NAWEA 2015 SYMPOSIUM

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Deborah Baldwin
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Baeder Alfred Gessow Rotorcraft Center Department of Aerospace

1 Aerodynamics and Aeroacoustics of Spanwise Wavy Trailing Edge Flatback Airfoils: Design Improvement Seung Joon Yang James D. Baeder Alfred Gessow Rotorcraft Center Department of Aerospace Engineering, University of Maryland NAWEA 2015 SYMPOSIUM Spanwise Wavy Trailing Edge Airfoil 1/ 31 NAWEA 2015

2 Outline Introduction Flatback airfoil drag and noise emission Numerical methods Wavy trailing edge design Wavy trailing edge modification Results and discussions Aerodynamic Characteristics Aeroacoustic Characteristics Conclusion Spanwise Wavy Trailing Edge Airfoil 2/ 31 NAWEA 2015

3 Introduction and Motivation Wind power generation is proportional to square of rotor blade length! * IPCC Special Report on Renewable Energy Sources and Climate Change Mitigation 2011 Higher power generation requires larger blades Demands a structurally robust blade Thicker Airfoil at inboard sections (~40% of blade span!) Spanwise Wavy Trailing Edge Airfoil 3/ 31 NAWEA 2015

4 Introduction: Flatback Airfoil Aerodynamics Sharp TE Flatback TE Advantages Flatback TE airfoil has superior lift performance; delayed stall on upper surface Structurally robust blade design compared to sharp TE airfoil * Baker, Experimental Analysis of Thick Blunt Trailing Edge wind turbine Airfoils, 2006 Spanwise Wavy Trailing Edge Airfoil 4/ 31 NAWEA 2015

5 Introduction: Flatback Airfoil Aerodynamics Sharp TE Flatback TE Flatback TE Sharp TE Increase in drag Disadvantages Flatback TE airfoil suffers from higher drag, lower max L/D Higher acoustical signature compared to sharp TE * Baker, Experimental Analysis of Thick Blunt Trailing Edge wind turbine Airfoils, 2006 Spanwise Wavy Trailing Edge Airfoil 5/ 31 NAWEA 2015

Introduction: Flatback Airfoil Noise Emission Flatback TE AoA 4, Re = 3,000,000 Strong vortex shedding Sharp TE Noise spectrum Vortex shedding pattern Disadvantages Flatback TE airfoil high tonal

6 Introduction: Flatback Airfoil Noise Emission Flatback TE AoA 4, Re = 3,000,000 Strong vortex shedding Sharp TE Noise spectrum Vortex shedding pattern Disadvantages Flatback TE airfoil high tonal noise Generated by pressure fluctuations at TE because of strong nearly 2-D spanwise coherent vortex shedding * Dale E. Berg and M. Barone, Aerodynamic and Aeroacoustic Properties of a Flatback Airfoil, WINDPOWER 2008, Houston, 2008 Spanwise Wavy Trailing Edge Airfoil 6/ 31 NAWEA 2015

Introduction: Spanwise Wavy Trailing Edge Modification Baseline Proposed solution Introduce streamwise vorticity to disintegrate/breakdown spanwise coherent vortex structure Can the trailing edge

7 Introduction: Spanwise Wavy Trailing Edge Modification Baseline Proposed solution Introduce streamwise vorticity to disintegrate/breakdown spanwise coherent vortex structure Can the trailing edge geometry be modified to reduce drag and noise while maintaining aerodynamic efficiency? Other solutions Splitter plate, serrated TE add on devices * Seung Joon Yang and James D. Baeder, Aerodynamic Drag and Aeroacoustic Noise Mitigation of Flatback Airfoil with Spanwise Wavy Trailng Edge, 33 rd Wind Energy Symposium at Scitech 2015, Kisimmee, FL, 2015 Spanwise Wavy Trailing Edge Airfoil 7/ 31 NAWEA 2015

Numerical methods RANS LES hybrid method (OVERTURNS, GPURANS3D) - Laminar Turbulent transition modeling ; γ Re θθ transition model with S-A turbulence model - Delayed Detached Eddy Simulation (DDES)

8 Numerical methods RANS LES hybrid method (OVERTURNS, GPURANS3D) - Laminar Turbulent transition modeling ; γ Re θθ transition model with S-A turbulence model - Delayed Detached Eddy Simulation (DDES) - Spatial reconstruction ; 5 th order WENO - Time marching; Diagonal Alternating Direction Implicit (DADI) - GPU accelerated computation ; Deepthought II cluster at UMD (40 gpu nodes) ; Nvidia Tesla K20m GPUs Nvidia Tesla K20m Processor core 2496 Processor core clock Memory Memory clock Band width 706 MHz 5 GB 2.6 GHz 208 GB/sec * Deepthought II Cluster at UMD, College Park Spanwise Wavy Trailing Edge Airfoil 8/ 31 NAWEA 2015

Mesh Generation 271 x 141 x 61 ~ 2.33 milion grid points for each airfoil geometries 50C distance away in the normal to surface direction resolution; Δy/c ~ 5.0µ (y+ ~ 0.

