acousticsworkshop1kw副本.docx

acousticsworkshop1kw副本.docx

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acousticsworkshop1kw副本

Note:

ThisworkshopprovidesinstructionsintermsoftheABAQUSKeywordsinterface.IfyouwishtousetheABAQUSGUIinterfaceinstead,pleaseseethe“Interactive”versionoftheseinstructions.

PleasecompleteeithertheKeywordsorInteractiveversionofthisworkshop.

Goals

Whenyoucompletethisworkshop,youwillbeableto:

∙Determinethenaturalfrequenciesofanacousticdomain.

∙Determinethesteady-statedynamicresponseofanacousticsystem.

∙Defineaclassicalacousticplane-waveabsorbingboundary.

∙Defineacousticexcitationloads.

∙ViewrealandimaginarycomponentsofacousticpressurewithABAQUS/Viewer.

∙UseABAQUS/Viewertocreatepathplotsofacousticoutputdata.

Introduction

Asimpleacousticmodelofasectionofairductwillbeusedtointroducesomeofthebasicanalysistechniquesthatcanbeappliedtoanacousticmesh.Inthisworkshopyouwilladdimportantmodelingdetailstocompletethesuppliedanalysisinputfilethatalreadycontainsbasicmodelingdatasuchasnode,element,andmaterialdefinitions,fortheairductsectionshowninFigureW1–1.Themodelforthisworkshoprepresentsonlytheairinsidetheduct.Therefore,theacousticnaturalboundaryconditionofinfiniteimpedanceatameshboundaryimpliesrigidductwalls.Youwillperformnaturalfrequencyextractionanalyseswithdifferentsetsofacousticboundaryconditionsassignedtotheendsoftheduct.Youwillthenperformaseriesofsteady-statedynamicresponseanalysesonthemodelusingdifferentexcitationmethods.Thesteady-statedynamicanalyseswillalsoincludethedefinitionofacousticplane-waveabsorbingboundaryimpedanceatoneendoftheducttoinvestigatepostprocessingoftravelingwavesforfrequency-domainsolutions.

TheairductsectionmodelshowninFigureW1–1issuppliedtoyouintermsofaninputfile,whichusestheABAQUSparametricinputcapability.Theparametervaluesdefinedintheinputfileshouldnotbechangedforthisworkshop.However,aftercompletionofthisworkshopyoumaywanttoconsidermodifyingthemeshingparameterstoperformadditionalconvergenceandaccuracystudies.Theairductsectionis4.25mlongandhasarectangularcross-sectionthatis0.2mby0.125m.Theacousticmediumisairwithadensityof1.225kg/m3andasoundspeedof340m/s.Thus,theeffectivebulkmodulusis141610Pa.Themodelisdesignedtoprovidereasonablequantitativeresultsforanalysesupto400Hzandutilizesfirst-orderacousticelementsthatare0.085mlong.Thismeshrefinementcorrespondsto10elements（nodaldivisions）peracousticwavelengthataresponsefrequencyof400Hz.Thelargestcross-sectionaldimensionoftheductissignificantlylessthanone-halfthe400Hzacousticwavelength;therefore,theductcross-sectioncanbemodeledwithasingleelement.

FigureW1–1Modelofasimpleairductsection.

Note:

Ingeneral,second-orderacousticelementsaremoreaccuratethanfirst-orderacousticelementsforthesamemeshnodaldensity.However,first-orderelementscanprovidenearlyasgoodanapproximationtoasinusoidalacousticpressurewaveascansecond-orderelementsaslongasthenodaldistributionsaresimilar.Therefore,whenperformingacousticanalysesinwhichdeterminingthepressurefieldisofprimaryinterest,first-orderelementsareoftenusedbecausetheyareeasiertoworkwithintermsofmeshing,definingacousticloads,andinputfileediting.Themainadvantageofsecond-orderacousticelementsisthattheyprovideforapiecewiselinearapproximationofthepressuregradients（and,thus,theparticlevelocities）,whilethefirst-orderelementsproduceapiecewiseconstantapproximation.Therefore,formesheswithsimilarnodaldensities,thesecond-orderelementswillprovidesignificantlybetterquantitativeresultsforitemssuchasacousticintensity（power）thatrequireaccurateestimatesofpressuregradientsandparticlevelocities.Usingfirst-orderacousticelementstodetermineapressurefieldisanalogoustousingfirst-orderstress-displacementelementstodeterminestiffness,whileusingsecond-orderacousticelementstodetermineacousticintensityisanalogoustousingsecond-orderstress-displacementelementstodetermineastress/strainconcentration.

Case1:

Naturalfrequencyextraction（rigid-rigidends）

ThefirstanalysisforWorkshop1involvesextractingthenaturalfrequenciesoftheairductsectionshowninFigureW1–1forthecasewherebothendsoftheductutilizethenaturalboundaryconditionassociatedwithanacousticmesh.Therefore,forthiscaseno*BOUNDARYor*CLOADoptionsareappliedtotheacousticpressuredegreeoffreedom.Thenaturalboundaryconditionassociatedwiththesurfaceofanacousticmeshcorrespondstoinfiniteboundaryimpedance.Themeshboundarycanthereforebeenvisionedasaninfinitelyrigidsurface.

