Aeroelastic effects of large blade deflections for wind turbines

Abstract

The objective of the present work is to present the latest results in effects of including large blade deflections in aeroelastic calculations and to quantify the errors of the linearized approach for a modern flexible mega-watt sized turbine. In this paper three nonlinear approaches have been used to quantify the effects of large deflections. One approach is a rather simple extension of the already existing aeroelastic code HAWC, which changes the calculation basis so that small deflections are assumed around an initially large deflected blade shape. The second approach is based on a multibody formulation of the beam element used in HAWC, where nonlinear effects are included as a result of modelling the blade using several interconnected bodies. The third approach is a corotational finite element formulation where each element can be regarded as linear and the nonlinear effects are included by translation and rotation of the local element coordinate system. The results from the three nonlinear approaches are compared with results obtained by the existing HAWC, and the impact on the following main effects (due to large deflections) are addressed: 1. The reduction of the effective radius. 2. The reduction of power production due to the reduced effective radius. 3. The effect of increased torsional inertia of the blade due to the deflected shape. 4. The change of blade frequencies due to changes in inertia and coupling effects.