Aeroelastic Simulation of Wind Turbine Dynamics
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Aeroelastic Simulation of Wind Turbine Dynamics by Anders Ahlstr¨om April 2005 Doctoral Thesis from Royal Institute of Technology Department of Mechanics SE-100 44 Stockholm, Sweden Akademisk avhandling som med tillst˚and av Kungliga Tekniska h¨ogskolan i Stock- holm framl¨agges till offentlig granskning f¨or avl¨aggande av teknologie doktorsexamen fredagen den 8:e april 2005 kl 10.00 i sal M3, Kungliga Tekniska h¨ogskolan, Valhalla- v¨agen 79, Stockholm. c Anders Ahlstr¨om 2005 Abstract The work in this thesis deals with the development of an aeroelastic simulation tool for horizontal axis wind turbine applications. Horizontal axis wind turbines can experience significant time varying aerodynamic loads, potentially causing adverse effects on structures, mechanical components, and power production. The needs for computational and experimental procedures for investigating aeroelastic stability and dynamic response have increased as wind turbines become lighter and more flexible. A finite element model for simulation of the dynamic response of horizontal axis wind turbines has been developed. The developed model uses the commercial finite element system MSC.Marc, focused on nonlinear design and analysis, to predict the structural response. The aerodynamic model, used to transform the wind flow field to loads on the blades, is a Blade-Element/Momentum model. The aerody- namic code is developed by The Swedish Defence Research Agency (FOI, previously named FFA) and is a state-of-the-art code incorporating a number of extensions to the Blade-Element/Momentum formulation. The software SOSIS-W, developed by Teknikgruppen AB was used to generate wind time series for modelling different wind conditions. The method is general, and different configurations of the structural model and various type of wind conditions can be simulated. The model is primarily intended for use as a research tool when influences of specific dynamic effects are investigated. Verification results are presented and discussed for an extensively tested Danwin 180 kW stall-controlled wind turbine. Code predictions of mechanical loads, fatigue and spectral properties, obtained at different conditions, have been compared with measurements. A comparison is also made between measured and calculated loads for the Tjæreborg 2 MW wind turbine during emergency braking of the rotor. The simulated results correspond well to measured data. Keywords: wind turbine; aeroelastic modelling; rotor aerodynamics; structural dynamics; MSC.Marc; AERFORCE; SOSIS-W i F¨orord I skrivande stundar ¨ det n¨ast intill fem ˚ar sedan jag b¨orjade som doktorand p˚aKTH och det sl˚ar mig hur fort dessa ˚ar har passerat. F¨orklaringenar ¨ att jag f¨or det mesta har haft ett b˚ade roligt och stimulerande jobb. Undantagenar ¨ f¨orst˚as n¨ar allt har g˚att, f¨or att uttrycka det milt, mindre bra. Som turar ¨ har jag d˚ahaftmin handledare att fr˚aga om r˚ad. Anders, utan dina fantastiska kunskaper och ditt st¨od skulle den h¨ar tiden k¨ants betydligt mycket l¨angre och tr˚akigare. Jag vill ocks˚a passa p˚a att tacka Ingemar Carl´en och Hans Ganander p˚aTeknik- gruppen AB, samt Jan-Ake˚ Dahlberg p˚a FOI, som alla har varit till stor hj¨alp i projektet. Jag villaven ¨ tacka Anders Bj¨orck f¨or hj¨alpmeddenaerodynamiska modellen och f¨or din medverkan i referensgruppen. Tack ocks˚a tillovriga ¨ medlemmar i referensgruppen, Lennart S¨oder, Sven-Erik Thor och Anders Wikstr¨om. Ett stort tack vill jag rikta till Kurt S. Hansen och Stig Øye p˚a DTU f¨or alla fr˚agor ni villigt besvarat. Ett stort tack riktar jag ocks˚a till Gunner Larsen f¨or ett varmt v¨alkomnande till Risø. Lunchen har utan avvikelse inmundigats klockan 11.30 p˚an˚agon finare restaurang. N¨ar man inmundigar en finare lunchar ¨ det viktigt med trevligt s¨allskap. Det har jag haft och det vill jag tacka mina lunchkompisar f¨or. Alla skratt har definitivt bidragit till den h¨oga trivselfaktorn h¨ar p˚amekanik. Tack till alla v¨anner p˚a mekanik och forna byggkonstruktion f¨or att ni gjort tiden som doktorand till en mycket positiv upplevelse. Tack till Energimyndigheten f¨or ekonomiskt st¨od. Slutligen vill jag tacka min familj och minalskade ¨ Jeanette. Stockholm, Februari 2005 Anders Ahlstr¨om iii Contents Abstract i F¨orord iii List of symbols xi List of figures xiii List of tables xvii 1 Introduction 1 1.1Background................................ 1 1.2Scopeandaims.............................. 1 1.3 Outline of thesis . 2 2 Wind turbine technology and design concepts 3 2.1Windpowerfromahistoricalpointofview............... 3 2.2 General description and layout of a wind turbine . 4 2.3Blades................................... 5 2.3.1 Material.............................. 5 2.3.2 Numberofblades......................... 