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

116 Pages·2015·19.25 MB·English

Master Thesis of Science
Design of a blade section featuring pas-
sive load alleviation capabilities resulting
from variable stiffness morphing struc-
tures
With a focus on large, modern wind turbines.
Wolf D.K. Cavens
May 19th 2015
European Wind Energy Master - EWEM- Rotor Design Track

Design of a blade section featuring passive load
alleviation capabilities resulting from variable
stiffness morphing structures
With a focus on large, modern wind turbines.
Master of Science Thesis
For obtaining the degree of Master of Science in Engineering Wind
Energy at Technical University of Denmark and in Aerospace
Engineering at Delft University of Technology.
Wolf D.K. Cavens
May 19th 2015
European Wind Energy Master - EWEM
DUWIND - Delft University of Technology
Risø - Denmark Technical University
Copyright (cid:13)c Wolf D.K. Cavens
All rights reserved.
European Wind Energy Master - EWEM
Of
Rotor Design Track
The undersigned hereby certify that they have read and recommend to the European Wind
Energy Master - EWEM for acceptance a thesis entitled “Design of a blade section featur-
ing passive load alleviation capabilities resulting from variable stiffness morphing
structures” by Wolf D.K. Cavens in partial fulfillment of the requirements for the degree
of Master of Science.
Dated: May 19th 2015
Supervisor:
Dr. Andres Arrieta of ETH Zu¨rich
Supervisor:
Dr. N´estor Ramos Garc´ıa of DTU
Supervisor:
Dr. Ir. Roeland de Breuker of TU Delft
Reader:
Prof. Dr. Gerard J.W. van Bussel of TU Delft

Summary
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. [1] and Kuder et al. [2] and is based on the DU
93-W-210 airfoil. The bi-stable element design by Kuder et al. [2] was carried over.
The aerodynamic codes XFOIL, Q3UIC and RFOIL were compared and validated with wind
tunneldataandmoreadvancedComputationalFluidDynamicsresults. TheaeroelasticAbaqus
model by Kuder et al. [2] was modified in order to accommodate for this project.
The effects of an extreme operating gust were studied and quantified. 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
flaphingemoment,thebi-stablesnapsfromthestifftotheflexiblestatewhichallowsthetrailing
edgeflaptomorphpassivelyandhencealleviateload. Theflaphingemomentbehaviourofsmall
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
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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.
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Acknowledgements
I wish to thank the following persons;
My supervisors Dr. Arrieta, Dr. Garc´ıa and Dr. Ir. de Breuker for their guidance and support.
Dr. Arriata in special for giving me the opportunity to work as a visiting student at ETH
Zu¨rich.
Dr. Garcia in special for letting me use the aerodynamic codes Q3UIC and Q3UIC AERO.
Izabela Kuder for the aeroelastic model and her advice regarding Python scripting for Abaqus.
Dr. Grasso from ECN for supplying a copy of the aerodynamic code RFOIL.
Ir. Timmer for providing valuable and unpublished airfoil data for DU airfoils.
Iva Hrogovan for the conformal airfoil mapping tool although it did not use it in the end.
Finally, I would like to thank the IDEA League for the financial support.
Delft, The Netherlands Wolf D.K. Cavens
May 19th 2015
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