1. Aerodynamic and Aeroacoustic Properties of a Flatback Airfoil: An Update
ASME Wind Energy Symposium
Orlando, FL
5 January 2009
Matthew Barone and Dale Berg
Wind Energy Technology Department
Sandia National Laboratories

2. Flatback Airfoils: Background
Flatback airfoil shapes have been proposed for the inboard region of wind turbine blades.
Thickness is added about a given camber line – different from “truncated” airfoil.
Benefits
Structural benefit of larger sectional area and larger moment of inertia for a given maximum thickness.
Aerodynamic benefits of larger sectional Clmax , larger lift curve slope, and reduced aerodynamic sensitivity to leading edge soiling.
Drawbacks
Increased drag due to separated base flow.
Introduction of an aerodynamic noise source due to trailing edge vortex shedding.
References: C.P. van Dam et al., SAND , SAND , J. Solar Energy Engineering, 2006.
2009 ASME Wind Energy Symposium

3. Flatback Noise: Is it important?
Flatbacks are used inboard where flow velocities are low
Noise intensity scales with velocity to the fifth or sixth power
Vortex-shedding tone is at low frequencies, Hz
Current noise standards emphasize A-weighted noise measurements
A-weighted noise emphasizes the middle of the human hearing range, and de-emphasizes high and low-frequency content
However…
Low-frequency noise in the range Hz is sometimes addressed by distinct community noise regulations
Tonal noise is often perceived as more annoying than broadband noise; vortex-shedding can generate tones
Low-frequency noise propagates efficiently
2009 ASME Wind Energy Symposium

4. 2009 ASME Wind Energy Symposium
Goals
Determine aerodynamic properties of a flatback airfoil at Reynolds number typical of inboard region of a utility scale wind turbine blade.
Assess the effect of a simple splitter plate trailing edge attachment on the drag and vortex-shedding noise of a flatback airfoil at this Reynolds number.
Compare aerodynamic predictions using Computational Fluid Dynamics to experimental data for both a sharp trailing edge airfoil and a flatback version of that airfoil
Measure the trailing edge vortex-shedding noise for a flatback airfoil.
2009 ASME Wind Energy Symposium

5. Flatback model with Splitter Plate
Wind Tunnel Models
30% thick DU97-W-300 airfoil
36-in chord
Steel frame, fiberglass surface
80 pressure taps per airfoil
Pressure and suction surfaces
3 Model configurations
1.7% thick trailing edge (“sharp”)
10% thick trailing edge (“flatback”)
Flatback with splitter plate
Profiles accurately measured
Flatback Model
Flatback model with Splitter Plate
2009 ASME Wind Energy Symposium

6. 2009 ASME Wind Energy Symposium
Instrumentation and Test Conditions Virginia Tech Stability Wind Tunnel
Kevlar Wall
Instrumentation
Surface pressures measured with scanivalve. Lift obtained by integrating surface pressures.
Wake pressures measured with traverse system. Drag determined from wake profiles.
Noise data obtained with 63 microphone phased array
Test Conditions
Clean surface
Tripped boundary layer
0.5 mm thick zig-zag tape
Three Reynolds numbers
Rec = 1.8, 2.4 & 3.2 X 106
Model in Wind Tunnel
Phased Array
2009 ASME Wind Energy Symposium

7. Location of Microphone Array
2009 ASME Wind Energy Symposium

8. Experimental Test Program History
Experimental program initiated Autumn 2007
Wind tunnel testing in the Virginia Tech Stability Wind Tunnel
Test setup and instrumentation described in Berg & Zayas, AIAA
Challenges encountered in the relatively new test facility
Kevlar walls admit some transpiration mass flow
Solid blockage large relative to previous tests in this facility
Flatback trailing edge vortex-shedding noise frequency below the cutoff frequency of the foam anechoic treatment.
Follow-on Testing Autumn 2008
Aerodynamics
Limited set of measurements were made with the DU97-W-300 in the solid wall test section
Mass flow vs. pressure drop relationship for the Kevlar walls was measured
Classical linear porous wall interference and blockage corrections applied
In progress: more sophisticated wall corrections based on a panel code, including Kevlar wall deflection
Aeroacoustics
Correction of low-frequency noise measurements was derived
2009 ASME Wind Energy Symposium

9. DU97-W-300 Lift and Pitching Moment
TU Delft data taken in the TU Delft low-speed, low-turbulence wind tunnel.
Timmer and van Rooij. J. Solar Energy Engineering, 125:488, 2003.
2009 ASME Wind Energy Symposium

