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

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 2008-2008, SAND 2008-1782, J. Solar Energy Engineering, 2006. 2009 ASME Wind Energy Symposium

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, 50-250 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 20-150 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

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

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

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

Location of Microphone Array 2009 ASME Wind Energy Symposium

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-2008-1455 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

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

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

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

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

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

Flatback Acoustic spectra, aoa=4 deg. Loud tone measured with St = f*h/U = 0.24 . Splitter plate reduces the peak SPL by 12 dB and shifts the peak frequency higher, to St = 0.30 2009 ASME Wind Energy Symposium

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

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

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

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

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

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

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

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

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

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

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

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

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 12-16 dB and increases Strouhal number to 0.30. 2009 ASME Wind Energy Symposium

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