Edgewise vibrations in stall

Results: Basically, the results confirm that the edgewise vibrations are caused by negative aerodynamic damping, i.e. energy supplied to the vibrating blade by the aerodynamic forces. The results show that the occurrence of edgewise vibrations are controlled primarily by properties related to 4 principal areas, which influence partly the aerodynamic damping, partly the structural damping and partly the properties of the complete structure with respect to either damping or amplification of an existing edgewise vibration: 1. The aerodynamic characteristics of the blade, represented by the static and the dynamic lift and drag coefficients. 2. The structural characeristics of the blade, which influence the mode shape and the natural frequency. Especially the orientation of the principal bending axes are of great importance, because the aerodynamic damping depends heavily on the resulting direction of the vibration. 3. The material and structual properties, which influence the structural damping of the blade. 4. The properties of the supporting structur, i.e. the nacelle and the tower, which partly modify the mode shape of the blade and partly contribute to the resulting damping of the structure. The influence of a wide range of characteristics within these areas has been investigated, both by use of quasi steady submodels and by integrated aeroelastic codes. The investigations confirm that the above mentioned areas are of substantial importance. The experience from the calcuations and the comparison with full scale measurements show that the absolute accuracy of the theoretical results may be limited - especially due to the aerodynamic models - and the prediction of the very small amounts of damping in questin are insecure. However, the relative changes of the damping associated with changing of characteristics related to the main areas above can be predicted with good accuracy. In summary it can be concluded that a design change, which in a qualified aeroelastic calculation results in a reduced vibration, with great probability will result in a reduction of the same order of magnitude on a real turbine. Based on these results it is therefore recommended that the designer makes widely use of thorough aeroelastic calculations, which investigate the influence of the properties listed above, and that the final design choices are based on these calculations. In general the structural damping should be chosen as high as possible. By overall choice of the most favourable properties, it is possible to obtain a good safety margin against edgewise vibrations. Specifically, it has been shown that airfoil and structural design properties can be obtained, which eliminate the appearance of edgewise vibrations