9 Mesh Generation 271 x 141 x 61 ~ 2.33 milion grid points for each airfoil geometries 50C distance away in the normal to surface direction resolution; Δy/c ~ 5.0µ (y+ ~ 0.8) Validation! 0.5C in span direction 104 grid points at the trailing edge 102 grid points upper/bottom surface near the trailing edge * Seung Joon Yang and James D. Baeder, Aerodynamic Drag and Aeroacoustic Noise Mitigation of Flatback Airfoil with Spanwise Wavy Trailng Edge, 33 rd Wind Energy Symposium at Scitech 2015, Kisimmee, FL, 2015 Spanwise Wavy Trailing Edge Airfoil 9/ 31 NAWEA 2015

Baeder and Seung Joon Yang, Wavy Trailing-Edge Flatback Aerodynamics Using a GPU-Accelerated Navier-Stokes

10 Wavy trailing edge design: Previous Designs Baseline 1/2flatback-4cyc/C 3/4flatback-4cyc/C Wave formula y = y mmm y mmm local thickness = y 2 cos ω 2ππ l y mmm 4 cyc/c better than 2 cyc/c or 8 cyc/c * James D. Baeder and Seung Joon Yang, Wavy Trailing-Edge Flatback Aerodynamics Using a GPU-Accelerated Navier-Stokes Solver, EWEA Offshoer, Copenhagen, Denmark, 2015, March Spanwise Wavy Trailing Edge Airfoil 10/ 31 NAWEA 2015

Baseline Results Base-line cases A. FB3500-1750 (TE thickness 17.50% of C, flatback TE) B. FB3500-0462 (TE thickness 4.

11 Baseline Results Base-line cases A. FB (TE thickness 17.50% of C, flatback TE) B. FB (TE thickness 4.62% of C, sharp TE) FB FB Strong nearly 2-D spanwise vortex structure with flatback airfoil Weak spanwise vortex structure with sharp trailing edge airfoil * Seung Joon Yang and James D. Baeder, Aerodynamic Drag and Aeroacoustic Noise Mitigation of Flatback Airfoil with Spanwise Wavy Trailng Edge, 33 rd Wind Energy Symposium at Scitech 2015, Kisimmee, FL, 2015 Spanwise Wavy Trailing Edge Airfoil 11/ 31 NAWEA 2015

Previous Wavy Trailing Edge: Flowfield 3/4 flatback 4cyc/C 1/2 flatback 4cyc/C With shallow wavy pattern, still span-wise vortex structure With deeper wavy pattern, more stream-wise vortex structure

12 Previous Wavy Trailing Edge: Flowfield 3/4 flatback 4cyc/C 1/2 flatback 4cyc/C With shallow wavy pattern, still span-wise vortex structure With deeper wavy pattern, more stream-wise vortex structure However, relatively unstable flow at the wave troughs * James D. Baeder and Seung Joon Yang, Wavy Trailing-Edge Flatback Aerodynamics Using a GPU-Accelerated Navier-Stokes Solver, EWEA Offshoer, Copenhagen, Denmark, 2015 Spanwise Wavy Trailing Edge Airfoil 12/ 31 NAWEA 2015

Previous Wavy Trailing Edge: Aerodynamic Performance With shallow wavy pattern, higher lift and reduced drag With deeper wavy pattern, too much loss of lift * James D.

13 Previous Wavy Trailing Edge: Aerodynamic Performance With shallow wavy pattern, higher lift and reduced drag With deeper wavy pattern, too much loss of lift * James D. Baeder and Seung Joon Yang, Wavy Trailing-Edge Flatback Aerodynamics Using a GPU-Accelerated Navier-Stokes Solver, EWEA Offshoer, Copenhagen, Denmark, 2015 Spanwise Wavy Trailing Edge Airfoil 13/ 31 NAWEA 2015

14 Previous Wavy Trailing Edge: Potential Problems Previous wavy TE Loss of blade volume at troughs weaken blade structural strength? Wavy modification make any difficulty during manufacturing stage? * Seung Joon Yang and James D. Baeder, Aerodynamic Drag and Aeroacoustic Noise Mitigation of Flatback Airfoil with Spanwise Wavy Trailng Edge, 33 rd Wind Energy Symposium at Scitech 2015, Kisimmee, FL, 2015 Spanwise Wavy Trailing Edge Airfoil 1/ 31 NAWEA 2015

