1. Basic Study of Winglet Effects
   
Basic Study of Winglet Effects
On Aerodynamics and Aeroacoustics Using Large-Eddy Simulation
Masakazu Shimooka, Makoto Iida, and Chuichi Arakawa
The University of Tokyo
European Wind Energy Conference & Exhibition
Athens, Greece, 27 February – 2 March 2006

2. Purpose of this work
To optimize the tip shape for increasing public acceptance of wind energy.
To clarify winglet effects on aerodynamic performance, loads, noise.
To investigate a possibility of application to the blade design tools.

3. Outline of this work
Simulate the whole blade including the tip shape effects, using LES (Large-Eddy simulation) with 300 million grid points.
Investigate effects of differences of tip shapes on aerodynamics and aeroacoustics (Direct Noise Simulation).
・ 2 types of winglets whose installation angle is 0, 50 degree.
Introduce our current work (Detached-Eddy simulation) based on our knowledge of LES.

4. Related research (WINDMELⅢ）
Actual tip shape
Simulation results
Oliver Fleig, Chuichi Arakawa
23rd ASME Wind Energy Symposium
January 5 – 8, 2004, Reno, Nevada
Ogee tip shape

5. What is winglet ?
Examples of winglets for blades of rotation
・Tip vane by van Holten　（Wind turbine）
・Mie vane by Shimizu　（Wind turbine）
・Bladelet by Ito　（Marine propeller）
Increase of rotor output as results of experiments and numerical analysis
such as
・BEM (Blade Element Momentum method)
・VLM (Vortex Lattice Method)
Developed by Whitcomb
Diffuse tip vortices
Reduce induced drag
Increase thrust and lift force
In this work, We use Navier-Stokes simulation to resolve complex structure of tip vortices in detail.

6. Numerical method(1) - Flow field
・ Governing equation: Compressible Navier-Stokes equation
・ Turbulence model： LES Smagorinsky model
SGS
Smagorinsky Model (Cs = 0.15)
Van Driest Wall damping function
・ 3rd order Upwind Finite Difference scheme in space
・ 1st order Implicit Euler scheme in time

7. Numerical method(2) - Acoustic field
Near field
　　（1 to 2 chord lengths）
　・ Direct noise simulation
　・ sufficiently fine grids
　・ Accurate modeling of
　　non-linear effects and wall reflection, refraction, scattering in the near field
Far field
　・ Ffowcs Williams-Hawkings equation
　・ permeable integration surface which does not need to correspond with the body surface
Near field:
Direct noise simulation
By compressible LES
Far field:
Modeled
By Ffowcs Williams-Hawkings (FW-H)
equation ×

8. Boundary condition inflow Uniform flow at inlet
Convective boundary conditions at outlet
Wall: No-slip conditions; pressure and density extrapolated　　　　
Outer boundaries are very coarse to prevent reflection of high frequency acoustic waves:
Large rate of grid stretching and extreme distance between blade and outer boundaries
　・Half-sphere　
・Periodic plane a-b
・Radius of sphere is twice the blade span a b y x z
Rotation axis
Computational domain

9. Computational grid Total number of grid points, 300million ξ ξ ζ x
765 points，along the surface (ξ)
193 points，perpendicular to the surface (η)
2209 points，along the span direction (ζ)
Total number of grid points, 300million
Use 14 nodes (112 CPU) on Earth Simulator ξ ξ η ζ
・ Single O-grid
・ Minimum wall distance is 2×10-5 corresponding to y+=1 (wall resolved)
・ High concentration of grid points in the blade tip region
Direct noise simulation
25-30 grid points per wavelength x y z
Grid spacing of airfoil section (ζplane)

10. Simulation parameters and tip shapes
Re = 1.0x106
Reference is the chord length at tip
c = 0.23(m)，
　and the effective flow velocity at tip
Ueff = 61.74(m/s)
Mach = 0.18 at tip
Δt = 3.6x10-5c/Ueff
= 1.3x10-7(s)
50deg.
Ueff
0deg.
Ueff
Tip shape (top: 50deg., bottom: 0deg.)

11. Flow field - Tip vortex
0deg.
50deg.
Vorticity magnitude iso-surfaces

12. Pressure contours at the trailing edge
Smaller but more complex structure
0deg.
50deg.
・trailing edge at the very tip (y/c=1.0)
・Winglet diffuses tip vortices.

13. Vorticity magnitude contours at the near wake
y/c =2.0
y/c =1.8
y/c =1.6
50deg.
y/c =1.4
y/c =1.2
y/c =1.0 y z
0deg. x

14. ・Winglet reduces the strength of tip vortices .
0deg.
50deg.
50deg
Vorticity magnitude contours and iso-surface (|ω|=4.0)
・Winglet reduces the strength of tip vortices .

15. Spanwise velocity components contoursx z x z
0deg.
50deg.
・Spanwise velocity (w) component contours at y/c=0.7
・Reduced downwash effect, and Spread of wake in spanwise direction.

16. Rotational torque and Flap moment
Main blade
Winglet
Main blade
Winglet
Hub side
Tip side
Hub side
Tip side
・ Increase of rotational torque at the winglet and the main blade near the winglet.
・ Reduction of flap moment at the winglet.

17. Pressure distribution
Larger suction peak at the leading edge
More sufficient recovery of pressure at the trailing edge
50deg.
0 deg.
Suction side

18. Acoustic field – Near field
0deg.
50deg.
0deg.
50deg.
SPL (dB), ref: 2×10-5(Pa)
SPL (dB), ref: 2×10-5(Pa)
Point A
Point B
Frequency (Hz)
Frequency (Hz)
・ Point A is where the tip vortex is developed.
・ Point B is slightly downstream from the trailing edge of main blade near the winglet.

19. Acoustic field – Far field
(dB)
(dB)
Integration surface for FW-H equation
(yellow surface)
Smaller but more complex vortices
caused by winglet emit strong noise
In high frequency.
Blade
0deg.
50deg.
Far field overall sound pressure level (OASPL)
Integration from 1kHz to 12.5kHz
(2.3m downstream from rotor)

20. Current Work — Detached-Eddy Simulation — for NREL Phase VI

21. Pressure distribution (U∞=7.0m/s)
α=11.8°
α=10.1°
α=12.2°
α=7.4°
α=8.3°

22. Flow field (U∞=25.1m/s) U∞
Vorticity magnitude iso-surface (|ω|=0.2) and contours
Streamlines

23. Conclusions
We succeeded in capturing winglet effects in detail, using 300 million grid points in Earth Simulator.
- Diffuse and reduce tip vortices.
- Reduce downwash effect, and Spread wake in spanwise direction.
This simulation will be very useful for designing optimal tip shapes.
We have performed Detached-Eddy simulation as the first step for less computational costs
This simulation is based on our knowledge of grid dependence in LES.