themostcommontypeofscour problems contains loose alluvial soil that is quickly eroded. However overall scour in cohesivesoilshouldnotbeassumedtobelessthaninnon-cohesivesoil;thescouractually takeslongertoproduce.

Debrismayalsohaveamajoreffectonscouringinavarietyofways.Debriscanchangethe flowofwater, alteringtheangleofattackandrisinglocal scour. Debriscanalsomovethe entire waterways around the bridge resulting in increased water flow and scour in another area.

2.3:MECHANISM

Whentheflowofwaterpassesthepier,attheintersectionpointofflowandpieravertical stagnation line is received. In the approaching flow due to velocity difference, a pressure gradientisobtainedfromtoptothebottomoftheriver.Duetopressuregradientadownflow, its interactionwith separate boundarylayerand the main water flowaround the pier cause verticeswhichcoverthemaximumupstreampartofthepierasshowninfig2.3.1.Thetop viewofthesevorticeslookslikeahorseshoe.That’swhysuchvortexisknownashorseshoe vortex.Horseshoeisthemainfactorwhichisresponsibleforthescouringaroundthebridge pier.Alargeregionattheseparationzoneinthedownstreamofthepierisknownaswake.

Fig.2.3.1ScourHole

Fig.2.3.2VortexFormation

Along with the horseshoe vortex, there also exist vortices at wake region known as wake vortex.Thesevorticeselevateupthesedimentparticlefromtheriverbedandmakeascour holearoundthefoundationofthepier.

In the live bed condition (a condition which is characterized by bed materials being transported into the narrow opening from upstream of the bridge) the flow constantly transports sediment particles right into a neighborhood scour hole. When the scour hole formed,theenergyofhorseshoevortexisdiminished.Whentheinflowofthebedmaterial seemstobeequaltotheoutflowofbedmaterialthenthedepthofthescourholewillreach equilibriumstate.

Inthecaseof clearwatercondition (aconditionwhere approachflowis clearand thebed materialisnotinmotion)thescoringprocesswillhappenwhentheshearstressproducedby thevortexexceedsthecriticalshearstressofthebedmaterial.Inthisconditionnorefilling ofsedimentoccursintherecessionlimbofthefloodbecauseoftheshortageofsediment.

Thereforedownflow,horseshoevortex,andwakevortexareconsideredasthemainelements responsibleforthedevelopmentofthescourholearoundthefoundation.

LITERATUREREVIEWS

Foranyhydraulicstructurelikebridges,transmissiontower,etc.whichareinbetweenany waterbodyorwhichhavepiers,themaincauseforfailureasmentionedearlierisscouring causedbyheavyfloworfloodinthatwaterbody.So,it’sbasicallyduetoscouringaround that obstruction. To avoid the failure the flow must be studied in detail by studying and analyze all parameters. In previous years many papers have appeared which studied flow around piers using a small experimental model and software like Ansys Fluent and Open foam.

Important works on scouring have been given by Dargahi (1982) and Breuer’s (1977). Dargahididexperimentalworkandinvestigatedthecouplingbetweenflowfieldbystudying flowvisualizationandhisworkswerethenfollowedbymanyotherresearchers.

Deng (1992) developed one three-dimensional model with numerical simulation of the turbulent flow around an airfoil in which they studied the main characteristics of the horseshoevortexwhichisnecessaryforstudyingscouring.Thatwasoneofthefirstworks in CFD (Computational Fluid Dynamics) on this scientific field in which they used the Reynolds-averagedNavier-Stokes(RANS)equations.

The work on three-dimensional flow around the square and circular piers given by MingHsengTseng(1998)usingnumericalsimulationwhichshowedthattheflowfluctuationand patternsforsquareandcircularpiersarealmostsimilarbutthedragcoefficientinthecaseof square pier is larger than that of the circular one, indicating that the resistance to flow is higherinthecaseofthesquarepierandthedomainsofthewakevortexandthehorseshoe vortexinthecaseofthesquarepierarelargerthanthoseinthecaseofthecircularone.

Nils R. B. Olsen and Hilde M. Kjellesvig (2010) worked on the estimation of maximum local scour depth for Three-dimensional numericalflow modeling. In their work the water flowmodeliscoupledwithsedimenttransportmodelsthecombinedmodelisabletopredict localscour.Thesimulatedmaximumdepthwascomparedwithnumericalempiricalformulae result. The sediment transport is calculated with the convection-diffusion equation for the

sedimentconcentration.Astructurednon-orthogonalgridwasused.Thefinegridwasused tocalculatethemaximumscourdepth.Maximumscourdepthoccurredafterabout750000 seconds (208 hours). Thetime step was 100 seconds.The finalscourholeextended on all sidesofthecylinder, withmaximumdepthattheupstreamsideandminimumdepthatthe downstreamside.

