Modelling of Serrated Trailing Edges to Reduce Aerodynamic Noise in Wind Turbines Using ...

109 Pages·2010·14.33 MB·English

IT 10 053
Examensarbete 30 hp
Oktober 2010

Modelling of Serrated Trailing
Edges to Reduce Aerodynamic Noise
in Wind Turbines Using Computational
Fluid Dynamics
Carlos A. Arce-León
Institutionen för informationsteknologi
Department of Information Technology

Abstract
Modelling of Serrated Trailing Edges to Reduce
Aerodynamic Noise in Wind Turbines
Carlos A. Arce-León
Teknisk- naturvetenskaplig fakultet
UTH-enheten An analysis is pursued on how serrations fitted on a section of a wind turbine blade
affect noise generation and properties, seeking to ultimately verify its noise mitigation
Besöksadress: effects. The research is conducted using computational fluid dynamics with aid from a
Ångströmlaboratoriet
basic acoustic model following the theory developed by Proudman and Lilley, coupled
Lägerhyddsvägen 1
Hus 4, Plan 0 with an analysis of sound properties by the characterization of turbulence length
scales present in the flow. An outline of the numerical methods, aeroacoustic
Postadress: principles and the theory behind trailing edge noise generation is presented in order
Box 536
to achieve a more complete understanding of why noise is generated, with what
751 21 Uppsala
means it can be studied and how it can ultimately be modified. It is found that, for the
Telefon: case at hand, turbulent structures become complex and anisotropic due to the
018 – 471 30 03 presence of the serrations, thus cutting down on the effectiveness of the chosen
acoustic model. The turbulence length scale analysis is used to compliment this
Telefax:
method and, by coupling the results from both methods, a conclusion is reached on
018 – 471 30 00
that noise can be mitigated by using serrated trailing edges since they extend the
Hemsida: presence of a more varied range of turbulence length scales, thus reducing the effect
http://www.teknat.uu.se/student of constructive interference from pressure fluctuations generated by eddies shed
from the trailing edge of the blade.
Handledare: Peter Fuglsang
Ämnesgranskare: Per Lötstedt
Examinator: Anders Jansson
IT 10 053
Sponsor: LM Wind Power
Tryckt av: Reprocentralen ITC
iv
To all who in my past I hold dear, for every word and
gesture has helped me sculpt who I am, what I’ve
done and where I want to go.
To all, but to none a greater debt of gratitude than to
my parents, my brother and grandparents I owe.
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ACKNOWLEDGEMENTS
First and foremost, thanks to my parents; it’s only because of their support, education and
touring of the world that I’ve been able to make it this far.
Thanks to everyone that has ever given me the opportunity to learn and grow from their
insight, ideas and beliefs, be that in a classroom or over coffee some afternoon.
A special thanks to the supervisors of this research, Per L¨otstedt and Peter Fuglsang, for
the much needed insight, information, suggestions and especially for the time they’ve invested
in it. Thanks also to Jesper Madsen for taking the time to provide the crucial simulation data
used in this study.
To my friends, here and far away, for motivating a gasp of fresh air and a healthy escape
from the routine. Also for bringing back the pleasure of having music in my life again.
Thanks to professor Gunilla Kreiss, with whom it was an honour to work in a project which,
not only has greatly influenced the writing of this thesis, but also allowed me to sharply expand
my horizons into this world of computational science. To professors Stefan P˚alson and Michael
Thun´e for their very valuable guidance in this M.Sc. program.
A huge recognition of appreciation to prof. Herman Snel, the inspiration and catalyst that
introduced me to the art and science of wind energy and without whom this thesis and my
cooperation with LM Wind Power would never have been conceived.
I’m in great debt to the open source community, without which I truly believe that achieving
this research would never have been so rewarding and fun. Especial thanks to the developers of
LATEX1,Vim2 anditsLATEX-Suite,Octave3,GIMP4,Inkscape5 andBlender6.TotheGNU/Linux
community, and KitWare for their magnificent VTK7 and Paraview8 products. But most of all,
thanks to the people that have introduced me to all these amazing possibilities and motivated
me to take a jump in the deep end. Especially thanks to Dr.rer.nat. Marcelo Magall´on, who
introduced me to the world of computational science and whom I’ll always consider to be my
mentor back home, and Dr.rer.nat. Francisco Frutos, to whom, amongst many other things, I
owe the pleasure of first hearing about LATEX.
Lastly, I extend my deepest appreciation to both the University of Costa Rica and Uppsala
University for sheltering me in the academic environment I’ve always looked for, and by doing
so, helping me make so many dreams come true.
1www.latex-project.org
2www.vim.org
3www.gnu.org/software/octave/
4www.gimp.org
5www.inkscape.org
6www.blender.org
7www.vtk.org
8www.paraview.org
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CONTENTS
1 Introduction 1
1.1 Social Impact of Wind Turbine Noise . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.2 Sources of Wind Turbine Noise . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
2 Numerical Methods in CFD 11
2.1 The Navier-Stokes Equations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
2.2 Turbulence . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
2.3 Direct Numerical Simulation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
2.4 Large Eddy Simulation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
2.5 Reynolds Averaged Navier-Stokes Models . . . . . . . . . . . . . . . . . . . . . . 18
3 Aeroacoustics 23
3.1 The Lighthill Analogy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25
3.2 The Ffowcs-Williams and Hawkings Analogy . . . . . . . . . . . . . . . . . . . . 27
3.3 The Proudman/Lilley Approach . . . . . . . . . . . . . . . . . . . . . . . . . . . 30
3.4 Computational Aeroacoustics and its Issues Regarding Computational Expense . 31
4 Theory of Noise Generation by Trailing Edges 35
4.1 The Theory of Noise Generated by a Straight Trailing Edge . . . . . . . . . . . . 35
4.2 Computational Aeroacoustic Research on Straight Trailing Edge Sound Generation 40
4.3 Computational Aeroacoustic Research on Straight Trailing Edge Wind Turbine
Blades . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43
4.4 The Effect of Serrated Trailing Edges on Sound Generation . . . . . . . . . . . . 46
4.5 Trailing Edges Noise Generation and Experimental Research . . . . . . . . . . . 50
5 Case Set Up 55
5.1 Blade Geometry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55
5.2 2D Set Up . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 57
5.3 3D Set Up . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 59
6 Simulations and Results 65
6.1 The 2D Case . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 65
6.2 The 3D Case . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 72
7 Conclusions 83
7.1 Further Work . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 84
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A Definitions 87
A.1 Reynolds Number, Re . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 87
A.2 The Dimensionless Wall Distance, y+ . . . . . . . . . . . . . . . . . . . . . . . . . 87
A.3 The Dirac Delta Function, δ(x) . . . . . . . . . . . . . . . . . . . . . . . . . . . . 88
A.4 Green’s Function . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 88
A.5 Wedge Product, ∧ . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 89
References 90
Acronyms 95
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