Experimental and numerical investigations of 3D airfoil characteristics on a MW wind turbine

Experimental and numerical investigations of 3D airfoil characteristics on a MW wind turbineNiels Troldborg, Christian Bak, Niels N. Sørensen, Frederik Zahle, Helge Aa. Madsen, Srinivas Guntur, Pierre-Elouan Réthoré

Wind Energy Department, DTU Wind Energy, DK-4000 Roskilde, Denmark

February 4-7, EWEA 2013, Vienna, Austria2 DTU Wind Energy

Background

Aeroelastic models typically use airfoil data (CL, CD) to predict aerodynamic forces

)(tan 1

VrVz

)()(2

21

D

Lrel C

CcV

DL

f


Background

Airfoil data commonly obtained from wind tunnel measurements in 2D steady flow

Inflow

Test section


Background

However, airfoil characteristics on a wind turbine can be quite different from wind tunnel measurements: CL(α) varies with spanwise position 3D effects are important!

3D airfoil charcteristics from the NREL/NASA Ames test. (From: Bak et al. Three-Dimensional Corrections of Airfoil Characteristics Based on Pressure distributions. EWEC 2006.)


Objectives

Study 3D aerofoil characteristics on a modern MW wind turbine vs. 2D aerofoil characteristics using

Wind tunnel tests Full scale field experiment 2D airfoil computations (CFD) 3D rotor computations (CFD)

Evaluate quality of measurements and computations for studying 3D effects Validate measurements and CFD Verify that the datasets are suitable for future development/improvements of

3D airfoil correction models.


Experimental approach: The DANAERO MW project

Four blade sections with 64 pressure taps: Section 1: r/R=33%, NACA63-433 Section 2: r/R=48%, NACA63-424 Section 3: r/R=75%, NACA63-421 Section 4: r/R=93%, NACA63-418

Five-hole Pitot tubes at r/R=36%, 51%, 78% and 90%, respectively

High frequency measurements of wind speed and direction from nearby met mast

The Tjæreborg field experiment on the 2.3 MW NM80 turbine:

Wind tunnel tests in the LM Wind Power tunnel: Measurements on four aerofoils with the

same shape as the instrumented sections on the LM38.8 blade of the NM80 turbine, i.e. sections 1, 2, 3 and 4.


Computational approach: EllipSys2D/3D

256x128 grid points k-ω SST turbulence model Correlation based transition model Steady uniform inflow

3D rotor computations on the NM80 turbine 256x128x128 grid points on blade Total mesh 432x32^3 k-ω SST turbulence model Correlation based transition model Steady uniform inflow 21 computations with varying wind speed

and pitch angle

2D airfoil computations blade sections 1, 2, 3 and 4 of the NM80 turbine


Wind tunnel measurements vs 2D airfoil CFD

Section 1 (NACA63-433)





Wall effects in a wind tunnel Differences between measurements and CFD may be due to wall effects which are significant for thick aifoils

FFA airfoil 30% thickness at AoA = 3°


Wall effects in a wind tunnel

NACA63-418 at AoA = 15°

Differences between measurements and CFD may be due to wall effects which are significant for thick aifoils and airfoils at high AOA

FFA airfoil 30% thickness at AoA = 3°


Measurements on NM80 turbine vs 3D rotor CFD

Comparison of Cp distributions at V∞=6.1 and RPM=12.1

Section 1 NACA63-433r/R=33%





Determining the angle of attack from 3D CFD

Azimuthal averaging technique (AAT) Extract velocities in annular rings up and downstream of the rotor.

Compute azimuthal average for each ring

Interpolate average velocities from up and downstream to the rotor plane.


Determining the angle of attack from measurements

Extract 1-minute averages of measured distributions Bin average on flow angle measured by Pitot tube at r/R=0.78, i.e.

establish From the rotor computations determine the AoA for each blade section

using the AAT and establish Estimate the measured AoA by minimizing

where n=64 is the number of pressure taps around the airfoil.

expPC

expPC

)(AoAC cfdP

)(exppitotP AoAC


Comparison of 2D and 3D airfoil characteristics






Conclusions

Unique dataset for studying 3D airfoil characteristics in comparison with 2D characteristics on a modern MW wind turbine is established.

Suitable as a basis for future development/improvements of 3D correction models

3D effects known from smaller turbines are documented Increased in-board lift at high AOA Decreased lift slope at in-board/out-board sections

Comparison between measured and computed airfoil characteristics on the NM80 rotor generally reveals good agreement

Large differences between 2D airfoil CFD and wind tunnel measurements for thick airfoils and for airfoils at high AOA.

Likely to be due to wall effects in the wind tunnel

Experimental and numerical investigations of 3D airfoil characteristics on a MW wind turbine

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