Masakazu Shimooka, Makoto Iida, and Chuichi ArakawaThe University of Tokyo

Basic Study of Winglet Effects On Aerodynamics and Aeroacoustics Using Large-Eddy Simulation

European Wind Energy Conference & ExhibitionAthens, Greece, 27 February – 2 March 2006

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.

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.

Related research (WINDMELⅢ ）

Oliver Fleig, Chuichi Arakawa23rd ASME Wind Energy SymposiumJanuary 5 – 8, 2004, Reno, Nevada

60

80

100

0 2000 4000 6000 8000 10000 12000Frequency (Hz)

SP

L [d

B],

ref:

2×10

-5 P

a

Actual tip shape

Ogee type tip shape

Simulation results

Actual tip shape

Ogee tip shape

What is winglet ?

Developed by Whitcomb

Diffuse tip vortices Reduce induced drag Increase thrust and lift

force

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)

In this work, We use Navier-Stokes simulation to resolve complex structure of tip vortices in detail.

Numerical method(1) - Flow field

10

Rej jj

QF G

t x

0

, ,i

i j i j ij j ij

j kj k j

u

Q u F u u p G

E Hu u q

2

3ji k

ij t ijj i k

uu u

x x x

・ Governing equation: Compressible Navier-Stokes equation

・ Turbulence model ： LES Smagorinsky model

1/ 222t s ij ijC S S SGS

Smagorinsky Model (Cs = 0.15)

1 exp / 26.0g y

Van Driest Wall damping function

・ 3rd order Upwind Finite Difference scheme in space

・ 1st order Implicit Euler scheme in time

Numerical method(2) - Acoustic field

Far field:Modeled By Ffowcs Williams-Hawkings (FW-H) equation

×

Near field:Direct noise simulation By compressible LES

Near field　　（ 1 to 2 chord lengths ）　・ Direct noise simulation　・ sufficiently fine grids 　・ Accurate modeling of 　　 non-linear effects and wall reflectio

n, refraction, scattering in the near field

Far field　・ Ffowcs Williams-Hawkings equation　・ permeable integration surface whic

h does not need to correspond with the body surface

Boundary condition

Rotation axis

a

b

inflow

x

y

z

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

Computational domain

Computational grid

ξ

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

η

ζ

xyz

Grid spacing of airfoil section (ζplane)

・ 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

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)

Tip shape (top: 50deg., bottom: 0deg.)

Ueff

Ueff

50deg.

0deg.

Flow field - Tip vortex

50deg.

0deg.

Vorticity magnitude iso-surfaces

Pressure contours at the trailing edge 　

・ trailing edge at the very tip (y/c=1.0)

・ Winglet diffuses tip vortices.

0deg. 50deg.

Smaller but more complex structure

Vorticity magnitude contours at the near wake

y/c =1.0y/c =1.2

y/c =1.4y/c =1.6

y/c =1.8y/c =2.0

50deg.

0deg. x

y

z

50deg0deg.

Vorticity magnitude contours and iso-surface (|ω|=4.0)

・ Winglet reduces the strength of tip vortices .

50deg.

Spanwise velocity components contours

・ Spanwise velocity (w) component contours at y/c=0.7・ Reduced downwash effect, and Spread of wake in spanwise direction.

0deg.

50deg.

x

z

x

z

Rotational torque and Flap moment

30 32 34 36 38

0

0.01

0.02

Spanwise position (z/c)

Rot

atio

nal t

orqu

e (n

on-d

imen

sion

)

0deg 50deg

Hub

sid

e

Tip

sid

e

30 32 34 36 380

0.1

0.2

Spanwise position (z/c)

Flap

mom

ent (

non-

dim

ensi

on)

0deg 50deg

Tip

sid

e

Hub

sid

e

Winglet WingletMain blade Main blade

・ Increase of rotational torque at the winglet and the main blade near the winglet.

・ Reduction of flap moment at the winglet.

Pressure distribution

0 0.2 0.4 0.6 0.8 1

-5

-4

-3

-2

-1

0

y/chord

Cp

0deg 50deg

0 deg.

50deg.

Suction side

Larger suction peak at the leading edge

More sufficient recovery of pressure at the trailing edge

Acoustic field – Near field

1000 5000 10000100

120

140

160

180

1000 5000 10000100

120

140

160

180

Point A Point B

Frequency (Hz) Frequency (Hz)

SP

L (d

B),

ref

: 2×

10-5(P

a)

SP

L (d

B),

ref

: 2×

10-5(P

a)

0deg.50deg.

・ Point A is where the tip vortex is developed.

・ Point B is slightly downstream from the trailing edge of main blade near the winglet.

0deg.50deg.

Acoustic field – Far field

Integration surface for FW-H equation

(yellow surface)

(dB) (dB)

Far field overall sound pressure level (OASPL)

Integration from 1kHz to 12.5kHz(2.3m downstream from rotor)

Blade

50deg.0deg.Smaller but more complex vortices

caused by winglet emit strong noise

In high frequency.

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

Pressure distribution (U∞=7.0m/s)

0 0.5 1-2

0

2

4

6

y/chord

-Cp

calc. exp.

r/R=0.30

0 0.5 1-2

0

2

4

6 calc. exp.

y/chord

-Cp

r/R=0.47

0 0.5 1-2

0

2

4

6

y/chord

-Cp

calc. exp.r/R=0.63

0 0.5 1-2

0

2

4

6

y/chord

-Cp

calc. exp.

r/R=0.80

0 0.5 1-2

0

2

4

6

y/chord

-Cp

calc.r/R=0.95

α=7.4°α=8.3°

α=10.1°α=11.8°α=12.2°

Flow field (U∞=25.1m/s)

U∞

Vorticity magnitude iso-surface (|ω|=0.2) and contours Streamlines

Conclusions We succeeded in capturing winglet effects in detail, using 30

0 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.