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Abstract
Abstract. This article qualitatively shows the yaw stability of a free-yawing downwind turbine and the ability of the turbine to align passively with the wind direction using a model with 2 degrees of freedom. An existing model of a Suzlon S111 upwind 2.1 MW turbine is converted into a downwind configuration with a 5∘ tilt and a 3.5∘ downwind cone angle. The analysis shows that the static tilt angle causes a wind-speed-dependent yaw misalignment of up to −19∘ due to the projection of the torque onto the yaw bearing and the skewed aerodynamic forces caused by wind speed projection. With increased cone angles, the yaw stiffness can be increased for better yaw alignment and the stabilization of the free-yaw motion. The shaft length influences the yaw alignment only for high wind speeds and cannot significantly contribute to the damping of the free-yaw mode within the investigated range. Asymmetric flapwise blade flexibility is seen to significantly decrease the damping of the free-yaw mode, leading to instability at wind speeds higher than 19 m s−1. It is shown that this additional degree of freedom is needed to predict the qualitative yaw behaviour of a free-yawing downwind wind turbine.
Highlights
With the increase in wind turbine rotor size and the increase in rotor blade flexibility, downwind concepts where the rotor is placed behind the tower are re-experiencing an increase in research efforts
The analysis shows that the static tilt angle causes a wind-speed-dependent yaw misalignment of up to −19◦ due to the projection of the torque onto the yaw bearing and the skewed aerodynamic forces caused by wind speed projection
The lowest observed equilibrium yaw angle of −19.4◦ is reached at 20 m s−1
Summary
With the increase in wind turbine rotor size and the increase in rotor blade flexibility, downwind concepts where the rotor is placed behind the tower are re-experiencing an increase in research efforts. In situations where one side of a rotor under yawed inflow is loaded higher than the other, the resulting forces on the blades create a restorative yaw moment and could potentially align the rotor with the wind direction. These passive yaw systems have been investigated already in the 1980s and the early 1990s. Corrigan and Viterna (1982) studied the free-yaw performance of the two-bladed, stall-controlled MOD-0 100 kW turbine with different blade sets They observed that the turbine aligns with the wind direction at yaw errors between −45 and −55◦. On the other hand, was observed to have a negative influence on the yaw alignment
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