An efficient Approach for Determining the Flutter Characteristics of Rotor Blades for Multi-Megawatt Wind Turbines EWEC Presentation No. 627 CS4: Aerodynamics, Aeroelasticity and Loads Dipl.-Ing. Raphael Haas EUROS Entwicklungsgesellschaft für Windkraftanlagen mbH, Berlin

An efficient Approach for Determining the Flutter Characteristics of Rotor Blades for Multi-Megawatt Wind Turbines Raphael Haas – EUROS Entwicklungsgesellschaft für Windkraftanlagen mbH – Berlin – Germany Introduction – Problem definition EUROS manufactures blades ranging from 10m to 56m for WEC from 100kW to 5.0MW Trend in development of large rotor blades: longer, lighter and narrower blades. Aeroelastically critical designs Risk of flutter

An efficient Approach for Determining the Flutter Characteristics of Rotor Blades for Multi-Megawatt Wind Turbines Raphael Haas – EUROS Entwicklungsgesellschaft für Windkraftanlagen mbH – Berlin – Germany What is flutter ? Flutter – an explanation: resonant (unstable) motion interaction of structural mass, stiffness, and aerodynamic forces usually present above some limiting (rotor) speed.

An efficient Approach for Determining the Flutter Characteristics of Rotor Blades for Multi-Megawatt Wind Turbines Raphael Haas – EUROS Entwicklungsgesellschaft für Windkraftanlagen mbH – Berlin – GermanyOverview System equation and structural model Unsteady aerodynamics – Greenberg Unsteady aerodynamics – Loewy Reduction of system size Results – Development of damping Results – Influencing Factors

An efficient Approach for Determining the Flutter Characteristics of Rotor Blades for Multi-Megawatt Wind Turbines Raphael Haas – EUROS Entwicklungsgesellschaft für Windkraftanlagen mbH – Berlin – Germany System equation Matrices: M:mass matrix D:damping matrix K:stiffness matrix Inidices: t:inertia mass terms a:aerodynamic terms el:elastic stiffness terms g:pretension stiffening terms z:centrifugal stiffening terms

An efficient Approach for Determining the Flutter Characteristics of Rotor Blades for Multi-Megawatt Wind Turbines Raphael Haas – EUROS Entwicklungsgesellschaft für Windkraftanlagen mbH – Berlin – Germany Structural model (1)

An efficient Approach for Determining the Flutter Characteristics of Rotor Blades for Multi-Megawatt Wind Turbines Raphael Haas – EUROS Entwicklungsgesellschaft für Windkraftanlagen mbH – Berlin – Germany Structural model (2)

An efficient Approach for Determining the Flutter Characteristics of Rotor Blades for Multi-Megawatt Wind Turbines Raphael Haas – EUROS Entwicklungsgesellschaft für Windkraftanlagen mbH – Berlin – Germany Unsteady Aerodynamics–Greenberg(1)

An efficient Approach for Determining the Flutter Characteristics of Rotor Blades for Multi-Megawatt Wind Turbines Raphael Haas – EUROS Entwicklungsgesellschaft für Windkraftanlagen mbH – Berlin – Germany Unsteady Aerodynamics–Greenberg(2)

An efficient Approach for Determining the Flutter Characteristics of Rotor Blades for Multi-Megawatt Wind Turbines Raphael Haas – EUROS Entwicklungsgesellschaft für Windkraftanlagen mbH – Berlin – Germany Unsteady Aerodynamics – Loewy (1) Pict. source: Loewy: „A Two-Dimesional Approximation to the Unsteady Aerodynamics of Rotary Wings“ Journal of the Aeronautical Sciences, Vol. 24, No. 2, pp , Feb

An efficient Approach for Determining the Flutter Characteristics of Rotor Blades for Multi-Megawatt Wind Turbines Raphael Haas – EUROS Entwicklungsgesellschaft für Windkraftanlagen mbH – Berlin – Germany Unsteady Aerodynamics – Loewy (2)

