有限元模态分析-ANSYS

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1、ModalAnalysisModule2Module2ModalAnalysisA.Definemodalanalysisanditspurpose.B.Discussassociatedconcepts,terminology,andmodeextractionmethods.C.LearnhowtodoamodalanalysisinANSYS.D.Workononeortwomodalanalysisexercises.July22,2004Inventory#0021102-ModalAnalysisA.Definition&PurposeWhatismodalan

2、alysis?Atechniqueusedtodetermineastructure’svibrationcharacteristics:NaturalfrequenciesModeshapesModeparticipationfactors(howmuchagivenmodeparticipatesinagivendirection)Mostfundamentalofallthedynamicanalysistypes.July22,2004Inventory#0021102-ModalAnalysis…Definition&PurposeBenefitsofmodala

3、nalysisAllowsthedesigntoavoidresonantvibrationsortovibrateataspecifiedfrequency(speakers,forexample).Givesengineersanideaofhowthedesignwillrespondtodifferenttypesofdynamicloads.Helpsincalculatingsolutioncontrols(timesteps,etc.)forotherdynamicanalyses.Recommendation:Becauseastructure’svibra

4、tioncharacteristicsdeterminehowitrespondstoanytypeofdynamicload,alwaysperformamodalanalysisfirstbeforetryinganyotherdynamicanalysis.July22,2004Inventory#0021102-Generalequationofmotion:Assumefreevibrationsandignoredamping:Assumeharmonicmotion(i.e.)Therootsofthisequationarei2,theeigenvalue

5、s,whereirangesfrom1tonumberofDOF.Correspondingvectorsare{u}i,theeigenvectors.ModalAnalysisB.Terminology&ConceptsJuly22,2004Inventory#0021102-ModalAnalysis…Terminology&ConceptsThesquarerootsoftheeigenvaluesarewi,thestructure’snaturalcircularfrequencies(radians/sec).Naturalfrequenciesfiareth

6、encalculatedasfi=wi/2p(cycles/sec).ItisthenaturalfrequenciesfithatareinputbytheuserandoutputbyANSYS.Theeigenvectors{u}irepresentthemodeshapes-theshapeassumedbythestructurewhenvibratingatfrequencyfi.July22,2004Inventory#0021102-ModalAnalysis…Terminology&Concepts(cont.)ModeExtractionistheter

7、musedtodescribethecalculationofeigenvaluesandeigenvectors.ModeExpansionhasadualmeaning.Forthereducedmethod,modeexpansionmeanscalculatingthefullmodeshapesfromthereducedmodeshapes.Forallothermethods,modeexpansionsimplymeanswritingmodeshapestotheresultsfile.July2