6 2.3.3 Airfoildesign........................... 7 2.3.4 Lightningprotection....................... 7 2.3.5 Structural modelling . 7 2.4Tower................................... 8 2.4.1 Tubular steel towers . 8 v 2.4.2 Lattice towers . 8 2.4.3 Structural modelling . 8 2.5Hub.................................... 9 2.5.1 Structural modelling . 9 2.6Nacelleandbedplate........................... 9 2.6.1 Structural modelling . 9 2.7Brakingsystem.............................. 10 2.7.1 Aerodynamicbrakes....................... 10 2.7.2 Mechanicalbrakes........................ 10 2.7.3 Structural modelling . 11 2.8Yawmechanism.............................. 11 2.8.1 Structural modelling . 11 2.9Generator................................. 12 2.9.1 Constantspeedgenerators.................... 12 2.9.1.1 Twogenerators..................... 12 2.9.1.2 Polechanginggenerators................ 12 2.9.2 Variablespeedgenerators.................... 12 2.9.2.1 Variableslipgenerators................ 13 2.9.2.2 Indirectgridconnection................ 13 2.9.2.3 Directdrivesystem................... 13 2.9.3 Structural modelling . 14 2.10Powercontrol............................... 14 2.10.1Pitchcontrolledwindturbines.................. 15 2.10.2Stallcontrolledwindturbines.................. 15 2.10.3 Active stall controlled wind turbines . 16 2.10.4Othercontrolmechanisms.................... 16 2.10.5 Structural modelling . 16 2.11Gearbox.................................. 16 2.11.1 Structural modelling . 17 vi 3 Wind turbine design calculations 19 3.1Introduction................................ 19 3.2Presentwindturbinedesigncodes.................... 19 3.3Windfieldrepresentation......................... 21 3.4Rotoraerodynamics............................ 22 3.4.1 Bladeelementtheory....................... 22 3.5Loadsandstructuralstresses....................... 25 3.5.1 Uniformandsteadyflow..................... 26 3.5.2 Nonuniformandunsteadyflow.................. 28 3.5.3 Towerinterference........................ 28 3.5.4 Gravitational, centrifugal and gyroscopic forces . 28 4 Finite element modelling of wind turbines 31 4.1Generalfeaturerequirements....................... 31 4.2Elementconsiderations.......................... 32 4.3 Component modelling . 33 4.3.1 Tower............................... 33 4.3.2 Blades............................... 33 4.3.3 Drive train and bedplate modelling . 34 4.3.4 Pitchingsystem.......................... 36 4.3.5 Yawsystem............................ 37 4.3.6 Foundation . 38 4.4Solutionmethods............................. 38 4.4.1 Damping.............................. 38 4.4.2 Numericalmethodsandtolerances............... 39 4.5Programstructure............................ 41 4.5.1 Structural modelling . 41 4.5.2 Aerodynamic modelling . 42 4.5.3 Wind modelling . 44 4.5.4 Calculationscheme........................ 47 vii 4.5.4.1 Derivation of transformation matrices . 48 4.5.4.2 Information needed through input files . 48 5 Numerical examples 51 5.1Alsvikturbine............................... 51 5.1.1 Exampleofbladefailuresimulation............... 53 5.1.1.1 Results of blade failure simulation . 53 5.1.1.2 Conclusions....................... 56 5.2 Tjæreborg turbine . 56 5.2.1 Visualisation and description of an emergency stop of the Tjæreborg turbine . 57 5.2.2 Simulation of emergency brake situations based on a modified Tjæreborg turbine . 64 5.2.2.1 Introduction...................... 64 5.2.2.2 Results......................... 64 5.2.2.3 Conclusions....................... 67 5.3Commentsonsimulations........................ 68 6 Conclusions and future work 69 6.1Conclusions................................ 69 6.2Futureresearch.............................. 70 Bibliography 71 A Airfoil data 77 A.1 Lift and drag profiles of the Alsvik 180 kW wind turbine . 77 A.2 Lift and drag profiles of the Tjæreborg 2 MW wind turbine . 79 B Input files 81 B.1 Example of SOSIS-W input file . 81 B.2 The Alsvik input file . 82 viii Paper 1: Aeroelastic FE modelling of wind turbine dynamics 89 Paper 2: Emergency stop simulation using a FEM model developed for large blade deflections 115 Paper 3: Influence of wind turbine flexibility on loads and power production 141 ix List of symbols a tangential induction factor, 23 α angle of attack, 23 c blade cord length, 22 CD drag coefficient, 22 CL lift coefficient, 22 CN projected drag coefficient, 23 c(r) chord at position r,24 CT projected lift coefficient, 23 D drag force, 23 FN force normal to rotor plane, 23 FT force tangential to rotor plane, 23 L lift force, 22 N number of blades, 24 ω rotation speed, 23 φ angle between disc plane and relative velocity, 23 r radius of the blade, 23 σ solidify factor, 24 θ local pitch of the blade, 23 U∞ undisturbed air speed, 23 Vrel relative air speed, 22 xi List of Figures 2.1 The 1.250 MW Smith-Putnam