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Abstract
Abstract. The qualitative changes in damping of the first edgewise modes when an upwind wind turbine is converted into the respective downwind configuration are investigated. A model of a Suzlon S111 2.1 MW turbine is used to show that the interaction of tower torsion and the rotor modes is the main reason for the change in edgewise damping. For the forward whirl mode, a maximum decrease in edgewise damping of 39 % is observed and for the backward whirl mode, a maximum increase of 18 % in edgewise damping is observed when the upwind configuration is changed into the downwind configuration. The shaft length is shown to be influencing the interaction between tower torsion and rotor modes as out-of-plane displacements can be increased or decreased with increasing shaft length due to the phase difference between rotor and tower motion. Modifying the tower torsional stiffness is seen to give the opportunity in the downwind configuration to account for both a favorable placement of the edgewise frequency relative to the second yaw frequency and a favorable phasing in the mode shapes.
Highlights
Upwind wind turbines, where the rotor is placed in front of the tower relative to the wind, have been in the focus of research efforts in recent years
This paper focuses on the difference in edgewise damping when the Suzlon S111 2.1 MW wind turbine is changed from an upwind configuration into a downwind configuration
The section further shows the estimated edgewise damping as a function of wind speed for the fully flexible up- and downwind FF configurations as well as the up- and downwind rotor and tower torsion (RTT) configurations
Summary
Upwind wind turbines, where the rotor is placed in front of the tower relative to the wind, have been in the focus of research efforts in recent years. For downwind rotors, where the rotor is placed behind the tower this constraint is relaxed during normal operation and downwind rotors re-experience an increase in the research effort. The downwind concepts are known to show a higher fatigue load for the flapwise blade root moments compared to the upwind concepts due to the tower shadow effect. Glasgow et al (1981) measured a significant fatigue load increase in the flapwise bending loads for a downwind configuration compared to an upwind configuration of a 100 kW machine due to the velocity deficit of a truss tower. A fatigue load increase of around 20 % for the damage equivalent flapwise blade root bending moment was found by Reiso and Muskulus (2013) when comparing the 5 MW NREL reference turbine in a downwind configuration to the original upwind configuration
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