Sandia is a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin Company,for the United States Department of Energy’s National Nuclear Security Administration under contract DE-AC04-94AL85000.

Matthew Barone, Josh PaquetteSandia National Laboratories, Albuquerque, NM

Eric SimleyUniversity of Colorado, Boulder, CO

Monica ChristiansenPenn State University, State College, PA

Aeroacoustics and Aerodynamic Performance of a Rotor with Flatback Airfoils

2010 European Wind Energy ConferenceWarsaw, Poland

23 April 2010

Outline

Flatback Airfoils: Motivation and Introduction Flatback Airfoil Wind Tunnel Tests

Field Tests of the BSDS Rotor

Modeling of Flatback Rotor Noise

Motivation Wind turbine blade design is a multi-disciplinary optimization

problem• Cost of Energy is the ultimate objective function• Optimal aero-structural design may differ markedly from the

optimal aerodynamic and optimal structural designs Basic Design Question

• Blade aerodynamics dominate the outboard design• Structural requirements dominate the inboard design• What inboard blade shape provides an optimal structural

design without sacrificing too much aerodynamic performance?

Sandia Blade System Design Study (BSDS) Employed a multi-disclipinary, iterative design process

• Integrated blade design for aerodynamic performance, low weight, and manufacturability

Innovative blade design features• Flatback airfoils• Optimal design of a carbon fiber spar cap

9 m blades were fabricated by TPI Composites for testing

Flatback Airfoils Structural Advantages

• Structural benefit of larger sectional stiffness for given chord and thickness.

• Results in higher blade strength, lower blade weight.

Aerodynamic Advantages/Disadvantages• Sensitivity of lift to leading edge soiling is

reduced.• Drag is increased (although L/D may still

increase).• Increased aerodynamic noise due to blunt

trailing edge.

Flatback Airfoil Research at Sandia

Wind Tunnel TestsField Tests

Computational Modeling

Goal: Predict and

Quantify Noise and Drag of

Flatback Airfoils

Flatback Airfoil Wind Tunnel Tests

Wind Tunnel Experiments

• Virginia Tech Stability Wind Tunnel• Aeroacoustic test section• Beamforming microphone array• Airfoil surface pressure taps• Pitot tube wake surveys

• DU97-W-300 and DU97-flatback (10% trailing edge thickness)

• Flatback tested with/without splitter plate• Chord Reynolds numbers from 1.5 to 3.2

million• Several angles of attack

Facilities and Instrumentation

Tests Performed

Wind Tunnel Noise Measurements

U = 56 m/s

Findings from the wind tunnel tests• Flatback noise generated a

prominent tone• Tonal frequency and amplitude is

relatively insensitive to• Angle of attack• Boundary layer transition location

• Simple splitter plate attachment reduced noise by ~12 dB

Field Tests of the BSDS Rotor

c

h

BSDS Blade Geometry

flatback

34-m Pad

CTL B

RESERVOIR

ROAD

N0 100 200

Scale, ft

Prevailing Wind

2.5 Dia Lateral Spacing

Turbine

Anemometer Tower

Test Turbine and Instrumentation

• Site• 8.7 m/s average wind speed

at 80 m•Turbine

•Modified Micon 65•19 m Rotor Diameter•23 m Hub Height

• Instrumentation• Inflow

• Center and off-axis met towers, and nacelle

• Wind speed and direction• Power• Loads

• Tower, hub, and blade• Noise

• 32-microphone array centered one hub height upwind

USDA/SNL Micon Test Turbine

Acoustic Instrumentation

Microphone Array Schematic45 Total Sensor Locations, configurable to either a low-frequency or high-frequency array.

High-frequency Microphone Ellipse

Low-frequency Microphone Ellipse

Tower

Acoustic Measurements

Averaged Noise Maps at Different Blade Azimuth Positions

250 Hz – Entire Rotor 1250 Hz – Single Blade

Modeling of Flatback Rotor Noise

Rotor Performance ModelWTPerf Performance Model with CFD-

generated airfoil tables

Modeling of Flatback Noise SourceObserver• Brooks, Pope, and Marcolini (BPM) model

for blunt trailing edge noise

• Empirical model based on wind tunnel measurements

• Peak amplitude depends on the ratio of blunt trailing edge thickness to boundary layer thickness, h/d* .

• BPM only had data for h/d* < 1.

• Modified BPM model

• Scaling with flow velocity and blade dimensions unchanged

• Spectral shape function unchanged• Amplitude function modified based on

Virginia Tech wind tunnel data – more applicable to large h/d*

• Low-frequency directivity function used

Trailing Edge

Boundary Layer

Turbulent Wake

h

d*

Modeling of Rotor Noise

Blunt Trailing Edge Noise• Blade divided into span-wise sections• Local relative flow velocity obtained

from WTPerf model• Modified BPM model applied for each

section with a flatback airfoil Inflow Turbulence Noise

• Empirical model of Hubbard and Shepherd

Other airfoil self-noise sources are not currently considered

• Turbulent boundary layer trailing edge• Laminar vortex-shedding• Separation

Flatback Noise Source

Low-frequency

noise

BSDS Rotor Noise Predictions

Rotor-averaged Noise spectra for a ground observer one hub height upwind

Wind Speed = 8 m/s Wind Speed = 12 m/s

Utility-Scale Rotor Noise Predictions

WindPACT 1.5 MW Reference Turbine

• Rotor Diameter = 72 m• Hub height = 85.3 m• Wind Speed = 11 m/s• Blade Pitch = 2.6 deg.• Rotational Speed = 20 RPM

Rotor-averaged Noise spectra for a ground observer one hub height upwind

Summary Flatback airfoil technology can lead to lighter, more efficient

rotors Flatback rotor noise is being measured in a subscale field test

• Challenging due to competing hub noise Noise associated with the blunt trailing edge of flatbacks has

been studied using models informed by wind tunnel data• Existing BPM model may be over-conservative for flatback airfoil noise• Flatback airfoil noise is predicted to be lower than inflow turbulence

noise for both the subscale BSDS rotor and a reference 1.5 MW rotor with flatbacks

Ongoing Work Acoustically absorbing foam

panels will be added to the test turbine nacelle

• Attenuate hub noise• Isolate inboard blade noise

Splitter plate trailing edge attachment will be added to the BSDS blades.

• Examine effects on performance and noise.

BSDS blade with trailing edge splitter plate