Angelou, Nikolas

Berg, Jacob

Lange, Julia

Mann, Jakob

Mikkelsen, Torben Krogh

Sjöholm, Mikael

The here presented results are part of a paper that is submitted and accepted with minor revisions by the Boundary-Layer Meteorology journal. The wake zone behind the escarpment of the Bolund peninsula in the Roskilde Fjord, Denmark, has been investigated with the help of a continuouswave Doppler lidar. The instrument measures the line-of-sight wind speed 390 times per second in highly resolved 7-m tall profiles by rapidly changing the focus distance and beam direction. The profiles reveal the detailed and rapidly changing structure of the wake induced by the Bolund escarpment. The wake grows with distance from the escarpment, with the wake height depending strongly on the wind direction, such that the minimum height appears when the flow is perpendicular to the escarpment. The wake increases by 10% to 70% when the wind direction deviates ± 15 from perpendicular depending on the distance to the edge and to a lesser degree on the method by which the wake height is determined

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Downloaded from orbit.dtu.dk on: Nov 08, 2017Variations of the wake height over the Bolund escarpmentLange, Julia; Mann, Jakob; Angelou, Nikolas; Berg, Jacob; Sjöholm, Mikael; Mikkelsen, Torben KroghPublished in:Scientific Proceedings. EWEA Annual Conference and Exhibition 2015Publication date:2015Document VersionPublisher's PDF, also known as Version of recordLink back to DTU OrbitCitation (APA):Lange, J., Mann, J., Angelou, N., Berg, J., Sjöholm, M., & Mikkelsen, T. K. (2015). Variations of the wake heightover the Bolund escarpment. In Scientific Proceedings. EWEA Annual Conference and Exhibition 2015  (pp.145-146). European Wind Energy Association (EWEA).EWEA 2015 Scientific Proceedings145sphericflowwasmeasuredinseven,7-mhighver-ticalprofileswithdistancesbetween8mand31mfromthescanninglidar(Fig.2).Inadditiontothesevenverticalprofilesahorizontalarcextending±60◦wasscanned120mawayfromtheinstrument.Theline-of-sightwindspeedsoftheeighthprofilewereusedtodeterminetheundisturbedinflowwindspeedandwinddirection.Whilewesterlywinddirectionsprevailed,lidarmeasurementswererecordedcontinuouslyduringanalmost24hourlongmeasurementperiod.Figure1:PhotoofBolund,takensouthofthepenin-sula.-��-��-��-���-��-����������������������[�]������[�]��������������������������������������������������������Figure2:Thepositionandheightofthe7verticalprofilesscannedbythelidarrelativetotheBolundescarpment.ThepositionoftheWindScanneritselfisindicatedbythecircle.4MethodThecharacteristicoftheescarpment-inducedwakeheightisfurtherinvestigatedbyidentifyingtheboundarybetweentheturbulentwakelayerandthefreestreamflowabove.Duetothehighmeasurement-samplingrateaprecisedeterminationoftheinterfacebetweenthetwodistinctlydifferentlayersispossible.Wedeterminethewakeheightδusingthreedifferentmethods.1.Thefirstapproachdeterminesthedisplacementthickness,δ 1,thatisdefinedasthedistancethattheboundarylayerisdisplacedtocompensateforthereductioninflowrateonaccountofthewakeformation,whereu(z)istheline-of-sightwindspeedatheightzandu0isthefreestreamvelocity[11]δ 1=∞ ∫ 0(1−u(z)u0) dz.