Yin-hui Wang (2015) made analysis on water flow pressure and its behavior. They concludedthatwaterflowpressurecomesbetweenrangeofmethodsgivenbyGeneralCode forDesignofHighwayBridgesandLoadCodeforHarborEngineering.

There was also a work done on studying flow behavior around pier located at ⁰ channel junctiongivenRasoolGhobadian(2017)theirresultsshowedthatthebridgepiernarrowed flowatthecrosssectionaroundpierwhichledtobackwaterriseatupstreamsections.This 6 further caused the reduction of longitudinal velocity at upstream side. The results also showed that the secondary flow patterns for cross section downstream bridge pier are differentfromcommonsecondaryflowinopenchannelwithoutbridgepier.

Thesedimenttransportinthevicinityofabridgepierorabutmentisdominatednotonlyby thefrictionalshearstress,butalsobythree-dimensionalflowstructure:vortices,down-flow, turbulent and pressure fluctuations, etc. Yafei Jia, Mustafa Altinakar and M. Sukru Guney(2017) founded that RANS models cannot directly resolve actual turbulent fluctuations,subscalevortexandcoherentstructures,whichplayimportantrolesinpicking upsediment particles in scour holes. The deepest scouroccurs near the stagnation point at thefoot of thepier.In addition, theyinformedthattheturbulent fluctuationsaround apier playanimportantroleinentrainingbedsediment,andhavetobeconsideredinadditionto theconventionalshearstresswhenRANSmodelsareapplied.ObservationsmadebyYafei Jia, Mustafa Altinakar and M. Sukru Guney(2017) indicated that, near thebaseofthe pierandinthe scourhole,thevertical motionandthehorseshoe vortex carrythe turbulent fluctuationstothebottomofthehole;theseadditionalturbulentfluctuationswouldincrease the sediment entrainment around the base of the pier. The simulated results of Yafei Jia, MustafaAltinakarandM.SukruGuney(2017)flowfieldsaregenerallymoreaccuratein frontofthepierthanthatinthewakezone,itisexpectedthatthesimulatedscourinthefront ofthepierwillbemoreaccuratethanthatdownstream.Theholewasscouredbyanunsteady

flow, and the sediment in the channel bed was non-uniform and also the simulated scour depth development began a little later than the one observed. This may be because the simulatedhorseshoevortexortheturbulentfluctuationsatinitialtimearerelativelyweaker thanthephysicalmodel.

Localscouratapierisattributabletotheforcesexertedonthebedbythecomplex,highly three-dimensional3DandunsteadyflowfieldgeneratedbythepierG.Kirkil,R.Ettema.

RecentlyYilinYang(2018)gavedetailedstudyonscourholedevelopmentconcludingthat the size of the horseshoe vortex increased with the development of scour holes while the vorticity induced due to the horseshoe vortex decreased and a small individual horseshoe vortexwasformedaroundeachandeverypileinthemainscourhole.

CHAPTER4

METHODOLOGY

4.1:2-DMODELSINGLEPIER:INTRO

Inthepresentwork2-dimensionalmodelismadeintheAnsysfluentforlookinginto thevelocitycontoursandstreamlines.Stepsusedforgettingtheresultsweremodelling, meshingandsimulation.Thelocationofthepierswastakenrandomlybyjustkeeping inmindthepracticalityofmodelinreallife.

4.2:MODEL:DESCRIPTION

Inthissectiongeometryof2-DSinglepiermodelisexplained.Thedimensionsofthe modelareshowninTable4.2.1.

Table4.2.1:Dimensionof2-Dmodel

Lengthofchannel 8meters

Widthofchannel .600meter

Diameterofpier .075meter

Distanceofpierfrominlet 3.925meter

Thesketchofthesinglepiermodelandsurfacecreatedfromthesketcharegivenfig 4.2.1andfig4.2.2respectively.

Fig4.2.1:Sketchof SinglePierModel

Fig4.2.2:Surfacecreatedfromsketch

4.3:MODEL:MESHING

Inthissectionmeshingofthepresentmodelisexplained.Themeshingwasdoneusing fourcommandswhicharetrianglemethods,edgesizing,bodysizingandinflationfor obtainingveryfinemeshandgettingtheelementsunder5lakhssothatsimulationcan proccedfurtherwithoutinterruptionbecauseinstudent’sversiontheelementsshould beunder5lakhs.Thedetailingofeachparameterisasunder.

ThepurposeofAllTriangleMethodistogenerateeachelementintotriangleshapeso that uniformity can be maintained. After applying theAll-TriangleMethod each and everyelementsofthesurfacebodygetsmeshedintriangleshape.

