1. WT2: the Wind Turbine in a Wind Tunnel Project C. L. Bottasso, F
WT2: the Wind Turbine in a Wind Tunnel Project C.L. Bottasso, F. Campagnolo Politecnico di Milano, Italy Spring 2010

2. Outline Project goals The wind tunnel at the Politecnico di Milano
Wind turbine model scaling and configuration
Aerodynamics
Blade manufacturing
Simulation environment
Data acquisition, control and model management system
Conclusions and outlook

3. Project Goals
Goals: design, manufacture and test an aeroelastically-scaled model of the Vestas V90 wind turbine
Applications:
Testing and comparison of advanced control laws and supporting technologies (e.g. wind and state observers)
Testing of extreme operating conditions (e.g. high speed high yawed flow, shut-down in high winds, etc.)
Tuning of mathematical models
Testing of system identification techniques
Aeroelasticity of wind turbines …
Possible extensions:
Multiple wind turbine interactions
Aeroelasticity of off-shore wind turbines (with prescribed motion of wind turbine base)
Effects of terrain orography on wind turbines

4. The Politecnico di Milano Wind Tunnel
1.4MW Civil-Aeronautical Wind Tunnel (CAWT):
13.8x3.8m, 14m/s, civil section:
turbulence < 2%
with turbulence generators = 25%
- 13m turntable
4x3.8m, 55m/s, aeronautical section:
turbulence <0.1%
open-closed test section

5. The Politecnico di Milano Wind Tunnel
Turn-table
13 m
Turbulence (boundary layer) generators
Low speed testing in the presence of vertical wind profile
Multiple wind turbine testing (wake-machine interaction)
High speed testing
Aerodynamic characterization (Cp-TSR-β & CF-TSR-β curves)

6. Outline Project goals The wind tunnel at the Politecnico di Milano
Wind turbine model scaling and configuration
Aerodynamics
Blade manufacturing
Simulation environment
Data acquisition, control and model management system
Conclusions and outlook

7. Model Scaling
Criteria for definition of scaling (using Buckingham Π Theorem):
Best compromise between:
Reynolds mismatch (quality of aerodynamics)
Speed-up of scaled time (avoid excessive increase of control bandwith)
Aeroelastic effects: correct relative placement of frequencies wrt rev harmonics, correct Lock number
Quantity
Scaling factor
Length Ratio
1/45
Time Ratio
1/22.84
Velocity Ratio
1/1.97
Power Ratio
1/15477
Rotor Speed Ratio
22.84
Torque Ratio
1/353574
Reynolds Ratio
1/88.64
Froude Ratio
11.6
Mach Ratio V2
V90
Rotor Diameter
2 [m]
90 [m]
Blade Length
977.8 [mm]
44 [m]
Rotor Overhang
75.1 [mm]
3.38 [m]
Hub Height
1.78 [m]
79.94 [m]
Rotor Speed
367 [rpm]
16 [rpm]
Nominal Power
193.8 [W]
3 [MW]
Nominal Torque
5.06 [Nm]
1790 [KNm]
Average Reynolds
 5÷6 e4
 4÷5 e6
Reynolds mismatch:
Use low-Re airfoils (AH79 & WM006) to minimize aerodynamic differences
Keep same chord distribution as original V90 blade, but
Adjust blade twist to optimize axial induction factor

8. Electronic board for blade strain gages (6 force/moment components)
V2 Model Configuration
Electronic board for blade strain gages
CONICAL SPIRAL TOOTHED GEARS
Rotor radius = 1m
Height = 2.8 m
Up-tilt = 6 deg
Balance
(6 force/moment components)

9. Main shaft with torque meter
V2 Model Configuration
Pitch actuator control units:
Faulhaber MCDC-3003 C
30 V – 10 A Max
Position and speed
Cone = 4 deg
Main shaft with torque meter
Slip ring Moog AC6355:
36 Channels
250 V – 2 A Max
Conical spiral gears
Torque actuator:
Portescap Brushless B
Pn = 340 W
Planetary gearhead
Torque and speed control
Pitch actuator:
Faulhaber 1524
Zero backlash gearhead
Built-in encoder IE 512

10. V2 Model Configuration

11. V2 Model Configuration
Wind turbine model shown without nacelle and tower covers, for clarity

12. Outline Project goals The wind tunnel at the Politecnico di Milano
Wind turbine model scaling and configuration
Aerodynamics
Blade manufacturing
Simulation environment
Data acquisition, control and model management system
Conclusions and outlook