(1)2.Thesecondapproachidentifiestheheightofthemaximumgradientoftheline-of-sightwindspeed,δ 2,ofeachverticalscan[12,Sect.2.2.2]δ 2=argmax z[ du(z)dz] .(2)3.Thethirdapproachidentifiestheheightatwhichtheaveragebetweentheintegralofthetwoat-mosphericlayersisthegreatest,δ 3,whichre-sembles[13]and[12,Sect.2.2.3].Here,z topisthetopoftheprofile,δ 3=argmax z 1z top−zztop ∫ zu(z)dz−1 zz ∫ 0u(z)dz  .(3)VariationsofthewakeheightovertheBolundescarpmentpresentingauthor:JuliaLangeco-authors:JakobMann,NikolasAngelou,JacobBerg,MikaelSjo¨holm,TorbenMikkelsenDTUWindEnergyTechnicalUniversityofDenmarkRisøCampusOctober11,20151AbstractTheherepresentedresultsarepartofapaperthatissubmittedandacceptedwithminorrevisionsbytheBoundary-LayerMeteorologyjournal.ThewakezonebehindtheescarpmentoftheBolundpeninsulaintheRoskildeFjord,Denmark,hasbeeninvestigatedwiththehelpofacontinuous-waveDopplerlidar.Theinstrumentmeasurestheline-of-sightwindspeed390timespersecondinhighlyresolved7-mtallprofilesbyrapidlychangingthefocusdistanceandbeamdirection.TheprofilesrevealthedetailedandrapidlychangingstructureofthewakeinducedbytheBolundescarpment.Thewakegrowswithdistancefromtheescarpment,withthewakeheightdependingstronglyonthewinddirection,suchthattheminimumheightappearswhentheflowisperpendiculartotheescarpment.Thewakeincreasesby10%to70%whenthewinddirectiondeviates±15◦fromperpendiculardependingonthedistancetotheedgeandtoalesserdegreeonthemethodbywhichthewakeheightisdetermined.Keywords:Bolund,Wakeheight,Complexflow,WindScanner2IntroductionToobtainhighqualityresultsinnumericalandphys-icalmodellingforwindenergypurposes,isitimpor-tanttoverifythesemodelswithreliablerealworldmeasurements[1,2,3,4,5,6].Bolund,aniso-latedflat-toppedhillwithsteepsidesintheRoskildeFjord,Denmark,(Fig.1)servesassuchabaselinereferenceforvariousstudieswithrespecttonumer-icalandphysicalmodellingsinceamastbasedat-mosphericexperimentwasconductedbyDTUWindEnergyduringwinter2007-2008[7,8].Toobtainamorecomprehensiveunderstandingoftheflowpat-ternovertheBolundpeninsula,especiallyclosetothesurface,acomplementaryfieldexperimentontheBolundpeninsulawasconducted.InOctober2011alaseranemometer,inthefollowingcalledWindScanner[9,10],wasplacedonthepeninsula20minlandfromthewestwardfacingescarpment.3ApproachTheWindScanner,alignedonthe270◦axis,wasop-eratedduringwesterlywindconditionstoscantheareadownstreamoftheBolundedge.Theatmo-EWEA 2015 Scientific Proceedings146References[1]P.S.JacksonandJ.C.RHunt.Turbulentflowoveralowhill.QuarterlyJournaloftheRoyalMeteorologicalSociety,101:929–955,1975.[2]J.C.R.Hunt,S.Leibovich,andK.J.Richards.Turbulentshearflowsoverlowhills.QuarterlyJournaloftheRoyalMeteorologicalSociety,114:1435–1470,1988.[3]FotiniKatopodesChowandRobertL.Street.EvaluationofTurbulenceClosureModelsforLarge-EddySimulationoverComplexTerrain:FlowoverAskerveinHill.JournalofAppliedMe-teorologyandClimatology,48(5):1050–1065,May2009.[4]F.A.Castro,J.M.L.M.Palma,andA.SilvaLopes.SimulationoftheAskerveinFlow.Part1:ReynoldsAveragedNavier–StokesEquations(keTurbulenceModel).Boundary-LayerMete-orology,107(3):501–530,2003.