Edgesizingwasthesecondcommandused.Thepurposeofthiscommandistocreate veryfinemeshmorethanthebodysothatflowcanbestudiedverynicelyaroundthe pier. The cylinder’s edge was selected and then the element size of 1 mm was used. Afterapplyingthisfinermeshwasobtainedascomparedtootherpartonsurfacebody. Body sizing was further used in order to get fine mesh in the body of the geometry whichisveryhelpfulforobtaininggoodorthogonalquality.

Inflationwasthelastcommandused.TheInflationcommandisthemostimportantfor observingtheflowbyusingfineanduniformmesharoundthepier.Inthiscommand surface was selected in geometry and edge was selected in boundary. First layer thickness was selected as this is a 2D model and the first layer thickness is selected sameastheelementsizeintheedgesizingwhichwas1mm.Thenumberofinflation layerswereselectedas60andgrowthrateof3.Fifty-fiveinflationlayersmeansthat total of 60 inflation layers will be concentric around the pier with smallest element starting from the inside boundary ofcylinder and increasing outside with thegrowth rateof3. The motive was to obtain good mesh on surface body, fine mesh around both piers, good mesh quality and to obtain elements under 5 lakhs. After applying all this commands,themeshqualityof.61andelements202079wasachievedasshowninfig from4.3.1tofig4.3.3.

Theexplanationofthesimulationofthepresentmodelisexplainedinthenextsection.

Fig4.3.1:MeshingofModel

Fig4.3.2:CloseviewaroundthePier

Fig4.3.3:Inflationlayersaroundpier

4.4:MODEL:SIMULATION

ThenumericalmodelingtechniqueANSYS–CFD(Fluent)wasadoptedforthepresent study. Volumeoffluid (VOF)k-ωturbulencemodelsconsideringtheconditionsfor openchannelwasusedfortheanalysis.Atinletboundaryuniformvelocityof0.463m/s wasapplied.

Theboundaryconditionsofsinglepierforthesimulationareshowninfig4.4.1.

Fig4.4.1:BoundaryConditionforSinglePier

Thereferencevalueswhichwereinputinthesetupareasshownintable4.4.1:

Table4.4.1:ReferencevaluesforSinglePier

AreaDensity 1000kg/m3

Asitisa2-Dmodelthecross-sectionareacannotbeconsideredandhencepressure alsowasnotconsidered.

Solutionmethodsforsinglepierisshownintable4.4.2.

Characteristicslength 0.075m Temperature 293K Velocity 0.463m/s Viscosity 0.0007kg/m-s Scheme

Table4.4.2:Solutionmethodsforsinglepier

Similarly,calculationmethodsforsinglepierareshownintable4.4.3.

Table4.4.3:CalculationmethodsforSinglePier

No.oftimesteps 300

Sizeoftimesteps 0.1sec

Iterationsineachtimesteps 20

ReportingInterval 1

Numberoftime stepsweretaken300andsizeofeachtimestepwas 0.1secondthat meansthattotalof300*0.1=30secondsofvideowillberecordedand20iterationswill be in each time steps that means 20 breaks will be used in each time step for the formation of that step. Higher the iterations in each step will make more smooth transitionintheanimationvideosandreportingintervalwaschosen1whichmeansat each interval file will be generated. Using this calculation data, the size of the files generatedwas35GBandwhichnearlytook6hoursandafterthatanimationwasalso smoothenoughtostudytheflow.

Fig4.4.2showstheinletandoutletofthemodel.

Fig4.4.2:InletandOutlet

4.5:3-DMODELSINGLEPIER-1:INTRO

Simulation was processed in Ansys Fluent. The modelling was completed in the “design modeler”.Atfirstthe2-Dsketchwasplottedasperrequireddimensionandthenextrusion wascarriedoutforcorrespondingdimensions.Inthedesignmodelerthesymmetrycommand was usedtohalvedthemodelalongaverticalplanepassingthroughthecenterofpierand alongthedirectionoftheflow.Afterusingsymmetry,theresultantdimensionandnumber of elements got reduced to half of geometry and in the end of simulation it reflected the simulatedresultandobtainedthewholedomain.Thisreducedtheprocessingtimeofmeshing andsimulation.

4.6:MODEL:DESCRIPTION

Due to computational unavailability, it wasn’t feasible to run the simulation trials on the actualexperimentalmodel, asit consumes alot timeandmemory.So, it was concludedto trythedifferentscenarios,whichincluded smallerdimensionscomparativelytotheactual experimentalmodel.Whiletaking modelsofdifferentdimensionstheturbulentbehaviorof the water flow was ensured. Once the methodology got concluded , the simulation was supposedtobesetwiththeactualexperimentalmodel.

Thedescriptionof3-Dsinglepiermodelisshownintable4.6.1.

