NAWEA 2015 SYMPOSIUM - Virginia Tech · 2020. 10. 6. · Edge Flatback Airfoils: Design Improvement NAWEA 2015 SYMPOSIUM ... square of rotor blade length! ... Lower half cut 1/2 flatback,

Spanwise Wavy Trailing Edge Airfoil 1/ 31 NAWEA 2015

Aerodynamics and Aeroacoustics of Spanwise Wavy Trailing Edge Flatback Airfoils: Design Improvement

NAWEA 2015 SYMPOSIUM

Seung Joon Yang James D. Baeder

Department of Aerospace Engineering, University of Maryland Alfred Gessow Rotorcraft Center


Outline

Wavy trailing edge design Wavy trailing edge modification

Conclusion

Introduction Flatback airfoil drag and noise emission Numerical methods

Results and discussions Aerodynamic Characteristics Aeroacoustic Characteristics


Introduction and Motivation

* IPCC Special Report on Renewable Energy Sources and Climate Change Mitigation” 2011

Higher power generation requires larger blades

Wind power generation is proportional to

square of rotor blade length!

Demands a structurally robust blade Thicker Airfoil at inboard sections (~40% of blade span!)


Introduction: Flatback Airfoil Aerodynamics

* Baker,”Experimental Analysis of Thick Blunt Trailing Edge wind turbine Airfoils”, 2006

Advantages

Sharp TE Flatback TE

Flatback TE airfoil has superior lift performance; delayed stall on upper surface Structurally robust blade design compared to sharp TE airfoil


Introduction: Flatback Airfoil Aerodynamics

Flatback TE

Sharp TE

Disadvantages

Flatback TE airfoil suffers from higher drag, lower max L/D Higher acoustical signature compared to sharp TE

Increase in drag

* Baker,”Experimental Analysis of Thick Blunt Trailing Edge wind turbine Airfoils”, 2006

Sharp TE Flatback TE


Introduction: Flatback Airfoil Noise Emission

AoA 4°, Re = 3,000,000

* Dale E. Berg and M. Barone,”Aerodynamic and Aeroacoustic Properties of a Flatback Airfoil”, WINDPOWER 2008, Houston, 2008

Flatback TE

Sharp TE

Strong vortex shedding

Disadvantages

Flatback TE airfoil high tonal noise Generated by pressure fluctuations at TE because of strong nearly 2-D

spanwise coherent vortex shedding

Noise spectrum Vortex shedding pattern


Introduction: Spanwise Wavy Trailing Edge Modification

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”, 33rd Wind Energy Symposium at Scitech 2015, Kisimmee, FL, 2015

Baseline Proposed solution


Numerical methods RANS – LES hybrid method (OVERTURNS, GPURANS3D) - Laminar – Turbulent transition modeling ; γ − 𝑅𝑒𝜃𝜃 transition model with S-A turbulence model - Delayed Detached Eddy Simulation (DDES) - Spatial reconstruction ; 5th order WENO - Time marching; Diagonal Alternating Direction Implicit (DADI) - GPU – accelerated computation ; Deepthought II cluster at UMD (40 gpu nodes) ; Nvidia Tesla K20m GPUs

* Deepthought II Cluster at UMD, College Park

Nvidia Tesla K20m

Processor core 2496

Processor core clock 706 MHz

Memory 5 GB

Memory clock 2.6 GHz

Band width 208 GB/sec


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) 0.5C in span direction 104 grid points at the trailing edge

102 grid points upper/bottom surface near the trailing edge

Validation!