1.Entertheworkingdirectoryforthisworkshop:

../acoustics/keywords/workshop1

2.Thefileacoust-ws1-template.inpcontainsaparametricmodeloftheairductsection.Copyacoust-ws1-template.inptoafilenamedws1-1.inp.Openws1-1.inpinatextfileandreviewitscontents.

3.Addthe*STEPoptionalongwithanappropriatedescriptivesubheading.

4.Addthe*FREQUENCYoptiontoextractthemodel’snaturalfrequencies.ItisrecommendedthattheLanczoseigensolverbeselectedviatheEIGENSOLVERparameter.Theextractionofnaturalfrequenciesforanacousticmeshinrealworldproblemswilllikelyinvolverelativelylargemodels,forwhichtheLanczoseigensolverismoreefficientthanthesubspaceiterationeigensolver.Inaddition,theLanczoseigensolverwillallowyoutoselectaprecisefrequencyrangefortheeigenvalueextraction.Extractthenaturalfrequenciesintherangefrom5Hzto400 Hzwithashiftpointcorrespondingto100Hz.

*FREQUENCY,EIGENSOLVER=LANCZOS

25,5.0,400.0,10000.0

5.Basicoutputrequestsforthisanalysisarecontainedintheinputfileanddonotneedtobemodified.However,notethattheoutputvariableidentifierfortheacousticpressureisPOR.

6.Savethemodifiedinputfilews1-1.inpandruntheABAQUS/Standardanalysis.

7.Reviewtheprintedoutputfilews1-1.datinatexteditor.ComparethenaturalfrequencieslistedintheEigenvalueOutputtabletotheexactclassicalvaluesforthisproblem.Theclassicalsolutionisforafundamentalmodeof40Hzwithanintervalof40Hzbetweensuccessivemodes.

8.Aftertheanalysiscompletes,startanABAQUS/Viewersessionandopentheoutputdatabasefilews1-1.odb.ViewtheacousticmodeshapesbycreatingcontourplotsofthepressurevariablePOR（PlotContours）.

9.DefineapathalongthelengthoftheductsectionandcreateaplotofPORalongthispathusingtheprocedurebelow.

a.Fromthemainmenubar,selectToolsPathCreate.

b.IntheCreatePathdialogbox,acceptthedefaultpathname（Path-1）andtype（Nodelist）.ClickContinue.

c.IntheEditNodeListPathdialogbox,clickAddBefore.

d.Intheviewport,selectthestartandendpointsforthepath,asshowninFigureW1–2.ClickDoneinthepromptarea.

e.ThetableintheEditNodeListPathdialogboxnowcontainstheselectednodelabels.ClickOKtocompletethepathdefinition.

f.Fromthemainmenubar,selectToolsXYDataCreate.

g.IntheCreateXYDatadialogbox,choosePathandclickContinue.

h.IntheXYDatafromPathdialogbox,toggleonIncludeintersectionstoobtainX–Ydataatlocationswherethepathintersectsthemodelaswellasatthepointsthatmakeupthepath.

i.IntheYValuesportionoftheXYDatafromPathdialogbox,clickStep/Frame.IntheStep/Framedialogboxthatappears,choosethemodeofinterestandclickOK.

j.IntheCreateXYDatadialogbox,clickPlottogeneratethepathplot.

FigureW1–2containsexamplesofbothacontourandapathplotforthefifthmode.

FigureW1–2Fifthacousticmodeshapeofthe

airductsectionwithrigid-rigidends.

CASE2:

Naturalfrequencyextraction（rigid-freeends）

Asstatedearlier,thenaturalboundaryconditionassociatedwiththesurfaceofanacousticmeshcorrespondstoinfiniteboundaryimpedance.Thisimpliesthattheacousticparticlevelocitynormaltotheboundarysurfaceiszero.Theboundarysurfacecan,therefore,bethoughtofasrigid.Thenaturalboundaryconditionischaracterizedbyazeropressuregradient,asillustratedintheCase1analysisbythepathplotshowninFigureW1–2.Assigningavaluetotheacousticpressuredegreeoffreedomataboundingsurfacerepresentsakinematicboundaryconditionandcorrespondstozeroboundaryimpedance.Thistypeofboundaryconditionallowsacousticparticlestomovefreelyacrosstheboundingsurface.Theacousticparticlevelocityattheboundingsurface,characterizedbythepressuregradient,isthenunknowninamanneranalogoustoareactionforceinstructuralmechanics.Forthisanalysiscaseyouwilldeterminethenaturalfrequenciesoftheairductsectioninwhichoneendhasinfiniteboundaryimpedance（rigid）andtheotherhaszeroboundaryimpedance（free）.

1.CopyyourCase1inputfile（ws1-1.inp）toafilenamedws1-2.inpandopenthenewfileinatexteditor.

2.Applyazeropressureboundarycondition（kinematic）tothenodesatEnd1oftheairductsection（seeFigureW1–1）.The*BOUNDARYoptionisusedwithdegreeoffreedom8（pressure）settozeroforthenodesetDUCT_END1.

*BOUNDARY

node_number_or_set,8,8,0.0

3.SavethemodifiedinputfileandruntheABAQUS/Standardanalysis.

4.Reviewtheprintedoutputfilews1-2.datinatexteditor.ComparethenaturalfrequencieslistedintheEigenvalueOutputtabletotheexactclassicalvaluesforthisproblem.Theclassicalsolutionisforafundamentalmodeof20Hzwithan