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
February 4-7, EWEA 2013, Vienna, Austria3 DTU Wind Energy
Background
Airfoil data commonly obtained from wind tunnel measurements in 2D steady flow
Inflow
Test section
February 4-7, EWEA 2013, Vienna, Austria4 DTU Wind Energy
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.)
February 4-7, EWEA 2013, Vienna, Austria5 DTU Wind Energy
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.
February 4-7, EWEA 2013, Vienna, Austria6 DTU Wind Energy
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.
February 4-7, EWEA 2013, Vienna, Austria7 DTU Wind Energy
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
February 4-7, EWEA 2013, Vienna, Austria8 DTU Wind Energy
Wind tunnel measurements vs 2D airfoil CFD
Section 1 (NACA63-433)
Section 2 (NACA63-424)
Section 3 (NACA63-421)
Section 4 (NACA63-418)
February 4-7, EWEA 2013, Vienna, Austria9 DTU Wind Energy
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°
February 4-7, EWEA 2013, Vienna, Austria10 DTU Wind Energy
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°
February 4-7, EWEA 2013, Vienna, Austria11 DTU Wind Energy
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%
Section 2 NACA63-424r/R=48%
Section 3 NACA63-421r/R=75%
Section 4 NACA63-418r/R=93%
February 4-7, EWEA 2013, Vienna, Austria12 DTU Wind Energy
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.
February 4-7, EWEA 2013, Vienna, Austria13 DTU Wind Energy
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
February 4-7, EWEA 2013, Vienna, Austria14 DTU Wind Energy
Comparison of 2D and 3D airfoil characteristics
Section 1 (NACA63-433)
Section 2 (NACA63-424)
Section 3 (NACA63-421)
Section 4 (NACA63-418)
February 4-7, EWEA 2013, Vienna, Austria15 DTU Wind Energy
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