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1、外文翻譯--溢洪道外文原文OVERFLOWSPILLWAYAnoverflowspillwayisasectionofdamdesignedtopermitwatertopassoveritscrest.Overflowspillwaysarewidelyusedongravity,arch,andbuttressdams.Someearthdamshaveaconcretegravitysectiondesignedtoserveasaspillway.Thedesignofthespillwayfortowda
2、msisnotusuallycritical,andavarietyofsimplecrestpatternsareused.Inthecaseoflargedamsitisimportantthattheoverflowingwaterbeguidedsmoothlyoverthecrestwithaminimumofturbulence.Iftheoverflowingwaterbreakscontactwiththespillwaysurface,avacuumwillformatthepointofsepa
3、rationandcavitationsmayoccur.Cavitationsplusthevibrationfromthealternatesmakingandbreakingofcontactbetweenthewaterandthefaceofthedammayresultinseriousstructuraldamage.Cavitiesfilledwithvapor,air,andothergaseswillforminaliquidwhenevertheabsolutepressureoftheliq
4、uidisclosetothevaporpressure.Thisphenomenon,cavitations,islikelytooccurwherehighvelocitiescausereducedpressure.Suchconditionsmayariseifthewallsofapassagearesosharplycurvedastocauseseparationofflowfromtheboundary.Thecavity,onmovingdownstream,mayenteraregionwher
5、etheabsoluteismuchhigher.Thiscausesthevaporinthecavitytocondenseandreturntoliquidwitharesultingimplosion,orcollapse,extremelyhighpressureresult.Someoftheimplosiveactivitywilloccuratthesurfacesofthepassageandinthecrevicesandporesoftheboundarymaterial.Underacont
6、inualbombardmentoftheseimplosions,thesurfaceundergoesfatiguefailureandsmallparticlesarebrokenaway,givingthesurfaceaspongyappearance.Thisdamagingactionofcavitationsiscalledpitting.Theidealspillwaywouldtaketheformoftheundersideofthenappedofasharp-crestedweirwhen
7、theflowratecorrespondstotheimumdesigncapacityofthespillway.Moreexactprofilesmaybefoundinmoreextensivetreatmentsofthesubject.Thereversecurveonthedownstreamfaceofthespillwayshouldbesmoothandgradual;Aradiusofaboutone-fourthofthespillwayheighthasprovedsatisfactory
8、.Structuraldesignofanogeespillwayisessentiallythesameasthedesignofaconcretegravitysection.Thepressureexertedonthecrestofthespillwaybytheflowingwaterandthedragforcescausedbyfluidfri
