performance enhancement of wind turbine blades · 2013-10-18 · performance enhancement of wind...
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1
Performance Enhancement of
Wind Turbine Blades
Miki Amitay Professor of Aerospace Engineering, and
Director, Center for Flow Physics and Control (CeFPaC)
Rensselaer Polytechnic Institute
Troy, NY
2
Flow Control
Aerodynamic performance (circulation, separation, drag)
Internal flows (separation, head losses)
Heat transfer control (electronic/film cooling)
Mixing enhancement (combustion, noise)
Structural vibrations control
Virtual shaping of building; wind channeling
Building integrated wind
Applications
• Unsteady blowing
• Oscillating ribbon or flap
• Internal and external acoustic excitations
• Oscillating surface
active
passive
• Turbulators / surface roughness
Flow control mechanisms
fact ~ fnatural (
fshed)
• Synthetic jets (fact ~ 10.fnatural)
Flow control: Any mechanism or process through which the
flow is caused to behave differently than it normally would.
baseline w/control
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Devices
• TE Flaps
• Microtabs
• Synthetic Jets
• Active Flexible Wall
Actuators
• Piezoelectric
• Motors
• MEMS
• Fluidics
Flow Phenomena (Physics and modeling)
• Flow separation
• Fluid/Structure interactions (structural vibration)
• Sectional Lift
• Spanwise flows
• Noise sources
• Laminar/turbulent flows
Flow Phenomena Controls
• Neural Networks
• Adaptive
• Physical Model-Based
• Dynamic System-Based
• Optimal Control Theory
Sensors
• Conventional
• Optical
• MEMS
Active
Flow
Control
Triad
Active Flow Control Triad
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Motivation and Objectives
Objectives
Reduce the amplitude of blade structural vibrations using synthetic jet based
active flow control techniques.
Reduce blade vibrations by selectively reattaching the flow along the blade
span, thereby manipulating the aerodynamic load along the span.
Motivation
As wind energy production increases using large
wind turbine rotor diameters, the blades become more
susceptible to atmospheric phenomena that places higher
fatigue loads and thus structural vibrations, which directly
impact the operating life of the wind turbine.
Thus, turbine manufacturers seek to implement techniques
to reduce these loads and high amplitude vibrations.
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Extend the range of usable wind
Time
Blade tip
deflection
Reduce blades’ structural stress
Performance Enhancement using Flow Control
Synthetic Jets
Unforced
Forced
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Synthetic Jet Actuator
Piezoelectric disk
Glezer & Amitay, “Synthetic Jets”, Ann. Rev. Fluid Mech., 34, 2002
Amitay & Cannelle, “Evolution of Finite Span Synthetic Jets ”, Physics of Fluids, 18, 5, 2006
(fact ~ 10.fnatural)
• Zero-net-mass-flux (ZNMF)
• Allows momentum transfer to the flow
• Diaphragm and cavity are driven near resonance
• Small electric power input (~1Watt per actuator)
• No plumbing or any mechanical complexity is needed
• Low cost ($0.50 to $200)
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Wind Turbine Model
Synthetic jet orifices
Strain gauge
Accelerometer
Dynamic pressure
6-components load cell
S809 Airfoil Blade
Span - b = 450mm
Root chord - cr = 203mm
Taper ratio ct/cr = 0.68
Aspect ratio of 2.63
Array of synthetic jets (LE &TE):
LE at x/c = 0.25, TE at x/c = 0.9
w
jjj
AU
AUnC
2
21
2
Momentum coefficient:
C9x10-4 < < 1x10-2
Root
jets Middle
jets Tip
jets
Active Gurney Flaps
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Frequency [Hz]
PS
D
100
101
102
10-3
10-2
10-1
100
101
102
103
t [sec]
Tip
Deflectio
n[m
m]
0.2 0.4 0.6 0.8-3.5
-3
-2.5
-2
-1.5
-1
-0.5
0
Baseline
Forcing - Sine wave
(a)
(b)
Without control, the blade oscillates at its
structural mode with an amplitude of ~1mm
Tip deflection is significantly reduced when AFC
is applied
The power spectrum shows that the turbulent
kinetic energy is significantly reduced
Vibration Control: Tip Deflection & PSD
Test Conditions: Cμ = 2.24x10-3, = 16 , and ReU∞ = 1.6x105
Structural
Flow
(shedding)
C
PS
Da
tf s
tru
c
0 0.001 0.002 0.003 0.004 0.005 0.0060
1
2
3
4
5
6
7
8
9
Velocity Vector Field at y/b = 0.33
-1 -0.8 -0.6 -0.4 -0.2 0-0.4
-0.2
0
0.2
Baseline
x/clocal
z/c l
oca
l
-1 -0.8 -0.6 -0.4 -0.2 0-0.4
-0.2
0
0.2
Sinusoidal
actuation
• The baseline flow is fully separated.