Table4.6.1:3-DModelDescriptionSinglePier

Fig

Fig4.6.1:ModelwithoutSymmetryTool

Fig4.6.2:Geometry

Fig4.6.3:Modelafterusingsymmetrytool

4.7:MODEL:MESHING

Inthissectionmeshingofthepresentmodelisexplained.FLUENTiscapableofsimulating bothtwo-dimensionalandthree-dimensionalsituations.Bothstructuredandunstructured meshesarepossible.Differenttypesofelementsarepermittedsuchasquadrilateralsand trianglesfor2Dsimulationsandhexahedra,tetrahedra,polyhedral,prismsandpyramidsfor 3Dsimulations.

“Multizone”methodofmeshingwasusedinthestructuredmeshingofthemodel.Along withfacesizingatthetopandbottomfaceofmodelandedgesizingatedgeswasprovided toattainthetotalnumberofelementsaround1.7lakhsand1.8lakhsofnodes.Element qualityofthemeshingwas0.76whichisaconsideredasagoodmesh.Afterapplyingallthe commandsstructuredmeshmodelisshowninfig4.7.1.

Theexplanationofthesimulationofthepresentworkisexplainedinthenextsection.

Fig4.7.1:Modelwithstructuredmesh

4.8:MODEL:SIMULATION

ThenumericalmodellingtechniqueAnsys-CFD(Fluent)wasadoptedforthepresentstudy. Multiphase-EulerianModelwithtwophaseswasselectedwithk-epsilonstandardviscous model. Boundaryconditionsforthemodelisshowninfig4.8.1.

Fig4.8.1:BoundaryConditions

Solutionmethods,referencevaluesandcalculationsareshownintable4.8.1,4.8.2and 4.8.3respectively.

Forthephasedeterminationwaterandsilicon-solidassedimentwereselectedasprimaryand secondaryphase.Waterwassettoprimaryphaseastheinterestofstudywastostudyflow behaviorofwateraroundwater.TheSIMPLEalgorithmusesarelationshipbetweenthe velocityandpressurecorrectionstoenforcemassconservationandtoobtainthepressure fieldformultiphasesflow.Thevelocitiesaresolvedbyphasesinasegregatedfashion.Fluxes arereconstructedatthefaceofthecontrolvolumeandthenapressurecorrectionequationis builtbasedonthetotalcontinuity.So,tomakethisdual-phasemodelworkwiththisscheme, thepatchingwasdoneforthesecondaryphasei.e.,sediment.

Table4.8.1:SolutionMethodsfor3-DSingle Pier-1

Phasedetermination

WaterandSediment Scheme PhasecoupledSIMPLE Discretization LeastSquarecell

Momentum

SecondOrderUpwind Volumefraction FirstOrderUpwind

Turbulentkineticenergy SecondOrderUpwind

TurbulentDissipationrate SecondOrderUpwind Initializationmethod StandardInitialization

Thepatchingcommandallowstopatchdifferent valuesofflowvariablesintodifferentcells. Heightofsedimentwassetto50mmonly.Forotherschemesliketurbulentkineticenergy, Specific dissipation rate and momentum Second order Upwind was selected to reduce the values of residuals , and consequently good simulation result was achieved. Standard initializationmethodwaschosenasgoingforthehybridmethodrequiresahigh-performance computer.Thesimulationtook36hourstocomplete.

Table4.8.2:ReferenceValuesfor3-DSinglePier-1

Table4.8.3:Calculationfor3-DSinglePier-1

No.oftimesteps 500

Sizeoftimesteps 0.01sec

Iterationsineachtimesteps 20

4.9:3-DMODELSINGLEPIER-2:Model-Description

Inorder to estimatethesedimentflowsalong withtheturbulentcharacteristics,aroundthe bridgepier,numberoftrialswereperformed.Thereasonforsuchtrialswereneededdueto unavailability of the experimental data mostly for sediments. There were also some operational as well as computational difficulties while performing the simulation of the model.

In this section 3-D model single pier-2 is explained. While finalizing the methodology for the present work, it was observed that, either whole sediments were getting washed away fromthechanneloreventuallythefluentsolverwasgettingdiverged.Itwasobservedthat, the sediments in the channels were washed owing to several reasons – insufficient information on the sediment characteristics, dimensions of the model that are adopted, the flow parameters such as inlet flow velocity might be larger than the incipient velocity etc. Theinletflowvelocitylargerthantheincipientvelocityisresponsibleforthemovementof the sediments even at the entrance of the channel. Looking at the several possibilities, the numberoftrialswereperformedanditwasconcludedtoalterthedimensionsofthemodel. Asmentionedearlier,aninletflowvelocitymightbelargerthanincipientvelocity.However, aninletflowvelocitycannotbereducedbecausebydoingso,thetimerequiredtopassthe flowthroughdomainishigh.Whichconsequentlyincreasesthememoryusage.Butthereis limitation of available computational time and facility. Therefore, the dimensions of the modelwere changed and thedepthofthesedimentswereincreased.Also, thedepth ofthe modelwasincreasedwhilemaintainingthescenarioofmeshing,velocity,timestep,courant numberandlookingatthelimitationsofmemoryusage.

