Determination of damping of edgewise vibrations

Over the last 10 years negative aerodynamic damping of the natural edgewise vibrations of the blades have in several cases been observed to cause unstable vibrations in stall. The basic physical phenomena and main parameters influencing these damping conditions have been well described in the Danish EFP-96 project 'Edgewise vibrations in Stall'. In the present project a theoretical investigations of possibilities to excite the relevant natural vibrations will be carried out. Examples to be covered: 1) impulse on single blade by hitting with a mass, 2) simultaneous impulse on two blades by cutting a pre-stressed wire connecting them, 3) electromechanical exciter on more or several blades, 4) electromechanical exciter in the nacelle. The aim is description of a least one experimental method to determine the damping. An exciter will be manufactured and installed on a Bonus turbine with relevant measurement equipment. A number of measurement campaigns will be carried out in order to verify the aeroelastic predictions. This will concern the structural models (mode shapes, structural pitch and structural damping) as well as the aerodynamic models (profile coefficients and dynamic stall model). Effects will be tested of changed blade setting angel, rotational speed and aerodynamic (by means of stall strips on the leading edge). The results of the verifications will be described in guide-lines for choice of parameters for the aeroelastic codes