A Progress Report on the Characterization and Modeling of a Very Flexible Wind Turbine Design

The combination of increasing turbine rotor diameters and the desire to achieve long lifetimes has placed increased emphasis on understanding the response of flexible turbine structures in a turbulent inflow environment. One approach to increase fatigue lifetimes has been to design structures that can either shed or adequately absorb turbulent loads through the use of flexible rotors and support towers, and hubs and nacelles that exhibit multiple degrees of angular freedom. The inevitable result in such designs is a substantial increase in dynamic complexity. In order to develop a sufficient knowledge of such concepts, extensive measurements coupled with detailed analytical simulations of a flexible turbine design are required. The Wind Eagle 300 turbine, with its lightweight flexible rotor and hub, meets these criteria and is currently being investigated. In this paper we discuss a few early results from our recently completed field measurement effort. We found that the turbine rotor response was dominated by a once-per-revolution oscillation that was responsible for large cyclic variations in the output power. The available evidence points to a rotor imbalance related to structural differences in one of the blades and misalignment of the pitch angles. We also compared the variation in mean out-of-plane bending loads with wind speed with a conventional rigid hub design