外文翻译溢洪道

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1、外文原文OVERFLOWSPILLWAYAnoverflowspillwayisasectionofdamdesignedtopermitwatertopassoveritscrest.Overflowspillwaysarewidelyusedongravity,arch,andbuttressdams.Someearthdamshaveaconcretegravitysectiondesignedtoserveasaspillway.Thedesignofthespillwayfortowdamsisnotusuallycritical,andavarie

2、tyofsimplecrestpatternsareused.Inthecaseoflargedamsitisimportantthattheoverflowingwaterbeguidedsmoothlyoverthecrestwithaminimumofturbulence.Iftheoverflowingwaterbreakscontactwiththespillwaysurface,avacuumwillformatthepointofseparationandcavitationsmayoccur.Cavitationsplusthevibratio

3、nfromthealternatesmakingandbreakingofcontactbetweenthewaterandthefaceofthedammayresultinseriousstructuraldamage.Cavitiesfilledwithvapor,air,andothergaseswillforminaliquidwhenevertheabsolutepressureoftheliquidisclosetothevaporpressure.Thisphenomenon,cavitations,islikelytooccurwherehi

4、ghvelocitiescausereducedpressure.Suchconditionsmayariseifthewallsofapassagearesosharplycurvedastocauseseparationofflowfromtheboundary.Thecavity,onmovingdownstream,mayenteraregionwheretheabsoluteismuchhigher.Thiscausesthevaporinthecavitytocondenseandreturntoliquidwitharesultingimplos

5、ion,orcollapse,extremelyhighpressureresult.Someoftheimplosiveactivitywilloccuratthesurfacesofthepassageandinthecrevicesandporesoftheboundarymaterial.Underacontinualbombardmentoftheseimplosions,thesurfaceundergoesfatiguefailureandsmallparticlesarebrokenaway,givingthesurfaceaspongyapp

6、earance.Thisdamagingactionofcavitationsiscalledpitting.Theidealspillwaywouldtaketheformoftheundersideofthenappedofasharp-crestedweirwhentheflowratecorrespondstothemaximumdesigncapacityofthespillway.Moreexactprofilesmaybefoundinmoreextensivetreatmentsofthesubject.Thereversecurveonthe

7、downstreamfaceofthespillwayshouldbesmoothandgradual;Aradiusofaboutone-fourthofthespillwayheighthasprovedsatisfactory.Structuraldesignofanogeespillwayisessentiallythesameasthedesignofaconcretegravitysection.Thepressureexertedonthecrestofthespillwaybytheflowingwaterandthedragforcescau

8、sedbyfluidfrictiona