15 Design Improvements: Design #1 Can we remove wavy structure on upper surface? Lower half way cut (only Bottom surface wavy TE ) Previous wavy TE Camber recovery helps aerodynamic performance Lower half way cut wavy TE Larger blade volume with design #1 Better manufacturability Spanwise Wavy Trailing Edge Airfoil 2/ 31 NAWEA 2015

16 Design Improvements: Design #2 Can we start wavy structure closer to TE? 90%C cut (wavy TE at 90% of Chord) Previous wavy TE Can we get rid of 2-D spanwise vortex structure With the New Designs?? Wavy TE at 90% of chord Larger blade volume with design #2 Better manufacturability Spanwise Wavy Trailing Edge Airfoil 3/ 31 NAWEA 2015

Improved Wavy Trailing Edge Designs A. Lower half cut 3/4 flatback B. Lower half cut 1/2 flatback C. 90C cut 3/4 flatback D. 90C cut 1/2 flatback Structurally enhanced designs A.

17 Improved Wavy Trailing Edge Designs A. Lower half cut 3/4 flatback B. Lower half cut 1/2 flatback C. 90C cut 3/4 flatback D. 90C cut 1/2 flatback Structurally enhanced designs A. Lower half cut 3/4 flatback (min. TE thickness 15.38% of C) B. Lower half cut 1/2 flatback (min. TE thickness 13.12% of C) C. 90C cut 3/4 flatback (min. TE thickness 13.12% of C) D. 90C cut 1/2 flatback (min. TE thickness 8.75% of C) Spanwise Wavy Trailing Edge Airfoil 4/ 31 NAWEA 2015

18 Results and Discussion: Flowfield (Iso-vorticity mag.) FB (flatback TE) Lower half cut 3/4 flatback Lower half cut 1/2 flatback FB (sharp TE) 90C cut 3/4 flatback 90C cut 1/2 flatback Lower half cut 3/4 flatback, still has spanwise coherent vortex structure Lower half cut 1/2 flatback, has more streamwise vorticity 90C cut designs work better to break up spanwise vortex Spanwise Wavy Trailing Edge Airfoil 5/ 31 NAWEA 2015

shedding patterns along span. With shallow wave (3/4 flatback), strong 2-D coherent vortex structure.

19 Results and Discussion: Flowfield (Vorticity contours) Aerodynamic characteristics; Trailing edge vortex shedding pattern Lower half cut 3/4 flatback Crest Lower half cut 1/2 flatback Crest Trough Trough Both Lower half cut airfoils have similar vortex shedding patterns along span. With shallow wave (3/4 flatback), strong 2-D coherent vortex structure. With deep wave (1/2 flatback), vortex strength now weaken and vortex core is formed at further downstream. Spanwise Wavy Trailing Edge Airfoil 6/ 31 NAWEA 2015

20 Results and Discussion: Flowfield (Vorticity contours) Aerodynamic characteristics; Trailing edge vortex shedding pattern Lower half cut 3/4 flatback Lower half cut 1/2 flatback Trough Crest Trough Crest Both Lower half cut airfoils have similar vortex shedding patterns along span. With shallow wave (3/4 flatback), strong 2-D coherent vortex structure. With deep wave (1/2 flatback), vortex strength now weaken and vortex core is formed at further downstream. Spanwise Wavy Trailing Edge Airfoil 7/ 31 NAWEA 2015

cut designs, now entirely different vortex shedding patterns at the crest and trough.

21 Results and Discussion: Flowfield (Vorticity contours) Aerodynamic characteristics; Trailing edge vortex shedding pattern 90C cut 3/4 flatback Crest 90C cut 1/2 flatback Crest Trough Trough With 90%C cut designs, now entirely different vortex shedding patterns at the crest and trough. Now vortex structure is more like 3-D, affected by streamwise vorticity. Spanwise Wavy Trailing Edge Airfoil 8/ 31 NAWEA 2015

22 Results and Discussion: Flowfield (Vorticity contours) Aerodynamic characteristics; Trailing edge vortex shedding pattern 90C cut 3/4 flatback 90C cut 1/2 flatback Trough Crest Trough Crest With 90%C cut designs, now entirely different vortex shedding patterns at the crest and trough. Now vortex structure is more like 3-D, affected by streamwise vorticity. Spanwise Wavy Trailing Edge Airfoil 9/ 31 NAWEA 2015

23 Results and Discussion: Lift vs. AoA Lower half cut 3/4 flatback Lower half cut 1/2 flatback 90C cut 3/4 flatback 90C cut 1/2 flatback Lower half cut 3/4 flatback, only small amount of lift loss. Lower half cut 1/2 flatback, 90C 3/4 flatback, some lift loss, but not a lot. 90C 1/2 flatback, too much loss of lift. (not eligible to be an improved design) Spanwise Wavy Trailing Edge Airfoil 10/ 31 NAWEA 2015

