# Masakazu Shimooka, Makoto Iida, and Chuichi Arakawa The University of Tokyo

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

Basic Study of Winglet Effects On Aerodynamics and Aeroacoustics Using Large-Eddy SimulationEuropean 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 workSimulate 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 (WINDMELOliver Fleig, Chuichi Arakawa23rd ASME Wind Energy SymposiumJanuary 5 8, 2004, Reno, NevadaActual tip shapeOgee tip shape

What is winglet ?Developed by WhitcombDiffuse tip vorticesReduce induced dragIncrease thrust and lift forceExamples of winglets for blades of rotation Tip vane by van HoltenWind turbine Mie vane by ShimizuWind turbine Bladelet by ItoMarine 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 Governing equation: Compressible Navier-Stokes equation Turbulence model LES Smagorinsky model 3rd order Upwind Finite Difference scheme in space 1st order Implicit Euler scheme in time

Numerical method(2) - Acoustic fieldNear field1 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

Boundary conditionRotation axisabinflowxyzUniform flow at inletConvective boundary conditions at outletWall: No-slip conditions; pressure and density extrapolatedOuter 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 grid765 pointsalong the surface ()193 pointsperpendicular to the surface ()2209 pointsalong the span direction ()

Total number of grid points, 300million

Use 14 nodes (112 CPU) on Earth Simulator Grid spacing of airfoil section (plane)

Single O-grid Minimum wall distance is 210-5 corresponding to y+=1 (wall resolved) High concentration of grid points in the blade tip region

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

t = 3.6x10-5c/Ueff = 1.3x10-7(s)

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

Flow field - Tip vortex50deg.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.

Vorticity magnitude contours at the near wakey/c =1.0y/c =1.2y/c =1.4y/c =1.6y/c =1.8y/c =2.050deg.0deg.xyz

50deg0deg.Vorticity magnitude contours and iso-surface (||=4.0) Winglet reduces the strength of tip vortices . 50deg.

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

Rotational torque and Flap momentHub sideTip sideTip sideHub sideWinglet

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 distribution0 deg.50deg.Suction side

Acoustic field Near fieldFrequency (Hz)Frequency (Hz)SPL (dB), ref: 210-5(Pa)SPL (dB), ref: 210-5(Pa) Point A is where the tip vortex is developed. Point B is slightly downstream from the trailing edge of main blade near the winglet.

Acoustic field Far fieldIntegration surface for FW-H equation(yellow surface)Far field overall sound pressure level (OASPL)Integration from 1kHz to 12.5kHz(2.3m downstream from rotor) 50deg.0deg.

Current Work Detached-Eddy Simulation for NREL Phase VI

Pressure distribution (U=7.0m/s)=7.4=8.3=10.1=11.8=12.2

Flow field (U=25.1m/s)UVorticity magnitude iso-surface (||=0.2) and contoursStreamlines

ConclusionsWe 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.