[5]A.SilvaLopes,J.M.L.M.Palma,andF.A.Castro.SimulationoftheAskerveinflow.Part2:Large-eddysimulations.Boundary-LayerMete-orology,125(1):85–108,July2007.[6]B.Conan.WindResourceAssessmentinCom-plexTerrainbyWindTunnelModelling.PhDthesis,2012.[7]A.Bechmann,J.Berg,M.S.Courtney,H.E.Jørgensen,J.Mann,andN.N.Sørensen.TheBolundExperiment:OverviewandBackgroundRisø-R-Report.TechnicalReportJuly,2009.[8]J.Berg,J.Mann,A.Bechmann,M.S.Courtney,andH.E.Jørgensen.TheBolundExperiment,PartI:FlowOveraSteep,Three-DimensionalHill.Boundary-LayerMeteorology,141(2):219–243,July2011.[9]M.Sjo¨holm,N.Angelou,P.Hansen,K.H.Hansen,T.Mikkelsen,S.Haga,J.A.Silgjerd,andN.Starsmore.Two-DimensionalRotorcraftDownwashFlowFieldMeasurementsbyLidar-BasedWindScannerswithAgileBeamSteer-ing.JournalofAtmosphericandOceanicTech-nology,31(4):930–937,April2014.[10]N.Angelou,J.Mann,M.Sjo¨holm,andM.Courtney.Directmeasurementofthespectraltransferfunctionofalaserbasedanemometer.ReviewofScientificInstruments,83(3):33111,2012.[11]J.O.Hinze.Turbulence.McGraw-Hillse-riesinmechanicalengineering,1975.ISBN:0070290377.[12]S.Emeis,H.P.Frank,andF.Fiedler.Modifi-cationofairflowoveranescarpment.ResultsfromtheHjardema˚lexperiment.Boundary-LayerMeteorology,74(1-2):131–161,April1995.[13]K.J.Davis,N.Gamage,C.R.Hagelberg,C.Kiemle,D.H.Lenschow,andP.P.Sullivan.AnObjectiveMethodforDerivingAtmosphericStructurefromAirborneLidarObservations.JAtmosOcean.,17(11):1455–1468,November2000.TheresultsofthewakeheightidentificationsofallthreemethodsarepresentedinFig.3.Allthreemethodsmanagetoidentifyawakeheight,al-thoughtheactualheightdiffersbetweenthemeth-ods.Method1givesthehighestvalueofthewakeheights.Figure3:Theline-of-sightprojectedwindspeedofprofile3,12mawayfromtheWindScannerlastingfor30swiththedefinedwakeheightsusingthreedifferentmethods.Thecalculatedwakeheightforeachprofileloca-tioncanbeplacedinrelationtotheundisturbedwinddirectionandspeed(Fig.4).Withincreasingdis-tancefromtheescarpment,thewakeheightsshowastrongerdependenceonthewinddirection.Thelowestwakeheightsofeveryprofileislocatedatawinddirectionof270◦.Dependingonthedistancefromtheescarpmentthewakeheightincreasesbe-tween10%and80%whenthewinddirectiondevi-atesfromwest,eithertothenorthorthesouth±15◦ .Atlargerdirectiondeviationstheheightseemsconstant.����������������������������������[°]��������������[�]�����������������������������������Figure4:Dependenceofthedeterminedwakeheightandthewinddirection.Thesolidlinesde-picttheaveragewakeheight.TheprofilenumberincreaseswithincreasingdistancefromtheWind-Scanner.Thewakeheightiscalculatedthroughthedefinitionofthedisplacementthickness.5ConclusionThenewremotesensingbasedwindprofilemea-surementsprovideauniquedatasetforvalidationofunsteadyflowmodellingovercomplexterrainforwindenergy.Basedontheanalysisofthehighfre-quencyatmosphericmeasurementswitharapidlyscanningcontinuous-waveDopplerlidararelation-shipbetweentheescarpmentinducedwakeheightandthewinddirectioncouldbeshown.

Variations of the wake height over the Bolund escarpment

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Variations of the wake height over the Bolund escarpment

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