Modal Analysis of Wind Turbine Blades

Abstract Modalanalysishasbeenusedtoidentifynaturalfrequencies,damp-ingcharacteristicsandmodeshapesofwindturbineblades.Differentexperimentalprocedureshavebeenconsideredandthemostappropri-ateofthesehasbeenselected.AlthoughthecomparisonisbasedonmeasurementsonaLM19mblade,therecommendationsgivenarebelievedtobevalidforothertypesofwindturbinebladesaswell.Thereliabilityoftheselectedexperimentalanalysishasbeenquantifiedbyes-timating the unsystematicvariations in the experimental findings. Satisfactoryresults have been obtained for natural frequencies, damping characteristics andforthedominatingdeflectiondirectionoftheinvestigatedmodeshapes.Fornon-dominating deflection directions of the investigated mode shapes, however, theobservedexperimental uncertainty may be considerable – especially for the tor-sionaldeflection.TheexperimentalanalysisoftheLM19mbladehasbeencomparedwithresultsfromaFE-modelingofthesameblade.Forsomeofthehighermodessubstantialdiscrepanciesbetween the naturalfrequencies originatingfrom the FE-modelingandthe modal analysis,respectively, areobserved.Comparing the mode shapes(normalized with respect to the tip deflection in the dominating deflection di-rection) good agreement has been demonstrated for the dominating deflectiondirection. For the non-dominating deflection directions, the qualitative features(i.e.theshape)ofmeasuredandcomputedmodesshapesareingoodagreement,whereastheyquantitativelymaydisplayconsiderabledeviations.Finally,suggestionsofpotentialfutureimprovementsofthe experimentalpro-cedurearediscussed.ISBN87–550–2696–6ISBN87–550–2697–4(Internet)ISSN0106–2840Print:PitneyBowesManagementServicesDenmarkA/S,2002