EFFECTS OF UNSTEADY AERODYNAMICS ON VERTICAL-AXIS WIND TURBINE ...

123 Pages·2014·14.31 MB·English

EFFECTS OF UNSTEADY AERODYNAMICS ON
VERTICAL-AXIS WIND TURBINE PERFORMANCE
BY
PETER KOZAK
Submitted in partial fulfillment of the
requirements for the degree of
Master of Science in Mechanical and Aerospace Engineering
in the Graduate College of the
Illinois Institute of Technology
Approved
Advisor
Chicago, Illinois
May 2014

ACKNOWLEDGMENT
I would like to thank my advisor, Professor Dietmar Rempfer, for his guidance
and support. Many of the ideas and concepts contained in this thesis originate from
our discussions, for which I am tremendously grateful. I would also like to express
my gratitude to Professors David Williams and Kevin Meade for taking the time
and effort to serve on my thesis committee. My colleagues in the Fluid Dynamics
Research Center and IIT in general are owed a debt of gratitude for their willingness
to serve as sounding boards for my ideas.
Throughout this research, I’ve relied on a great deal of encouragement from
my family and friends. Over the past year, they have shown incredible patience and
understanding, while having to share my time and attention with this work. For that,
I am truly thankful.
iii
TABLE OF CONTENTS
Page
ACKNOWLEDGEMENT . . . . . . . . . . . . . . . . . . . . . . . . . iii
LIST OF TABLES . . . . . . . . . . . . . . . . . . . . . . . . . . . . vi
LIST OF FIGURES . . . . . . . . . . . . . . . . . . . . . . . . . . . . x
LIST OF SYMBOLS . . . . . . . . . . . . . . . . . . . . . . . . . . . xi
ABSTRACT . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . xiv
CHAPTER
1. INTRODUCTION . . . . . . . . . . . . . . . . . . . . . . . 1
1.1. Background . . . . . . . . . . . . . . . . . . . . . . . 1
1.2. Previous Research . . . . . . . . . . . . . . . . . . . . 16
1.3. Scope . . . . . . . . . . . . . . . . . . . . . . . . . . 25
2. METHODOLOGY AND VALIDATION . . . . . . . . . . . . 28
2.1. Finite Volume Simulation . . . . . . . . . . . . . . . . . 28
2.2. Estimating the Effective Angle of Attack . . . . . . . . . 41
3. OBSERVATIONS OF UNSTEADY EFFECTS . . . . . . . . . 48
3.1. Summary . . . . . . . . . . . . . . . . . . . . . . . . . 48
3.2. Performance Limiting Phenomena . . . . . . . . . . . . . 58
4. TURBINE PERFORMANCE IMPROVEMENTS . . . . . . . 67
4.1. Procedure for Turbine Optimization . . . . . . . . . . . . 67
4.2. Fixed Non-Zero Blade Pitch . . . . . . . . . . . . . . . . 69
4.3. Variable Pitch . . . . . . . . . . . . . . . . . . . . . . 73
5. CONCLUSION . . . . . . . . . . . . . . . . . . . . . . . . 85
5.1. Summary . . . . . . . . . . . . . . . . . . . . . . . . . 85
5.2. Future Research Topics . . . . . . . . . . . . . . . . . . 90
APPENDIX . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 92
A. SUGGESTED CRITERIA FOR VAWT SIMULATIONS . . . . 92
A.1. Overset Stability Criteria . . . . . . . . . . . . . . . . . 93
A.2. Accurate Wall Treatment Criteria . . . . . . . . . . . . . 93
iv
B. EFFECTIVE ANGLE OF ATTACK CORRELATION CURVES
FOR SEVERAL AIRFOILS . . . . . . . . . . . . . . . . . . 94
B.1. NACA 0012 . . . . . . . . . . . . . . . . . . . . . . . 95
B.2. NACA 0015 . . . . . . . . . . . . . . . . . . . . . . . 97
B.3. NACA 0021 . . . . . . . . . . . . . . . . . . . . . . . 99
C. EQUATIONS FOR BLADE PITCH . . . . . . . . . . . . . . 101
C.1. Iteration 1 . . . . . . . . . . . . . . . . . . . . . . . . 102
C.2. Iteration 2 . . . . . . . . . . . . . . . . . . . . . . . . 103
C.3. Iteration 3 . . . . . . . . . . . . . . . . . . . . . . . . 103
BIBLIOGRAPHY . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 103
v
LIST OF TABLES
Table Page
1.1 Advantages / Disadvantages of VAWT Analysis Methods . . . . . 18
2.1 VAWT Geometry . . . . . . . . . . . . . . . . . . . . . . . . . 28
2.2 VAWT Simulation Parameters . . . . . . . . . . . . . . . . . . 37
2.3 Oscillating Airfoil Simulation Parameters . . . . . . . . . . . . . 45
4.1 Pitch Offset Performance . . . . . . . . . . . . . . . . . . . . . 69
4.2 Harmonic Pitch Cases . . . . . . . . . . . . . . . . . . . . . . 76
4.3 Harmonic Pitch Performance . . . . . . . . . . . . . . . . . . . 77
4.4 Variable Pitch Performance . . . . . . . . . . . . . . . . . . . . 80
5.1 VAWT Performance Evaluation . . . . . . . . . . . . . . . . . . 88
B.1 NACA 0012 C Data . . . . . . . . . . . . . . . . . . . . . . . 95
P
B.2 NACA 0015 C Data . . . . . . . . . . . . . . . . . . . . . . . 97
P
B.3 NACA 0021 C Data . . . . . . . . . . . . . . . . . . . . . . . 99
P
