Investigating Stall Flutter using a DS model-An application for HAWTs

As wind turbine blades become larger there is a tendency for the blade torsional stiffness to reduce, producing the possibility of dynamic instability at moderate windspeeds. While linearised methods can assess the envelope of allowable blade properties for avoiding classical flutter with attached flow aerodynamics, wind turbine aerofoils can experience stalled flow. Therefore, it is necessary to explore the possible effects of stall-flutter on blade stability. This paper aims to address methods for judging the stability of blade designs during both attached flow and stalled flow behaviour. This paper covers the following areas: i) Attached flow model A Beddoes-Leishman indicial model is presented and the choice of coefficients is explained in the context of Theodorsen's theory for flat-plate aerofoils and experimental results by Beddoes and Leishman. Special attention is given to the differing dynamic behaviour of the pitching moment due to flapping motion, pitching motion and dynamically varying inflow. (ii) Classical flutter analysis The time domain attached flow model is verified against a linear flutter analysis by comparing time domain results for a 3D model of a representative multi-megawatt turbine blade, varying the position of the centre of mass along the chord. The results show agreement to within 6% for a range of flutter onset speeds. (iii) Dynamic stall model On entering the stalled region, damping of torsional motion of an aerofoil section can become negative. A dynamic stall model which encompasses the effects of trailing edge separation and leading edge vortex detachment is presented and validated against published experimental data. (iv) Stall flutter The resulting time domain model is used in simulations validating the prediction of reduced flutter onset for stalled aerofoils. Representative stalled conditions for a multi-megawatt wind turbine blade are investigated to assess the possible reduction in flutter speed. A maximum reduction of 17% is observed for the particular blade investigated.