Design gridlines for passive instability suppression - Task-11 report

M.H. Hansen, T. Buhl

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    In these guidelines for passive instability suppression, eight relevant topics within aeroelastic stability of turbines are considered for the parameter variations: 1. Effect of airfoil aerodynamics: The airfoil aerodynamics given by the profilecoefficients for aerodynamic lift, drag, and moment are shown to have a direct effect on aerodynamic damping of blade vibrations. A redesign of the airfoils can improve the power performance of the rotor without loss of aerodynamic damping. 2. Effect offlap/edgewise frequency coincidence: The natural frequencies of the first flapwise and first edgewise blade bending modes become closer as the blades become more slender. This 1-1 resonance may lead to a coupling flap- and edgewise blade vibrations whichincreases the edgewise blade mode damping. 3. Effect of flap/edgewise whirling coupling: The aerodynamic damping of blade vibrations close to the rotor plane are generally lower than the aerodynamic damping of vibrations out of the rotor plane. Astructural coupling between the flapwise and edgewise whirling modes can increase the overall aerodynamic damping by adding more out of plane blade motion to the edgewise whirling modes. 4. Effect of torsional blade stiffness: A low torsional bladestiffness may lead to flutter where the first torsional blade mode couples to a flapwise bending mode in a flutter instability through the aerodynamic forces. 5. Can whirl flutter happen on a wind turbine? Whirl flutter is an aeroelastic instabilitysimilar to blade flutter. Whirl flutter can occur on turbines with very low natural frequencies of the tilt and yaw modes (about 5 % of their original values). 6. Edgewise/torsion coupling for large flapwise deflections: The large flapwise deflection ofmodern slender blades lead to a geometric coupling of edgewise bending and torsion. The aeroelastic damping of the blade modes are affected by a flapwise prebend of the blade. 7. Effect of yaw error on damping from wake: The wake behind the rotor has aninfluence on the aerodynamic damping of the turbine mode due to the dynamic behavior of the induced velocities from the wake. When the turbine is operating with an yaw error, a small change in the aerodynamic damping of lower damped turbine modes isobserved that may be caused by change of wake geometry. 8. Effect of generator dynamics: The total damping of turbine modes involving drivetrain rotation, as the drivetrain torsion and lateral tower modes, are highly affected by the dynamic behavior ofthe generator torque. The aeroelastic damping of these modes changes if the generator is operated at constant speed (e.g. asynchronous generators), constant torque, or constant power (e.g. double-fed induction machines).
    Original languageEnglish
    Number of pages55
    ISBN (Print)87-550-3545-0
    Publication statusPublished - 2006
    SeriesDenmark. Forskningscenter Risoe. Risoe-R


    • Risø-R-1575
    • Risø-R-1575(EN)

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