poli di mi tecnicolanotecnicolano wt 2 : the wind turbine in a wind tunnel project c.l. bottasso, f....

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WT2:the Wind Turbine in a Wind Tunnel

Project

C.L. Bottasso, F. CampagnoloPolitecnico di Milano, Italy

Spring 2010

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POLITECNICO di MILANO Poli-Wind Research Lab

OutlineOutline

• 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

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Project GoalsProject 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 - …

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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

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Turbulence (boundary layer) generators



Turn-table

13 m

• 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)

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OutlineOutline

• Project goals



• Aerodynamics





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Model ScalingModel Scaling

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

QuantityScaling 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 1/1.97

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

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

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CONICAL SPIRAL TOOTHED GEARS

Rotor radius = 1m

Balance (6 force/moment

components)

Height = 2.8 m

Up-tilt = 6 deg

Electronic board for blade strain

gages

V2 Model ConfigurationV2 Model Configuration

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Conical spiral gears

Main shaft with torque meter

Pitch actuator control units:• Faulhaber MCDC-

3003 C• 30 V – 10 A Max• Position and speed

Slip ring Moog AC6355:• 36 Channels• 250 V – 2 A Max

Torque actuator:• Portescap Brushless

B1515-150 • Pn = 340 W• Planetary gearhead • Torque and speed control

Cone = 4 deg


Pitch actuator:• Faulhaber 1524• Zero backlash

gearhead• Built-in encoder IE

512

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Wind turbine model shown without nacelle and tower covers, for clarity

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OutlineOutline

• Project goals



• Aerodynamics





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CP

TSR

Region II1/2P<Pr

Ω=Ωr

Region III

P=Pr

Ω=Ωr

BEM Predicted Aerodynamic Performance


Region IICPopt

λopt

βopt

• Good agreement in full load region III

• Poorer agreement in partial load regions II and II1/2, due to higher drag of V2 airfoils

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POLITECNICO di MILANO Poli-Wind Research Lab Filippo Campagnolo



CF

TSR

Region II1/2P<Pr

Ω=Ωr

Region III

P=Pr

Ω=Ωr

Region IICPopt

λopt

βopt

Good agreement between thrust coefficients in the entire working region, due to good lift characteristics of V2 airfoils

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Aerodynamic IdentificationAerodynamic 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:

1. Measure power and thrust coefficients

2. Parameterize airfoil lift and drag coefficients

3. Identify airfoil aerodynamic parameters that best match wind turbine performance, using a BEM model of the rotor

(Work in progress, results expected summer 2010)

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Aerodynamic identification

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

Constrained optimization: • Goal: match CP & CF at tested TSR &

β• Unknowns: parameters describing

airfoil CL & CD characteristics• Rotor model: BEM

Experimental CP & CF coefficients

• Trim at varying pitch β and TSR

• Measure power CP and thrust CF

CD

a

a

Design dataIdentified data

CL

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OutlineOutline

• Project goals



• Aerodynamics





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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

Blade ManufacturingBlade Manufacturing

CAD model for CNC machining, with support tabs (+resin support)

Carbon blades (will include blade-root strain gage in 2nd blade set – May 2010)

FEM verification of strain gage sensitivity

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Blade ManufacturingBlade Manufacturing

Aero-elastically scaled blades:

• 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)

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Structuraloptimization

Design of the V2 Aero-elastically Scaled Composite Blade

WidthChordwise Position

Thickness

Sectional optimization variables (position, width, thickness)Span-wise shape function interpolation

Optimization

Cross sectional analysis

Equivalent beam model

ANBA (ANisotropic Beam Analysis) FEM cross sectional model:• Evaluation of cross sectional

stiffness (6 by 6 fully populated matrix)

Objective: size spars (width, chordwise position & thickness) for desired sectional stiffness within mass budgetCost function: sectional stiffness error wrt target (scaled stiffness)Constraints: lowest 3 frequencies

Rohacell core with grooves for the housing of carbon fiber spars

Thermo-retractable film

Carbon fiber spars for desired stiffness

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Filippo Campagnolo

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 33.1

Mass gap can be corrected with weights

Solid line: scaled reference values

Dash-dotted line: optimal sizing

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Filippo Campagnolo

Approach:

1. 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

2. Demonstration of technology on blade-like specimen (twist, variable chord)

Status: in progress

3. Manufacture wind turbine model blade

Status: to be done (expected end 2010)

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Characterization of material properties:

Specimen of uniform properties:

Results:

• Good matching of lowest natural frequencies

• Acceptable repeatability

• Good shape and finishing

Demonstration of Technology on Simple Specimen

Demonstration of Technology on Simple Specimen

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 %

Dynamic testing Static testing Temperature–dependent characterization

Carbon fiber spars

Airfoil cross section

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OutlineOutline

• Project goals



• Aerodynamics





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Controller

Sensor modelsVirtual plant

Cp-Lambda model

WindMeasureme

nt noise

SupervisorStart-up, power production,

normal shut-down, emergency shut-down, …

Pitch-torque controller

Simulation EnvironmentSimulation Environment

Comprehensive aero-elastic simulation environment: supports all phases of the wind turbine model design (loads, aero-elasticity, and control)

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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 61400 compliant (DLCs, wind models)

Cp-Lambda (Code for Performance, Loads, Aero-elasticity by Multi-Body Dynamic Analysis):Global aero-servo-elastic FEM model

• Rigid body

• Geometrically exact beam

• Revolute joint

• Flexible joint

• Actuator

ANBA (ANisotropic Beam Analysis) cross sectional model

Compute sectional stiffness

Recover cross sectional

stresses/strains

Simulation ModelsSimulation Models

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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

Simulation EnvironmentSimulation Environment

ModelParameters

AeroelasticSimulation

AeroelasticSimulation

ScalingLaws

InverseScaling Laws

Performance

Example: LQR controller outperforms PID by similar amount on V2 and V90

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OutlineOutline

• Project goals



• Aerodynamics





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Data Acquisition, Control and Model Management System

Data Acquisition, Control and Model Management System

Control PC:• Real time Linux OS (RTAI)• Supervisory control• Control logic:- Normal mode: pitch-torque control law- Trimming mode: RPM regulation and pitch setting

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 tunnel control panel

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

• Pitch demand• Torque

demand

Ethernet

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OutlineOutline

• Project goals



• Aerodynamics





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Conclusions and OutlookConclusions 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

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AcknowledgementsAcknowledgements

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

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