• Sinusoidal actuation results in almost complete flow reattachment.
Test Conditions: ReU∞ = 1.6x105, = 16
x/clocal
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Closed-Loop Control System
Dynamic pitch waveforms
To simulate a sudden change in
wind direction or wind gust
0 5 10 15 20 25 30 35 40
0
3
6
9
12
15
18
Pitch rate 1 deg/s
2 deg/s
4 deg/s
8 deg/s
t [sec] xPC Target
Control
Computer
Signal
Conditioner
Servo
Amplifier
Signal
Amplifier
Matlab /
Simulink PC
Ethernet
DC Motor Encoder
Strain gauge
Synthetic Jets
Root Strain Signal
AOA Motor Command
Waveform Generator
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ReU∞ = 1.6x105
Tip
def
lect
ion
am
pli
tud
e [m
m]
Baseline
Forced
AOA
0 3 6 9 120
3
6
9
12
15
18
0 3 6 9 120
0.04
0.08
0.12
0.16
0.2
0 3 6 9 12 15 180
0.04
0.08
0.12
0.16
0.2
0 3 6 9 12 15 180
3
6
9
12
15
18
0 10 20 300
3
6
9
12
15
18
0 10 20 300
0.04
0.08
0.12
0.16
0.20.20
0.16
0.12
0.08
0.04
0
t [sec]
Closed-Loop Control of Structural Vibrations
1 deg/s 2 deg/s 4 deg/s
• Without flow control, the deflection amplitude is near zero for 0 < < 15, followed by a
rapid increase (due to flow separation). Then, the vibrations amplitude decreases
back (with hysteresis) to zero following the pitch down motion.
• Using closed-loop control: the increase in the amplitude was detected; the jets were
activated, resulting in a significantly lower vibrations (due to flow reattachment) for all
ramp rates.
[d
eg
]
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S809 Airfoil Finite Span Blade
Span - b = 419 mm
Chord - c = 127 mm
Aspect ratio of 3.3
Two Jet Arrays
Forward array at xj/c = 0.1
Rear array at xj/c = 0.2
Instrumentation
Laser Vibrometer Measurement
Six Component Load Cell
Labview for motion control and
Data Acquisition
Pitching/Flapping Wind Tunnel Model
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Dynamic Pitch
410*8.4
5.5
14
f
o
A
o
k
Dynamic Pitch parameters
14
Dynamic Pitch
310*8.4
5.5
14
f
o
A
o
kDynamic Pitch parameters
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14 Degrees Dynamic Pitching up
Jets off
14 Degrees Dynamic Pitching up
Jets on
14 Degrees Dynamic Pitching down
Jets off
14 Degrees Dynamic Pitching down
Jets on Vtotal [m/sec]
Tota
l V
elo
city (
m/s
ec)
Tota
l V
elo
city (
m/s
ec)
PIV Data during Dynamic Motion
16
Hysteresis Reduction
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Passive and Active Control
The synthetic jet orifice (open but not actuated)
results in reduction in hysteresis - strategic
placement of the jet orifice can be used as a
passive device.