13. BEM Predicted Aerodynamic Performance
Good agreement in full load region III
Poorer agreement in partial load regions II and II1/2, due to higher drag of V2 airfoils
Region II1/2
P<Pr
Ω=Ωr
Region II
CPopt
λopt
βopt CP
Region III
P=Pr
Ω=Ωr
TSR

14. BEM Predicted Aerodynamic Performance
Good agreement between thrust coefficients in the entire working region, due to good lift characteristics of V2 airfoils CF
Region II
CPopt
λopt
βopt
Region II1/2
P<Pr
Ω=Ωr
Region III
P=Pr
Ω=Ωr
TSR
Filippo Campagnolo

15. Aerodynamic Identification
Goal: identification of airfoil aerodynamic characteristics
Application: blade redesign, choice of airfoils, understanding of rotor aerodynamics
Approach: use wind tunnel measurements of the wind turbine response
Pros:
Avoid testing of individual airfoils
Include 3D and rotational effects
Procedure:
Measure power and thrust coefficients
Parameterize airfoil lift and drag coefficients
Identify airfoil aerodynamic parameters that best match wind turbine performance, using a BEM model of the rotor
(Work in progress, results expected summer 2010)

16. Aerodynamic identification
Constrained optimization:
Goal: match CP & CF at tested TSR & β
Unknowns: parameters describing airfoil CL & CD characteristics
Rotor model: BEM
Aerodynamic
identification
Experimental CP & CF coefficients
2.Experimental power CP and thrust CF coefficients
3. Maximum Likelihood identification
4. Identified aerodynamic coefficients of airfoils
5. Redesign blade to improve matching wrt V90
1. Wind tunnel testing
Trim at varying pitch β and TSR
Measure power CP and thrust CF CL
Design data
Identified data a CD a

17. Outline Project goals The wind tunnel at the Politecnico di Milano
Wind turbine model scaling and configuration
Aerodynamics
Blade manufacturing
Simulation environment
Data acquisition, control and model management system
Conclusions and outlook

18. Blade Manufacturing Rigid blades:
Easier and faster to manufacture than aero-elastically scaled blades
Used for initial testing and verification of suitable aerodynamic performance
Implemented two manufacturing solutions:
1. CNC machining of light aluminum alloy 2. UD carbon fiber
Carbon blades (will include blade-root strain gage in 2nd blade set – May 2010)
FEM verification of strain gage sensitivity
CAD model for CNC machining, with support tabs (+resin support)

19. Blade Manufacturing Aero-elastically scaled blades: Solution:
Need accurate aerodynamic shape: classical segmented solution is unsuitable
Structural requirements: match at least lower three modes
Very challenging problem: only 70g of weight for 1m of span!
Solution:
Rohacell core with carbon fiber spars and film coating
Sizing using constrained optimization
(Work in progress, expected completion of blade set by end of 2010)

20. Design of the V2 Aero-elastically Scaled Composite Blade
Objective: size spars (width, chordwise position & thickness) for desired sectional stiffness within mass budget
Cost function: sectional stiffness error wrt target (scaled stiffness)
Constraints: lowest 3 frequencies
Carbon fiber spars for desired stiffness
Structural
optimization
Optimization
Cross sectional analysis
Equivalent beam model
Sectional optimization variables
(position, width, thickness)
Span-wise shape function interpolation
Rohacell core with grooves for the housing of carbon fiber spars
Width
Chordwise Position
ANBA (ANisotropic Beam Analysis) FEM cross sectional model:
Evaluation of cross sectional stiffness (6 by 6 fully populated matrix)
Thickness
Thermo-retractable film

21. Design of the V2 Aero-elastically Scaled Composite Blade
Solid line: scaled reference values
Dash-dotted line: optimal sizing
Mass gap can be corrected with weights
Modes
Reference [Hz]
Optimization procedure [Hz]
1st Flap-wise
23.2
23.1
2nd Flap-wise
59.4
59.1
1st Edge-wise
33.1
Filippo Campagnolo

22. Design of the V2 Aero-elastically Scaled Composite Blade
Approach:
Demonstration of technology on simple specimen:
Design specimen (uniform cross section, untwisted) of typical properties (mass, stiffness)
Characterize material properties
Manufacture specimen
Characterize specimen (mass, stiffness, frequencies, shape)
Verify accuracy wrt design
Status: completed
Demonstration of technology on blade-like specimen (twist, variable chord)
Status: in progress
Manufacture wind turbine model blade
Status: to be done (expected end 2010)
Filippo Campagnolo

