Design of a blade section featuring passive load alleviation capabilities resulting from variable stiffness morphing structures

The main objective of this MSc thesis project is to investigate the load alleviation potential of a variable stiffness bi-stable morphing blade section under extreme gust conditions by designing such a blade section. This project builds on the work of Arrieta et al. and Kuder et al. and is based on the DU 93-W-210 airfoil. The bi-stable element design by Kuder et al. was carried over. The aerodynamic codes XFOIL, Q3UIC and RFOIL were compared and validated with wind tunnel data and more advanced Computational Fluid Dynamics results. The aeroelastic Abaqus model by Kuder et al. was modified in order to accommodate for this project. The effects of an extreme operating gust were studied and quantifed. Especially at cutout conditions, the impact of a gust was found to be profound. Furthermore, dynamic stall effects during a gust were found to be significant at high angle of attack. It was demonstrated with a parametric study that trailing edge flaps can alleviate a significant amount of the load increase due to a gust. A morphing trailing edge mechanism was presented. This mechanism consists of a morphing trailing edge flap which is restrained from rotation by a bi-stable plate. Upon reaching a critical flap hinge moment, the bi-stable snaps from the stiff to the flexible state which allows the trailing edge flap to morph passively and hence alleviate load. The flap hinge moment behaviour of small flaps was found desirable for passive morphing and originates from boundary layer separation. A locally compressible profile skin was found to be required in order to achieve load alleviation. Therefore, a corrugated skin was implemented on the suction side of the profile near the trailing edge. Two morphing blade sections were presented, one with a large flap (20.59% chord) and one with a small flap (13.19% chord). The effectiveness of the bi-stable restraining mechanism was demonstrated. The small and large flap exhibit relatively similar amounts of load alleviation which indicates that the small flap is more efficient. The instantaneous lift reduction is around 7% at rated conditions and 17% at cutout conditions. This means that trailing edge flaps are less effective at high angle of attack in separated flow conditions. However, the amount of alleviated load increases after snap-through with increasing wind speed. Finally, the dynamic response of an instantaneously morphing flap was addressed. It can be concluded that passive load alleviation was achieved. One of the critical elements for this load alleviation was the implementation of a corrugated skin. It is deemed that the presented designs can be optimised further to achieve even more load alleviation. However, this study indicates that there is a potential for a morphing passive load alleviation mechanism which reduces the impact of extreme gusts and potentially allows for a reduction in blade mass which in turn can induce a reduction in wind energy cost.

Subject

Windenergy

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Student theses

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master thesis

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Thesis_Wolf_Cavens.pdf

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