InordertounderstandthefeasibilityoftheFLUENTsoftwaretoestimatethesediment transport,numberoftrialswereperformedduringthecourseofthisstudy.

Imageofthe3-Dsinglepier-2modelisshowninthefig4.9.1andthedescriptionofthesame modelisgivenintable4.9.1.

Fig4.9.1:

4.10MODEL:MESHING

Thissectionincludestheexplanationforthemeshingofthepresentwork.Totalnumberof elementswerearound8lakhsand7.7lakhsofnodes.Elementqualityofthemeshingwas 0.86whichisaconsideredasagoodmesh.Modelwithstructuredmeshisshowninfig 4.10.1.

Simulationmethodofthepresentworkisexplainedinthepresentsection.

Fig4.10.1:Modelwithstructuredmesh

4.11:MODEL:SIMULATION

ThenumericalmodellingtechniqueAnsys-CFD(Fluent)wasadoptedforthepresentstudy. Multiphase-EulerianModelwithtwophaseswasselectedwithk-epsilonstandardviscous model.Andthesamemethodologywasusedinthisworkalsoasusedinmodel-1.Ittook4 daystocompletethesimulations.

Table4.11.1,4.11.2and4.11.3describesthesolutionmethod,referencevaluesand calculationsforsinglepier-2modelrespectively.

Table4.11.1:SolutionMethodsforSinglePier-2

WaterandSediment Scheme PhasecoupledSIMPLE Discretization LeastSquarecell Momentum SecondOrderUpwind Volumefraction FirstOrderUpwind Turbulentkineticenergy SecondOrderUpwind TurbulentDissipationrate SecondOrderUpwind Initializationmethod StandardInitialization

Phasedetermination

Table4.11.2:ReferenceValuesforSinglePier-2

Table4.11.3:CalculationsforSinglePier-2

4.12:3D-FreeSurfaceModel-Description

In this section geometry of the 3-D free surface model is explained. Table 4.12.1 shows descriptionof3-Dfreesurfacemodel.

Table4.12.1:3D-FreeSurfaceModel

Totaldepth 0.60meter Width 0.60meter Length 4.0meter

Heightoffreesurfacefrombottomofbed 0.40meter Numberofelements 6700 Nodes 8228 Meshtype Structured MeshQuality 0.99

4.13:Model:Simulation

In this section thesimulation of the present modelis explained. The VOFmodel was used withk-epsilonturbulencemodel.AndVOFsub-modelslike‘openchannelflow’and‘open channel wave BC’ were activated. Water was set as the primary phase and air as the secondaryphase.Intheboundaryconditionsthefreesurfaceheightwasinputas0.40meter andthetopsurfacei.e.,thefreesurfacelevelwasgiventheboundaryconditionof pressure outlet as anoperatingconditionofdefaultatmosphericpressurevalue101325Pa.Themesh wasn’t settomuchfinebecausetheaimofthisworkwastogenerateavelocityprofilewhich could beimported to anothermodel forsediment transport simulation. Directfree surface modelcouldn’tbeusedinthemodelforsedimenttransportanalysis,doingsowouldrequire threephasesinteractionofair-water-sediment.So,theimportingoffreesurfaceprofilewas carriedtothepiermodel.PRESTOwasappliedasitissuitableforswirlingflow,thatwould happenaroundthepier.PhasecoupleSIMPLEissuitableforwiderangeofmultiphaseflow.

Fig4.13.1showstheimageofmodelwithboundaryconditions.

Fig.4.13.1:BoundaryConditions

Table4.13.1and4.13.2containssolutionmethodandcalculationsfor3-Dfreesurface model.

Table4.13.2: Calculationfor3D-FreeSurfaceModel

No.oftimesteps 144

Sizeoftimesteps 0.5sec

Iterationsineachtimesteps 20

4.14:3-DFreeSurfacePierModel-Description

Inthissectionthegeometryofthe3-Dfreesurfacepier modelisexplained.Inactual realliferiver,duringformationofscourholeorifthescourholeisalreadypresentwhenwater passesjustabovethescourhole,duetogravitythewatergoesindownwarddirectionasthere isscourhole.Apartofwatervolumegoesdownfromthetopsurfaceandthatportionwill bereplacedbytheair forsomesecondorapartofsecondandthenairwill bereplacedby the upcoming portion of the water. Now imagine water is passing through a hollow rectangularcanalwithallfoursidescoveredonlyandairisn’ttherewhenwatercompletely flows,justlikeourmodel-2.Inthiscasethewaterwon’tgodownbecausevacuumwon’tbe createdand airisn’tthere.So,itwilleitherpassthroughleftorrightsideofthepierrather thangoingdownwardcreatingscourhole.So,FreeSurfacemodellingisimportantinwhich thereshouldbeairdomainandwaves.

