Coupling between bending and twist has a significant influence on
the aeroelastic response of wind turbine blades. The coupling can arise from
the blade geometry (e.g. sweep, prebending, or deflection under load) or from
the anisotropic properties of the blade material. Bend–twist coupling can be
utilized to reduce the fatigue loads of wind turbine blades. In this study
the effects of material-based coupling on the aeroelastic modal properties
and stability limits of the DTU 10 MW Reference Wind
Turbine are investigated. The modal
properties are determined by means of eigenvalue analysis around a
steady-state equilibrium using the aero-servo-elastic tool HAWCStab2 which
has been extended by a beam element that allows for fully coupled
cross-sectional properties. Bend–twist coupling is introduced in the
cross-sectional stiffness matrix by means of coupling coefficients that
introduce twist for flapwise (flap–twist coupling) or edgewise (edge–twist
coupling) bending. Edge–twist coupling can increase or decrease the damping
of the edgewise mode relative to the reference blade, depending on the
operational condition of the turbine. Edge–twist to feather coupling for
edgewise deflection towards the leading edge reduces the inflow speed at
which the blade becomes unstable. Flap–twist to feather coupling for
flapwise deflections towards the suction side increase the frequency and
reduce damping of the flapwise mode. Flap–twist to stall reduces frequency
and increases damping. The reduction of blade root flapwise and tower bottom
fore–aft moments due to variations in mean wind speed of a flap–twist to
feather blade are confirmed by frequency response functions
