Aeroelastic Analysis of Pre-Curved Rotor Blades V. A. Riziotis, S. G. Voutsinas and D. I. Manolas National Technical University of Athens 9 Heroon Polytechniou str., 15780, Athens, Greece vasilis@fluid.mech.ntua.gr spyros@fluid.mech.ntua.gr manolasd@fluid.mech.ntua.gr E. S. Politis and P. K. Chaviaropoulos Centre for Renewable Energy Sources 19 th km Marathonos Av., 19009, Pikermi, Greece vpolitis@cres.gr tchaviar@cres.gr Abstract The aeroelastic behaviour of pre-curved rotor blades is analysed by means of two different state-of-the-art aeroelastic tools. The first one is a second order accurate beam model based on the Euler-Bernoulli formulation by Hodges that is presently extended to account for the build-in curvature of a blade. The second is a multi-body model, in which the blade is divided in a number of interconnected sub-bodies, each one of them considered as linear Euler-Bernoulli beam. Blade curvature is analysed both in the direction of the wind (pre-bend), as well as in the rotational direction (sweep). Simulations are reported for various pre-curved blade geometries on the 5 MW Reference Wind Turbine of the UPWIND project. By comparing the results obtained from the two models, the limitations related to the various modelling assumptions are highlighted. Also, the blade loads for the various pre-curved blade geometries are compared to those of their straight counterpart. In this way advantages and disadvantages, in terms of loads and stability, of pre-curved blades in comparison to conventional straight blades are demonstrated. Keywords: aeroelasticity, pre-bend, sweep, wind turbine blades 1 Introduction Large blades in the multi MW scale are flexible, so in order to avoid the blade hitting the tower, pre-bending the blades against the wind has become common practice. In this connection LM Glasfiber developed blade pre-bending solutions for blade load reduction besides avoiding collisions with the tower. While, this can be achieved by tilting or coning the rotor, it requires adjustments to the nacelle design in order to accurately position the centre of gravity. On the contrary, pre-bending will keep the nacelle compact. At the same time, the weight of the blades can be reduced since they can be more flexible as the amount of allowable bending is greater. This makes it possible to use less material overall and fewer processed materials, which has a beneficial impact on the cost as well. Moreover, lighter blades can be longer and therefore capable of producing more energy. Recent research at Sandia Laboratories has indicated that the coupling of the blade flapwise bending with the blade torsion can be very beneficial in mitigating loads [1]. The underlying physical mechanism is that as the outer part of the blade bends; it also twists so that the angles of attack get lower and therefore the aerodynamic loads decrease. A bending/torsion coupling of this kind can be achieved structurally by properly placing and orienting the fibres of the material, but also geometrically by skewing the blade elastic axis close to the tip. Then the loads acting at the tip will generate moment with respect to the blade pitch axis and therefore the sections close to the tip will twist. Blade sweep (in-plane curvature) and forward or backward blade pre-bending (out-of-plane curvature) will activate flap/torsion coupling. Recent reports from Sandia Laboratories [2] showed an increase in annual energy production by 10-12% for a new swept back blade (STAR) installed on a Zond 750 test turbine. The flap/torsion coupling of the swept blade was used as a means for passively reducing loads. The gain in loading allowed an increase of rotor diameter which was incorporated in the existing wind turbine design. The present work supplements and enhances previous work by the authors [3], which analyzed the important structural non-linearities on large MW flexible blades originating from large deflections. In [3], on the basis of aerodynamic