This model was made just after exploring the free-surface modelling and with the actual experimentalmodelaftergettingsomemorerelevantdata.Thissimulationrequiresalottime andmemory.

Table4.14.1givesthedescriptionof3-Dfreesurfacepiermodel.

Table4.14.1:3-DFreeSurfacePierModelDescription

Totaldepth 0.60meter

Width 0.60meter Length 7.0meter

DiameterofPier 0.075m

Heightoffreesurfacefrombottomofbed 0.40meter

Heightofthesedimentbed 0.177meter.

Numberofelements 10lakhs Nodes 10.5lakhs

Meshtype Structured MeshQuality 0.85

4.15:Meshing

In this section themeshing ofthe present model is explained.The structured meshing was doneinthismodel.Togetmorefinemesharoundthepier,sphereofinfluencewasusedwith elementsofsize9mmandtheradiusofinfluencewas1meter.Also alongtheoutercircular peripheryofpieredgesizingwasdonewith100asnumberofdivisions.

Fig4.15.1showsthemeshedimageofthemodel.

Simulationmethodforthepresentmodelisexplainedinthenextsection.

Fig4.15.1:Meshing

4.16:Simulation

ThismodellingwasdonesameastheEulerianmodelusedinthePiermodel-1andPier Model-2.Onlythingwasdoneistheimportingofthevelocityoutletvelocityprofileof3-D freesurfacemodeltotheinletvelocityofthepresentwork.

Forthephasedeterminationwaterandsilicon-solidassediment wereselectedasprimary andsecondaryphase.Waterwassettoprimaryphaseastheinterestofstudywastostudy flowbehaviorofwateraroundwater.TheSIMPLEalgorithmusesarelationshipbetween thevelocityandpressurecorrectionstoenforcemassconservationandtoobtainthe pressurefieldformultiphasesflow.Thevelocitiesweresolvedbyphasesinasegregated fashion.Fluxesarereconstructedatthefaceofthecontrolvolumeandthenapressure correctionequationisbuiltbasedonthetotalcontinuity.So,tomakethisdual-phasemodel workwiththisscheme,thepatchingwasdoneforthesecondaryphasenamelysediment.

Thepatchingcommandallowstopatchdifferent valuesofflowvariablesintodifferent cells.Heightofsedimentwasset 50mmonly.Forotherschemesliketurbulentkinetic energy,SpecificdissipationrateandmomentumSecondorderUpwindwasselectedto reducethevaluesofresiduals,andconsequently goodsimulationresultwasachieved.

Fig4.16.1showstheboundaryconditionsofthemodel.

Fig4.16.1:BoundaryCondition

Table4.16.1showsthesolutionmethodsfor3-Dfreesurfacepiermodel.

Table4.16.1:3-DFreeSurfacePierSolutionMethods

Fig4.16.2showsthepatchingofsedimentatbottomofthemodel.

Fig4.16.2:Patchingofsedimentatbottom Table4.16.2showsthecalculationsfor3-Dfreesurfacepiermodel.

Table4.16.2:3-DFreeSurfacePier Calculation No.oftimesteps 170 Sizeoftimesteps 0.5sec Iterationsineachtimesteps 30

ResultsandDiscussions

5.1:2-DsinglePierModel:ResultandDiscussion

Thefiguresfrom5.1.1to5.1.6showsvelocitycontourinthe2-dimensionalmodelat differenttimingofthesimulation,producedinCFD-Postprocessingaftersimulation intheFluent.Itcanbeobservedfromthefiguresthatsteadystateisreachedafter20 secofthesimulationasthevelocitycontourswerenotchanginganyfurther.

Furthertheformulationofwakevorticesandhorseshoevortexaroundthecircularpier couldbeseenintheabovefiguresandhenceitcouldbepredictedthataroundthepier therewouldbeoccurrenceofscouringaroundthepier.

Somestreamlinesoftheobtainedvelocitycontourswerealsogeneratedinordertoget a betterviewofflowing ofwaterand formulation ofvorticesaround the pier, which areshowninthefiguresfrom5.1.7to5.1.9respectively.

Fig5.1.7:Streamlineatt=0s

Fig5.1.8:Streamlineatt=10s

Fig5.1.9:Streamlineatt=20s

Thefiguresfrom5.1.10to5.1.14showspressurevariationaroundthecircularpierat differenttimeintervalsofthesimulationof2-dimensionalmodel.Theplotofpressure contourgivesusabetterunderstandingofwhatishappeningaroundthepierlikewater flowandformulationofvortices.Thesteadystateofthesimulationcanalsobeproved withthehelpofpressurecontours.