C.1 Variable Pitch 1 (N=8) . . . . . . . . . . . . . . . . . . . . . . 102
C.2 Variable Pitch 2 (N=5) . . . . . . . . . . . . . . . . . . . . . . 103
C.3 Variable Pitch 3 (N=6) . . . . . . . . . . . . . . . . . . . . . . 103
vi
LIST OF FIGURES
Figure Page
1.1 The contributions of all electricity sources in the United States dur-
ing the 2012 year [82]. Due to recent growth, wind energy is now
responsible for 3.5% of US electricity production, more than any
other non-hydroelectric renewable source. . . . . . . . . . . . . 2
1.2 Histogram showing exponential growth of wind energy’s contribu-
tion to the US electricity production [81]. . . . . . . . . . . . . 3
1.3 Themajorwindturbinetypesincludingthepropeller-typehorizontal-
axiswindturbine(HAWT),drag-basedSavoniusdesign,andthelift-
based Darrieus and H-rotor vertical-axis wind turbines (VAWTs).
VAWT diagrams originate from (Eriksson, 2008) [23]. . . . . . . . 4
1.4 The left subfigure shows the 2-D geometry and layout of a Darrieus
turbine or H-rotor. The blades are typically mounted at the quarter
or half-chord point equidistant from the others. The sub-figure on
the right demonstrates the velocity components and aerodynamic
forces on one of the blades located at an azimuthal angle of 90o. . 5
1.5 The maximum angle of attack present at an azimuthal angle of 90o
for a typical range of tip-speed ratios for a vertical-axis machine. . 7
1.6 Example of net torque curves for a single blade (a) and the entire
turbine (b) for a 3-blade VAWT at a tip-speed ratio of TSR = 2.5 9
1.7 2-D variation in the time averaged U-velocity for at turbine operat-
ing at TSR = 3.0. The figure shows that kinetic energy is lost as the
flow passes through the turbine, with the greatest drops occurring
as the flow passes through the blade path. There is also a variation
in the speed in the vertical direction. At the top of the turbine, the
blade pushes against the flow and slowing it down. At the bottom
of the figure, the blade pulls the flow faster. . . . . . . . . . . . 10
1.8 Actuator disk representation of a wind turbine. State 0 and State 3
are the far field conditions upstream and downstream and the States
1 and 2 are the flow in the vicinity (but outside) of the turbine. . . 11
1.9 Vertax Wind Ltd. proposed multi-megawatt turbines. These sea-
basedturbineswouldrelyonfewermovingpartsthanhorizontal-axis
machines, allowing a longer lifespan and less maintenance [39]. . . 15
1.10 A VAWT blade oriented with a positive (tow out) pitch β. . . . . 22
vii
2.1 Layout of (a) the physical domain of the FVM simulation with
boundary conditions and (b) the overset grids surrounding the tur-
bine blades. . . . . . . . . . . . . . . . . . . . . . . . . . . . 29
2.2 Boundary between the overset and underset grids. . . . . . . . . 31
2.3 Fully elliptic overset grid containing a VAWT blade surface. . . . 32
2.4 Topology of the overset grid layout used to maximize orthogonality
of the element edges. . . . . . . . . . . . . . . . . . . . . . . 33
2.5 Closeup view of the overset grid near the blade’s surface, including
the boundary layer. . . . . . . . . . . . . . . . . . . . . . . . 34
2.6 View of the overset grid near the turbine blades trailing-edge. . . . 35
2.7 Plots comparing the average power coefficient with respect to tip-
speed ratio for (a) all simulations and (b) only low Y+ wall treat-
ment FVM simulations. . . . . . . . . . . . . . . . . . . . . . 39
2.8 Lift coefficient with respect to angle of attack (neglecting stall) for
a NACA 0015 airfoil obtained using JavaFoil [34]. . . . . . . . . 42
2.9 Pressure coefficient with respect to location along the chord line at
angles of attack between 0 and 12o for a NACA 0015 airfoil obtained
using JavaFoil [34]. . . . . . . . . . . . . . . . . . . . . . . . 43
2.10 Correlation curves of the effective angle of attack with respect to
the pressure coefficient ratio at 0.20 c. . . . . . . . . . . . . . 44
2.11 Physical domain of the rotating NACA 0015 simulation with bound-
ary conditions. . . . . . . . . . . . . . . . . . . . . . . . . . 45
2.12 Validation of the oscillating airfoil simulation, comparing the (a) lift
and (b) drag with Piziali’s experimental results. . . . . . . . . . 46
2.13 Effectiveangleofattackwithrespecttothegeometricangleofattack
for Piziali (corrected for Re = 300,000) using the lift and that of the
FVM simulation using the pressure ratio. . . . . . . . . . . . . 47
3.1 Average power coefficient with respect to tip-speed ratio for the
