Flutter of Variations on a 5 MW Swept Wind Turbine Blade1

Abstract

Previous research has shown that as wind turbine rotor designs increase in diameter they approach flutter instabilities. Recent blade designs incorporate blade sweep to lower fatigue loads and increase rotor diameter for more energy production; however, the flutter margin may be reduced with sweep. In this work, flutter speeds are predicted and compared for turbines with straight and swept blades. For this study, a wind turbine analysis tool specifically developed for blade sweep (CurveFAST) was used, in addition to a tool developed for helicopter analysis: rotorcraft comprehensive analysis system (RCAS). The results showed that CurveFAST generally predicted a higher flutter speed and RCAS predicted a lower flutter speed compared to previous research. The difference may be attributed to the unsteady aerodynamics modeling. The results for an extended radius and swept 5 MW turbine showed that the flutter speed was most sensitive to the position of the blade center of mass axis. Using the selected design parameters, it was not possible to design a swept 5 MW blade with adequate flutter margin that would produce 5% more energy with the same loads as a baseline straight rotor. Further study of swept blades is required to determine if there is a size limit above which flutter cannot be avoided.