Fig5.1.10:Pressurecontouratt=0sFig5.1.11:Pressurecontouratt=1s

Fig5.2.1:SedimentTransportatt=0sec

Fig:5.2.2,5.2.3and5.2.4showsthesedimentbeingtransportedfromupstreamsideofthe pier.Andbyreferringtothelegendview,scouringwasnoticed.

Fig5.2.2:SedimentTransportatt=0.5sec.

Fig5.2.3:SedimentTransportatt=1.5sec

Fig5.2.4:SedimentTransportatt=2.5sec

5.3:Resultsforsedimenttransport

Afterthesimulationof8mmodel,theresultswereprocessedinPost-CFD.Theresultswere theIso-surfaceofthesedimentatdifferentvolumefraction,volumerenderingofsediment,

velocitystreamlineofwaterandVelocityvectorsofthewater.Theresultswereforthetime intervalt=0secondtot=72seconds.

AnIso-surfaceisa3Dsurfacerepresentationofpointswithequalvaluesina3Ddata distribution.Inthepresentmodel,iso-surfaceofthesedimentatdifferentvolumefractionof 0.2,0.3,0.4and0.5weregenerated.Iso-surfaceat0.3volumefractionisthesurface containing30%sedimentand70%water.

Thefiguresfrom5.3.1to5.3.6showsiso-surfaceofthemodelatdifferentsimulationtime.

Fig5.3.1:Isosurfaceattimet=2s.

Fig5.3.2:Iso-surfaceattimet=4s.

Fig5.3.3:Iso-surfaceattimet=6s.

Fig5.3.4:Iso-surfaceattimet=10s.

Fig5.3.5:Iso-surfaceattimet=15s.

Fig5.3.6:Iso-surfaceattimet=30s.

Asseenintheabovefigures,theiso-surfaceareofwavenatureandsameastheflow generatedinthecanalinreallife.Thereare4layersinthefigures.Thetoponeisofisosurfaceat0.2volumefraction,belowittheyare0.3volumefraction,0.4volumefraction, and0.5volumefractionrespectivelycomingfromtoptobottom.Astimepassesthesediment washesawayandreachestowardsthedownstreamsidefromtheupstreamside.

Onobservingtheiso-surfacenearpier,itwasseenthatthesedimentnearthepierwasata lowerlevelthanthesedimentintheotherpartofcanalwhichmeansscouringtookplacenear thepier.Forgettingmoreinformation,acontourwasplottedinzdirectiontakingisosurface asdomainasshowninfig5.3.7and5.3.8.Nearthepierbluecolorisseenontheiso-surface whichisatthedepthof0.14mandrestyellowandgreencolorisinthemajoritywhichisat thedepthof0.25mand0.22mrespectivelywhichmeansthatscouringtookplacenearthe pier.Butthedimensionsofthescourholecouldn’tbejustifiedfromiso-surfaceastheisosurfaceisgeneratedforaparticularvolumefraction.Theiso-surfaceinthefig5.3.7and5.3.8 areforvolumefraction0.3.

Fig5.3.7:Iso-surfaceatvolumefraction0.3nearpier.

Fig5.3.8:Iso-surfaceatvolumefraction0.3nearpier.

Volumerenderingisasetoftechniquesusedtodisplaya2Dprojectionofa3D discretelysampleddataset,typicallya3Dscalarfield.Thedifferencebetweeniso-surface andvolumerenderingisthatiso-surfaceisforaparticularvalueofvolumefractionwhereas thevolumerenderingisforarangeofvolumefraction.

Fig5.3.9and5.3.10showsvolumerenderingat2differenttime.

Fig5.3.9:Volumerenderingattimet=8s

Fig5.3.10:Volumerenderingattimet=12s

It’sclearlyobservedfromthefig5.3.9and5.3.10thatthevolumefractionaroundthepier islessthantheotherareawhichshowspresentofscourhole.Astimepassesthesizeof scourholeincreasesandthesedimentwashesawaytowardsthedownstreamside. Astreamlineisasmalllinethat’stangentialtothevelocitydirectionataparticularinstant andthecombinationofallsuchlinearethevelocitystreamline.Forthepredictionoftheflow patternofthewateraroundthepier,Velocitystreamlinewasused.

Forabetterviewtwoverticalplanesweretakentoobservetheflowaroundthepierandone clipplanewasusedasshowninfig5.3.11and5.3.12tocuttheunwantedpartofthecanal sothatfocuscouldbemorearoundthepier.

Fig5.3.11:2Verticalplanes

Fig5.3.12:2Verticalplanes

Figfrom5.3.13to5.3.16showsvelocitystreamlinesatseveraldifferenttime.

