Abstract
The downwind configuration of wind turbines offers benefits regarding the blade-tower clearance, as during operation the blade primarily bends away from the tower. Consequently, the blades can be designed with lower stiffness. For tubular towers, however, a significant deficit of the wind speed in the tower wake occurs, resulting in fatigue-inducing vibrations. For this reason, full-height lattice towers are considered the preferred support structures for wind turbines with a downwind rotor. This work estimates the tower shadow excitation of a downwind rotor blade from a tubular tower. To this end, the blade of a commercial 6 MW downwind turbine is modelled with finite-elements. The tower wake is described on the basis of Madsen’s model and for the unsteady aerodynamic interaction Küssner’s function is adopted. At below- and above-rated wind conditions, the tower wake-induced vibrations are compared with the response of a blade of an equivalent upwind rotor, considering both the tip deflections and the root moments, the latter on the basis of damage-equivalent moments, to obtain an indication of the expected difference in fatigue damage. The downwind blade experiences vibrations with considerable larger amplitudes, especially in the out-of-plane direction. From the damage-equivalent moments it can be inferred that the blades of the downwind rotor encounter a much faster accumulation of fatigue damage.
Highlights
With the increasing rotor dimensions, downwind rotor configurations may become an attractive alternative, compared to the conventional upwind rotor configurations
At below- and above-rated wind conditions, the tower wakeinduced vibrations are compared with the response of a blade of an equivalent upwind rotor, considering both the tip deflections and the root moments, the latter on the basis of damageequivalent moments, to obtain an indication of the expected difference in fatigue damage
The objective of the current study is a quantification of the tower shadow excitation for a downwind rotor behind a tubular tower, using a decoupled modelling approach and Madsen’s tower wake formulation
Summary
With the increasing rotor dimensions, downwind rotor configurations may become an attractive alternative, compared to the conventional upwind rotor configurations. The wake behind a tubular tower was modelled by means of a finite-volume discretisation of the incompressible Reynolds-averaged Navier-Stokes equations, for a representative tower diameter and wind speed It should be noted, that Madsen’s model does not account for tower-induced turbulence. The objective of the current study is a quantification of the tower shadow excitation for a downwind rotor behind a tubular tower, using a decoupled modelling approach and Madsen’s tower wake formulation To this end, the blade tip motion and the damage-equivalent root moment of a commercial two-bladed 6 MW downwind turbine are estimated, providing a basis for the further estimation of the induced fatigue and sound radiation. As no description of the control system was available, both the rotational speed and the pitch angle are kept constant in the simulations
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