1. European Wind Energy Conference and Exhibition 2010 Warsaw, Poland EWEC 2010 Warsaw 20-23 April 2010 Aeroelastic Analysis of Pre-Curved Rotor Blades V.A.Riziotis,S.G.Voutsinas and D.I. Manolas National Technical University of Athens School of Mechanical Engineering Fluids Section E.S. Politis and P. K. Chaviaropoulos Centre for Renewable Energy Sources Wind Energy Section

2. EWEC 2010 Warsaw 20-23 April 2010 2/19 Outline Rationale of pre-curving rotor blades Objectives of the work Alternative structural modeling of pre-curved blades –2 nd order beam theory –Multi-body formulation Comparative results for the 5MW reference wind turbine Conclusions Outlook

3. EWEC 2010 Warsaw 20-23 April 2010 3/19 Rationale of pre-curving Fore pre-bending has been originally introduced in order to increase the blade-tower clearance However analysis has shown that pre-curving the rotor blades activates flap-torsion coupling which can be very beneficial in mitigating loads Flap-torsion coupling can be also achieved by structural tailoring as indicated in recent research at Sandia Laboratories The underlying 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 Load reduction is always important due to its direct impact on the cost of energy (e.g. lowering the loads allows the increase of rotor diameter for the same given strength)

4. EWEC 2010 Warsaw 20-23 April 2010 4/19 Formulation of suitable formulations based on existing structural models, namely: –Hodges’ 2 nd order beam theory and –multi-body modelling Identification of the additional coupling terms in the structural loads Comparative analysis of aeroelastic simulations Assessment of the various pre-curved blade concepts Objectives of the work

5. EWEC 2010 Warsaw 20-23 April 2010 5/19 Alternative structural formulations Pre-curvature will generate a distributed change of the orientation of the beam sections which should be taken into account in the kinematics Stresses will be re-orientated resulting in a modification of the blade loading including additional couplings which allow load mitigation The structural formulation becomes non-linear even though the material properties remain linear

6. EWEC 2010 Warsaw 20-23 April 2010 6/19 2 nd order beam model Z e : the pre-bending

7. EWEC 2010 Warsaw 20-23 April 2010 7/19 Multi-body modelling has been introduced in order to take into account non-linear dynamics The same approach can be also used in order to approximate large displacements along the blade To this end the blade (major body) is divided into “sub- bodies” each considered as an independent component Non-linear matching of loading and kinematics is assumed at the connection points Multi-body modelling

8. EWEC 2010 Warsaw 20-23 April 2010 8/19 Coupling terms Bending-torsion coupling on pre-bent blades In case of large bending deflections additional non-linear terms will become significant:

9. EWEC 2010 Warsaw 20-23 April 2010 9/19 first edgewise second edgewise fore pre-bent blade (4.5 m tip deflection) Results

10. EWEC 2010 Warsaw 20-23 April 2010 10/19 Results first flapwise second flapwise aft swept blade (4.5 m tip deflection)

11. EWEC 2010 Warsaw 20-23 April 2010 11/19 Results flapwise deflection torsion angle pre-bent – Uwind=8m/s

12. EWEC 2010 Warsaw 20-23 April 2010 12/19 Results flapwise deflection torsion angle pre swept blade – Uwind=8m/s

13. EWEC 2010 Warsaw 20-23 April 2010 13/19 Results flapwise deflection torsion angle pre-bent 3m – Uwind=8m/s

14. EWEC 2010 Warsaw 20-23 April 2010 14/19 Results Equivalent loads

15. EWEC 2010 Warsaw 20-23 April 2010 15/19 Results

16. EWEC 2010 Warsaw 20-23 April 2010 16/19 The aeroelastic behaviour of pre-curved blades is analysed using two models: a 2 nd order beam model and a multi-body model The 2 nd order model is formulated with respect to the pre-curved blade geometry while the second is based on multi-body modelling Both models take into account the orientation changes along the blade. In the 2 nd order model however the additional terms will appear explicitly while in the multi-body model the modifications are implicitly seen only in the results Comparison of the two models indicates the consistency of the ordering scheme followed in the 2nd order model Conclusions

17. EWEC 2010 Warsaw 20-23 April 2010 17/19 Fore pre-bend besides increasing the blade-tower clearance, reduces torsion loads and increases blade stability Aft pre-bend is meaningless because it reduces the clearance of upwind rotors but also reduces the damping of the low damped edgewise modes. Pre-sweep of blades drives flapwise bending/torsion coupling. Aft pre-sweep gives rise to nose down torsion deformation and leads to reduced fatigue loads. The opposite effect is obtained for forward sweep. Conclusions

18. EWEC 2010 Warsaw 20-23 April 2010 18/19 This work was partly funded by the European Commission and by the Greek Secretariat for Research and Technology under contract SES6 019945 (UpWind Integrated Project). Acknowledgements

19. EWEC 2010 Warsaw 20-23 April 2010 19/19 Thanks for your attention END