Fig5.3.13:VelocityStreamlineatt=0s

Fig5.3.14:VelocityStreamlineatt=5s

Fig5.3.15:VelocityStreamlineatt=10s

Fig5.3.16:VelocityStreamlineatt=20s

Asshownintheabove4figuresthewaterwhilecomingtowardsthepierfromtheupstream sidedipsdownwardjustbeforethepierandthenflowsaheadwhichexactlyhappensinthe developmentofscourhole.Thedownwardflowstartsjustbeforethepierandaftercrossing thepierwaterflowstraighthorizontally.Thesamethinghappenswhenindividualhorseshoe vorticesareformedduringdevelopmentofscourhole.Also,thevelocitystreamlineshowed thattheturbulencyiscreatedintheflowaroundthepier.

Thevelocityvectorsarethesmallarrowsshowingthedirectionofthefluidparticle.They areobservedandstudiedtopredicttheflowandscouring.

Fig5.3.17showthevelocityvectorsinthecanalstartingfrominletandmovingtowards outlet.Aroundthepierthevelocityvectorsshowedmuchturbulentbehavior.

Fig5.3.17:Velocityvectorsinthecanal

Forobservinginabetterway,averticalplanewastakentoseethevelocityvectorsaround thepier.

Figfrom5.3.18to5.3.25showsvelocityvectorsatseveraldifferenttime.

Fig5.3.18:Velocityvectoratt=0s.

Fig5.3.19:Velocityvectoratt=2s.

Fig5.3.20:Velocityvectoratt=4s.

Theflowstartsandwhenreachingaroundthepier,waterdipsdownwardsattimet=2sas showninfigure5.3.19thenflowsstraightattimet=4sasshowninfig5.3.20.

Fig5.3.21:Velocityvectorsattimet=8s

Attimet=8s,thewatercoversalittledistancefromthepiertowardsthedownstreamside andthencomesbackformingalooptowardsthepiermakingshapelikeahorseshoeknows ashorseshoevortexwhichisresponsibleforthedevelopmentofscourhole.

Fig5.3.22:Velocityvectorsattimet=10s.

AsshowninFig5.3.22attimet=10s,thedevelopmentofhorseshoecontinuesandwater flowsinthecircularloopandmovesagaintowardsthedownstreamside.

Fig5.3.23:Velocityvectorsattimet=12s.

Fromtimet=12s,theloopgetssmallerandwaterflowsstraightjustbesidethelooptowards downstreamsideasshownintheFigure5.3.22.Thatisbecauseofthehighervelocityand washingofthesediment.So,thescouringdimensionscanbemeasuredattimet=10s.

Fig5.3.24:Velocityvectorsattimet=15s.

Fig5.3.25:Velocityvectorsattimet=30s.

Eventuallyasthetimepasses,theloopdisappearsgraduallyasshownintheFigure5.3.24 and5.3.25.

5.4:

3D-FreeSurfaceModel:ResultsandDiscussions

Thevelocitywaveprofilegeneratedisasshowninfigure5.4.1.

Fig5.4.1:VelocityProfile

5.5:3D-FreeSurfacePierModel:ResultsandDiscussions

This model is still running in the computer. Due to low memory capacity this simulation wouldtakearound 40-60 daystocomplete.So duetocomputationallimitationstheresults ofthismodelareatwait. Alsoin all the3-Dmodelsk-epsilonturbulence modelis usedin thepresentstudy,itshouldbereplacedbyk-ωturbulence,itgivesmoregoodresultsnearthe wallofpier,butrequiresmorememoryandcomputationaltime.

Conclusions

6.1For2-DSinglePiersSimulation

The 2-dimensional simulation of model helped in getting the brief information around the pier like pressure contours which gives a knowledge about the pressure in the vortices as pressure difference is one of the reason of vortices formation. With the help of velocity contour of the 2-dimensional simulation difference in the velocity around the pier can be easily seen which is helpful for getting into the phenomenon of vortices which leads into scouring.

6.2For3-DSinglePiersSimulation

By observing the of iso-surface at different volume fraction and volume rendering, it was concludedthatthesedimentwasinlesseramountnearthepierascomparedtotheotherpart ofcanal. The velocitystreamlineshowsthe downflowofthe waterwhen waterpassesjust fromtheupstreamsideandthenflowsinstraightlinetowardsdownstreamside.Thevelocity vectorsshowedthatwhenwater passesthepier,itcompletesawholeloopasshownin the Fig 5.3.22 at time t=10swhich are the characteristics of Horseshoe vortex. It is thereason for development of scour hole. As discussed earlier about the velocity, which couldn’t be reduced due to unavailability of computational time and facility, the scour hole was developing till 10 seconds after the sediments were getting washed away towards the downstreamsideduetohighervelocityandlackoffielddataforcomputation.

 The dimensions of scour hole across the width of the canal were 0.28m and across the length of the canal from upstream side till the pier was 0.13m and from pier towardsthedownstreamsidewas0.45mwhereasthedepthofscourholewas0.21m0.24mapproximately.

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