23. Demonstration of Technology on Simple Specimen
Characterization of material properties:
Specimen of uniform properties:
Results:
Good matching of lowest natural frequencies
Acceptable repeatability
Good shape and finishing
Dynamic testing
Static testing
Temperature–dependent characterization
Carbon fiber spars
Airfoil cross section
Modes (specimen A/B)
Percent Error
(specimen A/B)
236/246 Hz
4.5/0.3 %
329/339 Hz
3.1/6.1 %
545/570 Hz
1.9/6.3 %
604/627 Hz
5.1/1.2 %

24. Outline Project goals The wind tunnel at the Politecnico di Milano
Wind turbine model scaling and configuration
Aerodynamics
Blade manufacturing
Simulation environment
Data acquisition, control and model management system
Conclusions and outlook

25. Simulation Environment
Comprehensive aero-elastic simulation environment: supports all phases of the wind turbine model design (loads, aero-elasticity, and control)
Sensor models
Virtual plant Cp-Lambda model
Measurement noise
Wind
Supervisor
Start-up, power production, normal shut-down, emergency shut-down, …
Pitch-torque controller
Controller

26. Simulation Models Cp-Lambda highlights:
Geometrically exact composite-ready FEM beam models
Generic topology (Cartesian coordinates+Lagrange multipliers)
Dynamic wake model (Peters-He, yawed flow conditions)
Efficient large-scale DAE solver
Non-linearly stable time integrator
Fully IEC compliant (DLCs, wind models)
Cp-Lambda (Code for Performance, Loads, Aero-elasticity by Multi-Body Dynamic Analysis):
Global aero-servo-elastic FEM model
Compute sectional stiffness
ANBA (ANisotropic Beam Analysis) cross sectional model
Rigid body
Geometrically exact beam
Revolute joint
Flexible joint
Actuator
Recover cross sectional stresses/strains

27. Simulation Environment
Example: verify adequacy of model for the testing of control laws
Question: does testing of control laws on V2 lead to similar conclusions than V90 testing, notwithstanding differences in aerodynamics (Reynolds)?
Approach:
Choose comparison metrics
Simulate response of scaled and full-scale models
Compare responses upon back-scaling
Draw conclusions
Example: LQR controller outperforms PID by similar amount on V2 and V90
Aeroelastic
Simulation
Performance
Model
Parameters
Scaling
Laws
Aeroelastic
Simulation
Inverse
Scaling Laws

28. Outline Project goals The wind tunnel at the Politecnico di Milano
Wind turbine model scaling and configuration
Aerodynamics
Blade manufacturing
Simulation environment
Data acquisition, control and model management system
Conclusions and outlook

29. Data Acquisition, Control and Model Management System
Wind tunnel control panel
Pitch demand
Torque demand
Remote Control Unit:
Management of experiment (choice of control logic, choice of trim points, etc.)
Data logging, post-processing and visualization
Emergency shut-down
Wind turbine sensor readings:
Shaft torque-meter
Balance strain gages
Blade strain gages (May 2010)
Rotor RPM and azimuth
Blade pitch
Nacelle accelerometer
Wind tunnel sensor readings:
Wind speed
Temperature, humidity
Ethernet
Control PC:
Real time Linux OS (RTAI)
Supervisory control
Control logic:
- Normal mode: pitch-torque control law
- Trimming mode: RPM regulation and pitch setting

30. Outline Project goals The wind tunnel at the Politecnico di Milano
Wind turbine model scaling and configuration
Aerodynamics
Blade manufacturing
Simulation environment
Data acquisition, control and model management system
Conclusions and outlook

31. Conclusions and Outlook
Work is in progress on many fronts, no meaningful conclusions can be drawn at the moment
Work plan:
Initial entry in the wind tunnel by April 2010 (rigid blades, trimming control mode)
- Verification of functionality of all systems, troubleshooting, software debugging
- Verification of aerodynamic performance (measurement of CP-TSR-β & CF-TSR-β curves)
Second entry in May 2010 after fixes/improvements (rigid blades with root strain gages, trimming and normal control modes)
Aerodynamic identification: possible redesign of rotor blades to improve aerodynamic model fidelity (airfoils, transition strips, flaps, etc.)
Blade design and manufacturing:
Implement strain gages in composite rigid blades
Continue development of flexible composite blades
Add strain gages and/or fiber optics to flexible composite blades
Control and management system: complete and improve GUI and functionalities
Full model capabilities: expected end 2010

32. Acknowledgements Research funded by Vestas Wind Systems A/S
The authors gratefully acknowledge the contribution of S. Calovi and S. Cacciola, G. Galetto, L. Maffenini, P. Marrone, M. Mauri, M. Monguzzi, D. Rocchi, S. Rota, G. Sala of the Politecnico di Milano