1. Aeroacoustics and Aerodynamic Performance of a Rotor with Flatback Airfoils
2010 European Wind Energy Conference
Warsaw, Poland
23 April 2010
Matthew Barone, Josh Paquette
Sandia National Laboratories, Albuquerque, NM
Eric Simley
University of Colorado, Boulder, CO
Monica Christiansen
Penn State University, State College, PA

2. Outline Flatback Airfoils: Motivation and Introduction
Flatback Airfoil Wind Tunnel Tests
Field Tests of the BSDS Rotor
Modeling of Flatback Rotor Noise

3. 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?

4. 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

5. 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.

6. Flatback Airfoil Research at Sandia
Field Tests
Wind Tunnel Tests
Goal:
Predict and Quantify Noise and Drag of Flatback Airfoils
Computational Modeling

7. Flatback Airfoil Wind Tunnel Tests

8. Wind Tunnel Experiments
Facilities and Instrumentation
Virginia Tech Stability Wind Tunnel
Aeroacoustic test section
Beamforming microphone array
Airfoil surface pressure taps
Pitot tube wake surveys
Tests Performed
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

9. 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

10. Field Tests of the BSDS Rotor

11. BSDS Blade Geometry
flatback c h

12. 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

13. Acoustic Instrumentation
Tower
Microphone Array Schematic
45 Total Sensor Locations, configurable to either a low-frequency or high-frequency array.
Low-frequency Microphone Ellipse
High-frequency Microphone Ellipse

14. Acoustic Measurements
Averaged Noise Maps at Different Blade Azimuth Positions
250 Hz – Entire Rotor
1250 Hz – Single Blade

15. Modeling of Flatback Rotor Noise

16. Rotor Performance Model
WTPerf Performance Model with CFD-generated airfoil tables

17. Modeling of Flatback Noise Source
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
Observer
Trailing Edge
Boundary Layer
Turbulent Wake h d*

18. Modeling of Rotor Noise
Flatback Noise Source
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
Low-frequency noise

19. 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

20. Utility-Scale Rotor Noise Predictions
Rotor-averaged Noise spectra for a ground observer one hub height upwind
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

21. 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

22. Ongoing Work
BSDS blade with trailing edge splitter plate
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.