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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

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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

Deflection

[mm

]

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: C = 2.24x10-3, = 16 , and ReU = 1.6x10

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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

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f

o

A

o

k

Dynamic Pitch parameters

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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

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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 fo

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 fo

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 blades 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

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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