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Comparing Partial to Full Loop
410*8.45.514 f
o
A
o kDynamic Pitch parameters
Jets Start at = 14o
Activation of the flow control for only a portion
of the dynamic pitch cycle results in the same
performance as a full cycle actuation, but
without the loss at low pitch angles, and with
less input power!
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Pulse Modulation vs. Full Loop
410*8.45.514 f
o
A
o kDynamic Pitch parameters
Jets modulated at 260 Hz (F+ of 1)
Using pulse modulation, where the jets are
activated for only a portion of the time, results
in a significant reduction of the hysteresis with
a fraction of the input power.
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Summary
Active flow control, using synthetic jet actuators, has been shown to be
a viable means to enhance turbine blades’ performance
Using synthetic jets, the blade’s structural vibrations are significantly
reduced during static conditions
The effect of the synthetic jet was also explored during dynamic motion
of the blade, where hysteresis and structural were significantly reduced
The combination of these effects could lead to reduced maintenance
cost and improved power output
Thanks to Grad students: Keith Taylor (PhD student). Victor Maldonado (MS student)
Undergrad students: Marianne Monastero, Clay Harp, Hannah Sheldon
Research Engineer: Dr. Chia Leong
In parallel to the experiments, we conduct numerical study, led by Prof. Onkar Sahni.
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Tip Vibrations, = 18o, Rec = 220,000
Baseline Actuated
22
Primary
structural
frequency
Tip Vibrations, = 18o, Rec = 220,000
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Region I: the wind speed is too low for the turbine to generate power
Region II: (sub-rated power region): between the cut-in speed and rated speed. Here the
generator operates at below rated power (power is proportional to the cube of wind speed)
Region III: power output is limited by the turbine; this occurs when the wind is sufficient for
the turbine to reach its rated output power
Region IV: period of stronger winds, where the power in the wind is so great that it could be
detrimental to the turbine, so the turbine shuts down.
Typical Power Curve of Commercial Wind Turbines
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Most large turbines (O(MWs) in rated power) use variable-speed rotors combined
with active collective blade pitch to optimize energy yield and control loads.
In Region II, turbines tend to operate at a fixed pitch using variable rotor speed to
maintain an optimal tip-speed ratio and maximize energy capture.
In Region III, the rotor operates at near constant speed and the blades are pitched to
maintain the torque within acceptable limits.
Difficulties arise in turbulent winds when excessive loading (both extreme and fatigue
loads) occurs. Using current technology, it is difficult to mitigate these loads; pitching
of the entire blade is too slow and variable rotor speed allows shedding for some of the
high loads, but not all. The need to mitigate excessive loads has led to investigations
of new methods of control.
Variable-speed rotors and collective pitch are not capable of handling oscillatory or
fatigue loads. These loads occur as a result of rotor yaw errors, wind shear, wind
upflow, shaft tilt, wind gusts, and turbulence in the wind flow.
The traditional method of pitch control uses a collective mode, in which all blades are
adjusted simultaneously. Advanced methods of pitch control (cyclic pitch and
individual pitch) are being investigated.
Energy Optimization and Load Control
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Cyclic pitch control varies the blade pitch angles to alleviate the load variations
caused by rotor tilt and yaw errors to keep the power at a desired level
Individual pitch control adjusts the pitch angle of each individual blade
independently to minimize loads without affecting the power output.
The goal is to create two load-reducing systems (collective pitch and individual
pitch) that are independent.
There are two major concerns when considering individual pitch control:
1. The entire blade still must be pitched. The flow conditions along a long blade are
not uniform and therefore pitching the entire blade may not be ideal.
2. The pitching mechanism may be unable to act fast enough to relieve the oscillating
loads due to wind gusts (gusts have rise times on the order of seconds and last for
5 to 10 seconds)
Challenges: 1. Response time requirements to counter load perturbations
2. Larger pitch motors
3. Power required to operate the system
Cyclic Pitch and Individual Pitch Control
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