Aeroelastic stability predictions for a MW‐sized blade

Abstract

Classical aeroelastic flutter instability historically has not been a driving issue in wind turbine design. In fact, rarely has this issue even been addressed in the past. Commensurately, among the wind turbines that have been built, rarely has classical flutter ever been observed. However, with the advent of larger turbines fitted with relatively softer blades, classical flutter may become a more important design consideration. In addition, innovative blade designs involving the use of aeroelastic tailoring, wherein the blade twists as it bends under the action of aerodynamic loads to shed load resulting from wind turbulence, may increase the blade's proclivity for flutter.With these considerations in mind it is prudent to revisit aeroelastic stability issues for a MW-sized blade with and without aeroelastic tailoring. Focusing on aeroelastic stability associated with the shed wake from an individual blade turning in still air, the frequency domain technique developed by Theodorsen for predicting classical flutter in fixed wing aircraft has been adapted for use with a rotor blade. Results indicate that the predicted flutter speed of a MW-sized blade is slightly greater than twice the operational speed of the rotor. When a moderate amount of aeroelastic tailoring is added to the blade, a modest decrease (12%) in the flutter speed is predicted. By comparison, for a smaller rotor with relatively stiff blades the predicted flutter speed is approximately six times the operating speed. When frequently used approximations to Theodorsen's method are implemented, drastic underpredictions result, which, while conservative, may adversely impact blade design. These underpredictions are also evident when this MW-sized blade is analysed using time domain methods.

Stability investigation of an airfoil section with active flap control

“…(12). The resulting curve ( △ line) largely underestimates the section utter limit for most of the frequency ratios, as also observed on a rigid airfoil by Hansen [18] and on a full blade by Lobitz [17]. It is therefore concluded that the wake vorticity has a stabilizing e ect that can not be neglected in a stability analysis.…”

“…In Theodorsen and Garrick's formulation, the e ects on the aerodynamic forces from the vorticity shed into the wake are modeled by complex Bessel functions that depend on the utter frequency itself, therefore a recursive solution is required to determine the stability limit. A similar formulation is adopted by Lobitz [17], who modi es the model to determine the utter limits of an isolated wind turbine blade rotating in still air. Lobitz concludes that, for a 1.5 MW wind turbine blade, utter would occur if the rotor over-speeds up to double the nominal rotational speed.…”

Indicial lift response function: an empirical relation for finite‐thickness airfoils, and effects on aeroelastic simulations

“…As discussed for fatigue and ultimate loads, neglecting the circulatory lift dynamics in attached flow causes the largest variations in the simulated response. The stability limit encountered with the quasi-steady assumption is in fact much lower than in the other two cases; Lobitz [29] reported a similar result for the flutter limit of an isolated blade. The finite-thickness indicial lift response function results in slightly higher stability limits, but the difference from the flat plate case is rather small; variations of similar magnitude were reported in the flutter analysis of a 2D profile [30].…”

Modal properties and stability of bend–twist coupled wind turbine blades

“…Flap-twist to feather coupling reduces damping while damping increases for flaptwist to stall coupling. Lobitz (2004) investigates the flutter speed of an uncoupled and a bend-twist to feather coupled megawatt-sized wind turbine blade with quasi-steady and unsteady aerodynamic models, applying the Theodorsen's approach. Lobitz shows that quasi-steady flutter speeds are significantly lower than the flutter speeds obtained with unsteady aerodynamics.…”