外文翻译--溢洪道

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1、外文翻译--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