FVM simulation. . . . . . . . . . . . . . . . . . . . . . . . . 48
3.2 Blade torque coefficient with respect to azimuthal angle for various
tip-speed ratios. . . . . . . . . . . . . . . . . . . . . . . . . 49
3.3 Turbinetorquecoefficientwithrespecttoazimuthalangleforvarious
tip-speed ratios. . . . . . . . . . . . . . . . . . . . . . . . . . 50
viii
3.4 Effective angle of attack with respect to the azimuthal angle for var-
ious tip-speed ratios. Sudden drops in magnitude correspond to full
leading-edge separation and the jumps correspond to reattachment. 52
3.5 The sequence shows the vorticity magnitude distribution as the tur-
bine passes through one third of a cycle at a tip-speed ratio of TSR
= 3.0. The turbine maintains fully attached flow throughout the
entire cycle. . . . . . . . . . . . . . . . . . . . . . . . . . . . 53
3.6 The sequence shows the vorticity magnitude distribution as the tur-
bine passes through one third of a cycle at a tip-speed ratio of TSR
= 2.0. Around the 180o point, the blades undergo separation and
reattachment. . . . . . . . . . . . . . . . . . . . . . . . . . . 54
3.7 Average power coefficient curves for the BEM model and the FVM
simulation, with respect to tip-speed ratio. . . . . . . . . . . . . 56
3.8 Comparisonoftheeffectiveangleofattackforthestreamtube(BEM)
model and the FVM simulation. . . . . . . . . . . . . . . . . . 57
3.9 Close up view of the vorticity magnitude distribution around a tur-
bine blade at an azimuthal angle of 180o. (TSR = 2.0) Near the end
of the half-cycle, the blade stalls and the separation bubble is swept
away as the angle of attack changes from negative to positive. . . 59
3.10 TwoviewsoftheflowdownstreamofaVAWTbladeatanazimuthal
angle of 0o and for a tip-speed ratio of TSR = 2.0. . . . . . . . . 62
3.11 Close up view of the vorticity magnitude distribution for tip-speed
ratios of TSR = 2.0 and 3.0, demonstrating the higher likelihood of
blade / wake interaction at higher tip-speed ratios. . . . . . . . . 63
3.12 Close up view of the vorticity magnitude distribution around a tur-
bine blade as it passes through wake structures. (TSR = 3.0) . . . 65
4.1 Effective angle of attack curve for the TSR = 2.0 case. . . . . . . 67
4.2 Torque curve for the TSR = 2.0 case. . . . . . . . . . . . . . . 67
4.3 Theprocedureusedtodevelopaneffectivevariablebladepitchregime. 68
4.4 Comparisonoftheeffectiveangleofattackforseveralconstantblade
pitches. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 70
4.5 Comparison of the blade torque coefficient for several constant blade
pitches. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 71
4.6 Target angle of attack with respect to the azimuthal angle. . . . . 73
ix
4.7 Blade pitch vs azimuthal angle for the three iterations. . . . . . . 74
4.8 Harmonic variable blade pitch curve with a maximum pitch angle
of 5o. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 76
4.9 Blade torque coefficients with respect to azimuthal angle for the two
harmonic variable blade pitch cases. . . . . . . . . . . . . . . . 78
4.10 Effective angle of attack with respect to azimuthal angle for the two
harmonic variable blade pitch cases. . . . . . . . . . . . . . . . 79
4.11 Comparison of the effective angle of attack for the several variable
blade pitch iterations. . . . . . . . . . . . . . . . . . . . . . . 81
4.12 Comparison of the blade torque coefficients for the several variable
blade pitch iterations. . . . . . . . . . . . . . . . . . . . . . . 82
4.13 Comparison of the blade torque coefficients for the third iteration
variable pitch curve and the zero pitch case. . . . . . . . . . . . 83
4.14 Comparison of the effective angle of attack curve for the third iter-
ation variable pitch case and the desired angle of attack. . . . . . 84
5.1 Average power coefficients for several VAWT cases compared to a
Mod-5B horizontal-axis machine [75]. . . . . . . . . . . . . . . 89
B.1 NACA 0012 airfoil shape. . . . . . . . . . . . . . . . . . . . . 95
B.2 Pressure coefficient ratio versus angle of attack for the NACA 0012
blade. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 96
B.3 NACA 0015 airfoil shape. . . . . . . . . . . . . . . . . . . . . 97
B.4 Pressure coefficient ratio versus angle of attack for the NACA 0015
blade. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 98
B.5 NACA 0021 airfoil shape. . . . . . . . . . . . . . . . . . . . . 99
B.6 Pressure coefficient ratio versus angle of attack for the NACA 0021
blade. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 100
x