Wavy trailing edge design: Previous Designs

∆y = y𝑚𝑚𝑚 − y𝑚𝑚𝑚

local thickness =∆𝑦2

cos𝜔 ∗ 2𝜋𝜋

𝑙+ 1 + 𝑦𝑚𝑚𝑚

Wave formula

4 cyc/c better than 2 cyc/c or 8 cyc/c

1/2flatback-4cyc/C 3/4flatback-4cyc/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

Baseline


Baseline Results Base-line cases A. FB3500-1750 (TE thickness 17.50% of C, flatback TE) B. FB3500-0462 (TE thickness 4.62% of C, sharp TE)

FB3500-1750 FB3500-0462


Strong nearly 2-D spanwise vortex structure with flatback airfoil Weak spanwise vortex structure with sharp trailing edge airfoil


Previous Wavy Trailing Edge: Flowfield

3/4 flatback – 4cyc/C 1/2 flatback – 4cyc/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

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


Previous Wavy Trailing Edge: Aerodynamic Performance

* James D. Baeder and Seung Joon Yang,”Wavy Trailing-Edge Flatback Aerodynamics Using a GPU-Accelerated Navier-Stokes Solver”, EWEA Offshoer, Copenhagen, Denmark, 2015

With shallow wavy pattern, higher lift and reduced drag With deeper wavy pattern, too much loss of lift


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?


Design Improvements: Design #1 Can we remove wavy structure on upper surface?

Lower half way cut (only Bottom surface wavy TE )

Previous wavy TE

Lower half way cut wavy TE

Camber recovery helps aerodynamic performance

Larger blade volume with design #1 Better manufacturability


Design Improvements: Design #2 Can we start wavy structure closer to TE?

90%C cut (wavy TE at 90% of Chord)

Previous wavy TE

Wavy TE at 90% of chord

Can we get rid of 2-D spanwise vortex structure

With the New Designs??

Larger blade volume with design #2 Better manufacturability


Improved Wavy Trailing Edge Designs

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)

B. Lower half cut 1/2 flatback A. Lower half cut 3/4 flatback

D. 90C cut 1/2 flatback C. 90C cut 3/4 flatback


Results and Discussion: Flowfield (Iso-vorticity mag.)

Lower half cut 1/2 flatback Lower half cut 3/4 flatback

90C cut 3/4 flatback 90C cut 1/2 flatback

FB3500-1750 (flatback TE)

FB3500-0462 (sharp TE)

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


Results and Discussion: Flowfield (Vorticity contours) • Aerodynamic characteristics; Trailing edge vortex shedding pattern


Crest

Trough

Crest

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.




Crest

Trough

Crest

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.




Crest

Trough

Crest

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.




Crest

Trough

Crest

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.


Results and Discussion: Lift vs. AoA

Lower half cut 1/2 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)


Results and Discussion: Lift vs. Drag Polar




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)


Results and Discussion: Lift / Drag Map



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.


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 log10( 𝑝′2

𝑝𝑟𝑟𝑟2 ) , 1kHz sampling rates for 1 sec


Results and Discussion: Sound Pressure Level





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.


Results and Discussion: Noise Spectrum (by FFT)

peak [dB] peak [dB] peak [dB] peak [dB]

Const. thickness flatback TE: Tonal noise at low frequency range Noise peak is alleviated by the improved designs.


Results and Discussion / Overall • Overall (AoA 12°)

Min. TE thickness Cl Cd Cl/Cd Acoustics

15.38% of C 1.9789 0.0820 24 118

13.12% of C 1.8252 0.0475 38 103

13.12% of C 1.8283 0.0483 37 105

8.75% of C 1.6757 0.0490 34 99





Best Performance: 90%C cut ¾ flatback & Lower half cut ½ flatback!

High Drag High noise

Low Lift

Best Performance!


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


NAWEA 2015 SYMPOSIUM

THANK YOU

Acknowledgements UMD supercomputing resources – Use of Deepthought II computing cluster

Research sponsored by State of Maryland (MHEC/MEA)

Documents

NAWEA 2015 SYMPOSIUM - Virginia Tech · 2020. 10. 6. · Edge Flatback Airfoils: Design Improvement NAWEA 2015 SYMPOSIUM ... square of rotor blade length! ... Lower half cut 1/2 flatback,