水利电力英文翻译英文+中文

1OVERFLOWSPILLWAYAnoverflowspillwayisasectionofdamdesignedtopermitwatertopassoveritscrest.Overflowspillwaysarewidelyusedongravity,arch,andbuttressdams.Someearthdamshaveaconcretegravitysectiondesignedtoserveasaspillway.Thedesignofthespillwayfortowdamsisnotusuallycritical,andavarietyofsimplecrestpatternsareused.Inthecaseoflargedamsitisimportantthattheoverflowingwaterbeguidedsmoothlyoverthecrestwithaminimumofturbulence.Iftheoverflowingwaterbreakscontactwiththespillwaysurface,avacuumwillformatthepointofseparationandcavitationsmayoccur.Cavitationsplusthevibrationfromthealternatesmakingandbreakingofcontactbetweenthewaterandthefaceofthedammayresultinseriousstructuraldamage.Cavitiesfilledwithvapor,air,andothergaseswillforminaliquidwhenevertheabsolutepressureoftheliquidisclosetothevaporpressure.Thisphenomenon,cavitations,islikelytooccurwherehighvelocitiescausereducedpressure.Suchconditionsmayariseifthewallsofapassagearesosharplycurvedastocauseseparationofflowfromtheboundary.Thecavity,onmovingdownstream,mayenteraregionwheretheabsoluteismuchhigher.Thiscausesthevaporinthecavitytocondenseandreturntoliquidwitharesultingimplosion,orcollapse,extremelyhighpressureresult.Someoftheimplosiveactivitywilloccuratthesurfacesofthepassageandinthecrevicesandporesoftheboundarymaterial.Underacontinualbombardmentoftheseimplosions,thesurfaceundergoesfatiguefailureandsmallparticlesarebrokenaway,givingthesurfaceaspongyappearance.Thisdamagingactionofcavitationsiscalledpitting.Theidealspillwaywouldtaketheformoftheundersideofthenappedofasharp-crestedweirwhentheflowratecorrespondstothemaximumdesigncapacityofthespillway.Moreexactprofilesmaybefoundinmoreextensivetreatmentsofthesubject.Thereversecurveonthedownstreamfaceofthespillwayshouldbesmoothandgradual;Aradiusofaboutone-fourthofthespillwayheighthasprovedsatisfactory.Structuraldesignofanogeespillwayisessentiallythesameasthedesignofaconcretegravitysection.Thepressureexertedonthecrestofthespillwaybytheflowingwaterandthedragforcescausedbyfluidfrictionareusuallysmallincomparisonwiththeotherforcesactingonthesection.Thechangeinmomentumoftheflowinthevicinityofthereversecurvemay,however,createaforcewhichmustbeconsidered.Therequirementsoftheogeeshapeusuallynecessitateathickersectionthantheadjacentnooverflowsections.Asavingofconcretecanbeeffectedbyprovidingaprojectingcorbelontheupstreamfacetocontroltheflowinoutletconduitsthroughthedam,acorbelwillinterferewithgateoperation.Thedischargeofanoverflowspillwayisgivenbytheweirequation23CQLhWhereQ=discharge,orsec/3mtcoefficienCL=coefficienth=headonthespillway(verticaldistancefromthecrestofthespillwaytothereservoirlevel),2mThecoefficientCvarieswiththedesignandhead.Experimentalmodelsareoftenusedtodeterminespillwaycoefficient.EndcontractionsonaspillwayreducetheeffectivelengthbelowtheactuallengthL.Square-corneredpiersdisturbtheflowconsiderablyandreducetheeffectivelengthbythewidthofthepiersplusabout0.2hforeachpier.Streamliningthepiersorflaringthespillwayentranceminimizestheflowdisturbance.Ifthecross-sectionalareaofthereservoirjustupstreamfromthespillwayislessthanfivetimestheareaofflowoverthespillway,theapproachvelocitywithincreasethedischargeanoticeableamount.Theeffectofapproachvelocitycanbeaccountedforbytheequation2320g2VhQLCwhere0Vistheapproachvelocity.PROPERTIESOFCONCRETEThecharacteristicsofconcreteshouldbeconsideredinrelationtothequalityforanygivenconstructionpurpose.Theclosestpracticableapproachtoperfectionineverypropertyoftheconcretewouldresultinpooreconomyundermanyconditions,andthemostdesirablestructureisthatinwhichtheconcretehasbeendesignedwiththecorrectemphasisoneachofthevariouspropertiesoftheconcrete,andnotsolelywithaviewtoobtaining,say,themaximumpossiblestrength.Althoughtheattainmentofthemaximumstrengthshouldnotbethesolecriterionindesign,themeasurementofthecrushingstrengthofconcretecubesorcylindersprovidesameansofmaintainingauniformstandardofquality,and,infact,istheusualwayofdoingso.Sincetheotherpropertiesofanyparticularmixofconcretearerelatedtothecrushingstrengthinsomemanner,itispossiblethatasasinglecontroltestitisstillthemostconvenientandinformative.Thetestingofthehardenedconcreteinprefabricatedunitspresentsnodifficulty,sincecompleteunitscanbeselectedandbrokenifnecessaryintheprocessoftesting.Samplescanbetakenfromsomepartsofafinishedstructurebycuttingcores,butatconsideronecostandwithapossibleweakeningofthestructure.Itiscustoms,therefore,toestimatethepropertiesoftheconcreteinthestructureontheoasisofthetestsmadeonspecimensmoundedfromthefreshconcreteasitisplaced.ThesespecimensarecompactedandcuredinastandardmannergiveninBS1881in1970asinthesetworespectsitisimpossibletosimulateexactlytheconditionsinthestructure.Sincethecrushingstructureisalsoaffectedbythesizeandshapeofaspecimenorpartofastructure,itfollowsthatthecrushingstrengthofacubeisnotnecessarythesameasthatofthemassofexactlythesameconcrete.CrushingstrengthConcretecanbemadehavingastrengthincompressionofuptoabout80N/2mm,oreven3moredependingmainlyontherelativeproportionsofwaterandcement,thatis