10. 2009 ASME Wind Energy Symposium
DU97-W-300 Drag
2009 ASME Wind Energy Symposium

11. DU97-W-300 Pressure Distributions
AOA = 4 deg.
AOA = 8 deg.
2009 ASME Wind Energy Symposium

12. DU97-flatback Lift and Pitching Moment
Lift curve slope and maximum lift are increased for the flatback.
Splitter plate results in decrease in lift (not including splitter plate load).
2009 ASME Wind Energy Symposium

13. 2009 ASME Wind Energy Symposium
DU97-flatback drag
Flatback drag decreases with increasing angle of attack.
Splitter plate reduces the drag by 45-50%.
2009 ASME Wind Energy Symposium

14. Flatback Acoustic spectra, aoa=4 deg.
Loud tone measured with St = f*h/U =
Splitter plate reduces the peak SPL by 12 dB and shifts the peak frequency higher, to St = 0.30
2009 ASME Wind Energy Symposium

15. Acoustic Spectra, aoa=11 deg.
At higher aoa:
Tone amplitude decreases by 4 dB .
Effect of splitter plate similar to aoa=4 deg.
2009 ASME Wind Energy Symposium

16. Acoustic Spectra, tripped b.l. aoa=11 deg.
Tripping increased peak SPL by 4 dB and narrowed the peak.
Splitter plate more effective, reducing the peak SPL by 16 dB.
2009 ASME Wind Energy Symposium

17. Computational Fluid Dynamics (CFD) Aerodynamic Predictions
2009 ASME Wind Energy Symposium

18. 2009 ASME Wind Energy Symposium
CFD Method
SACCARA Reynolds-averaged Navier-Stokes finite volume code
Steady solutions using two established turbulence models
Spalart-Allmaras one-equation model (“SA” model)
Menter k-w two-equation model (“k-w” model)
Fine meshes (800 cells along airfoil surface, 240 cells across flatback base, y+ < 0.4)
Boundary layer transition locations calculated using XFOIL and then specified in RANS computations.
Assumption: splitter plate does not influence transition location
2009 ASME Wind Energy Symposium

19. 2009 ASME Wind Energy Symposium
DU97-W-300 CFD Predictions
SA model
k-w model
2009 ASME Wind Energy Symposium

20. Skin friction coefficient
DU97-W-300 CFD Predictions
Skin friction coefficient
k-w model
SA model
2009 ASME Wind Energy Symposium

21. DU97-W-300 Surface Pressure, aoa=4 deg.
SA model
k-w model
2009 ASME Wind Energy Symposium

22. DU97-W-300 Surface Pressure, aoa=8 deg.
SA model
k-w model
2009 ASME Wind Energy Symposium

23. DU97-W-300 Surface Pressure, aoa=12 deg.
SA model
k-w model
2009 ASME Wind Energy Symposium

24. DU97-flatback CFD predictions
SA model
k-w model
2009 ASME Wind Energy Symposium

25. 2009 ASME Wind Energy Symposium
DU97-flatback Drag
k-w model
SA model
2009 ASME Wind Energy Symposium

26. DU97-flatback with splitter plate
SA model
2009 ASME Wind Energy Symposium

27. 2009 ASME Wind Energy Symposium
Conclusions
Positive impact of flatback shape on lift curve slope and maximum lift maintained at Re = 3 million.
Flatback drag penalties are severe but a simple splitter plate attachment reduced the drag by 45-50%
The present CFD predictions of…
lift give good agreement for both airfoils except near stall.
pitching moment give decent agreement except near stall.
drag are poor, especially for flatback airfoil.
Acoustic measurements of flatback show a loud tone at Strouhal number of 0.24
Peak SPL is 4 dB lower at aoa=11 deg. than at aoa=4 deg.
Peak SPL is 4 dB higher at aoa=11 deg. for tripped b.l.
Splitter plate reduces peak SPL by dB and increases Strouhal number to 0.30.
2009 ASME Wind Energy Symposium

28. Status and Acknowledgements
Thanks to Prof. William Devenport, Prof. Ricardo Burdisso, and Aurelian Borgoltz at Virginia Tech
A comprehensive report is forthcoming in 2009 detailing:
Aerodynamic and aeroacoustic measurements at lower Reynolds number
Final corrected wind tunnel data using panel code method
Comparisons of aeroacoustic results to trailing edge noise theory
2009 ASME Wind Energy Symposium