An efficient Approach for Determining the Flutter Characteristics of Rotor Blades for Multi-Megawatt Wind Turbines Raphael Haas – EUROS Entwicklungsgesellschaft für Windkraftanlagen mbH – Berlin – Germany Static Condensation Reduction of system size (1) Pict. source: Gasch: „Strukturdynamik, Band 2: Kontinua und ihre Diskretisierung.“ Springer. Berlin

An efficient Approach for Determining the Flutter Characteristics of Rotor Blades for Multi-Megawatt Wind Turbines Raphael Haas – EUROS Entwicklungsgesellschaft für Windkraftanlagen mbH – Berlin – Germany Modal Condensation Reduction of system size (2) Pict. source: Gasch: „Strukturdynamik, Band 2: Kontinua und ihre Diskretisierung.“ Springer. Berlin

An efficient Approach for Determining the Flutter Characteristics of Rotor Blades for Multi-Megawatt Wind Turbines Raphael Haas – EUROS Entwicklungsgesellschaft für Windkraftanlagen mbH – Berlin – Germany Example blade EU80: Length:39m Weight:6.5 tons Diagrams First order elastic beam Half torsional stiffness (2nd o. beam) Full torsional stiffness (2nd o. beam) Results-Trend of damping coefficients(1)

An efficient Approach for Determining the Flutter Characteristics of Rotor Blades for Multi-Megawatt Wind Turbines Raphael Haas – EUROS Entwicklungsgesellschaft für Windkraftanlagen mbH – Berlin – Germany Damping coefficients vs. Rotor speed, first order elastic beam. Results-Trend of damping coefficients(2)

An efficient Approach for Determining the Flutter Characteristics of Rotor Blades for Multi-Megawatt Wind Turbines Raphael Haas – EUROS Entwicklungsgesellschaft für Windkraftanlagen mbH – Berlin – Germany Damping vs. rotor speed, 2nd order el. Beam, full torsional stiffness. Results-Trend of damping coefficients(3)

An efficient Approach for Determining the Flutter Characteristics of Rotor Blades for Multi-Megawatt Wind Turbines Raphael Haas – EUROS Entwicklungsgesellschaft für Windkraftanlagen mbH – Berlin – Germany Damping vs. rotor speed, 2nd order el. Beam, half torsional stiffness. Results-Trend of damping coefficients(4)

An efficient Approach for Determining the Flutter Characteristics of Rotor Blades for Multi-Megawatt Wind Turbines Raphael Haas – EUROS Entwicklungsgesellschaft für Windkraftanlagen mbH – Berlin – Germany Results – Influencing Factors Higher torsional stiffness significantly shifts the flutter onset rotor speed Usage of a second order FEM blade model is mandatory

An efficient Approach for Determining the Flutter Characteristics of Rotor Blades for Multi-Megawatt Wind Turbines Raphael Haas – EUROS Entwicklungsgesellschaft für Windkraftanlagen mbH – Berlin – Germany Present status and outlook Natural frequencies and modeshapes evaluated for each blade Fast and easy to use tool Integration into an iterative design loop

An efficient Approach for Determining the Flutter Characteristics of Rotor Blades for Multi-Megawatt Wind Turbines Raphael Haas – EUROS Entwicklungsgesellschaft für Windkraftanlagen mbH – Berlin – Germany

An efficient Approach for Determining the Flutter Characteristics of Rotor Blades for Multi-Megawatt Wind Turbines Raphael Haas – EUROS Entwicklungsgesellschaft für Windkraftanlagen mbH – Berlin – Germany Natural frequ. vs. rotor speed, cons. system – no aerodynamic loads.Natural frequ. vs. rotor speed, 2nd order el. Beam, no returning wake. Damping vs. rotor speed, 2nd order el. Beam, no returning wake. Results – No returning wake