24 Results and Discussion: Lift vs. Drag Polar Lower half cut 3/4 flatback Lower half cut 1/2 flatback 90C cut 3/4 flatback Lower half cut 3/4 flatback, only little amount of lift loss, but too much drag (not eligible as a drag reduced design) Lower half cut 1/2 flatback, 90C 3/4 flatback, some of lift loss, but not a lot and large drag reduction (down to 1/3 of the original flatback design) Spanwise Wavy Trailing Edge Airfoil 11/ 31 NAWEA 2015

25 Results and Discussion: Lift / Drag Map Lower half cut 1/2 flatback 90C cut 3/4 flatback Lower half cut 1/2 flatback, 90C 3/4 flatback have better aerodynamic performance than the original flatback aifoil for both moderate and high angle of attack. 90C 3/4 flatback has broader performance coverage than lower halfway cut 1/2 flatback. Spanwise Wavy Trailing Edge Airfoil 12/ 31 NAWEA 2015

Results and Discussion: Acoutic Measurement Details Aeroacoustic characteristics; measurement details - 3 pressure fluctuation measurement points at 3C distance from TE - 0.

26 Results and Discussion: Acoutic Measurement Details Aeroacoustic characteristics; measurement details - 3 pressure fluctuation measurement points at 3C distance from TE - 0.5C distances between 3 locations - Freestream M = 0.3, Re = 666,000, AoA = 12 SPL db = 10 log 10 ( p 2 p rrr 2 ), 1kHz sampling rates for 1 sec Spanwise Wavy Trailing Edge Airfoil 13/ 31 NAWEA 2015

27 Results and Discussion: Sound Pressure Level Lower half cut 3/4 flatback Lower half cut 1/2 flatback 90C cut 3/4 flatback 90C cut 1/2 flatback Noise emissions reduced about 20 db by the improved wavy trailing edge. Regarding aerodynamic performance, the lower half cut 1/2 and 90C 3/4 flatback may be the best designs acoustic-wise. Spanwise Wavy Trailing Edge Airfoil 14/ 31 NAWEA 2015

28 Results and Discussion: Noise Spectrum (by FFT) peak [db] Const. thickness flatback TE: Tonal noise at low frequency range Noise peak is alleviated by the improved designs. Spanwise Wavy Trailing Edge Airfoil 15/ 31 NAWEA 2015

29 Results and Discussion / Overall Overall (AoA 12 ) Min. TE thic kness Cl Cd Cl/Cd Acoustics Lower half cut 3/4 flatback High Drag High noise 15.38% of C Lower half cut 1/2 flatback 90C cut 3/4 flatback 13.12% of C Best Performance! 13.12% of C C cut 1/2 flatback Low Lift 8.75% of C Best Performance: 90%C cut ¾ flatback & Lower half cut ½ flatback! Spanwise Wavy Trailing Edge Airfoil 16/ 31 NAWEA 2015

30 Conclusions Aerodynamic Performance Larger blade volume with the lower half cut and 90%C cut wavy trailing edges 90%C cut wavy TE more effective to break down spanwise vortex compared to the lower half cut Lower half cut wavy TE 1/2 flatback: small lift loss & large drag reduction, consequently high L/D 90%C cut wavy TE 1/2 flatback: although dramatic drag reduction, too much lift loss, consequently low L/D 90%C cut wavy TE 3/4 flatback: small lift loss & large drag reduction, consequently high L/D Acoustic Noise Reduction Strong magnitude tonal noise peaks at low frequency (100~170 Hz) with const. Flatback airfoil, was reduced up to 20 db (mitigated down to the sharp TE noise level) by the improved designs Although best acoutic noise reduction design is the 90%C cut wavy TE 1/2 flatback, however, relatively worse aerodynamic performance. Best aerodynamic and aeroacoustic performance: Lower half cut wavy TE 1/2 flatback / 90%C cut wavy TE 3/4 flatback Future work Combine to investigate lower half 90%C cut wavy TE 1/2 flatback Spanwise Wavy Trailing Edge Airfoil 17/ 31 NAWEA 2015

31 NAWEA 2015 SYMPOSIUM THANK YOU Acknowledgements UMD supercomputing resources Use of Deepthought II computing cluster Research sponsored by State of Maryland (MHEC/MEA) Spanwise Wavy Trailing Edge Airfoil 18/ 31 NAWEA 2015

Effect of Wavy Trailing Edge on 100meter Flatback Wind Turbine Blade

Journal of Physics: Conference Series PAPER OPEN ACCESS Effect of Wavy Trailing Edge on 100meter Flatback Wind Turbine Blade To cite this article: SJ Yang and J D Baeder 2016 J. Phys.: Conf. Ser. 753 022060