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May 18, 2005 WindPower 2005 - Denver, Colorado

1

THE IMPACT OF COHERENT TURBULENCE ON WIND TURBINE AEROELASTIC RESPONSE AND

ITS SIMULATION

Neil D. Kelley1, Bonnie J. Jonkman1, Jan T. Bialasiewicz2, George N. Scott1, Lisa S. Redmond2

1National Renewable Energy Laboratory, Golden, Colorado

2University of Colorado at Denver, Denver, Colorado

May 18, 2005 WindPower 2005 Denver, Colorado

May 18, 2005 WindPower 2005 - Denver, Colorado 2

Outline • Research Objectives

• Brief Overview of Impact of Coherent Turbulence on Wind

Turbines Atmospheric scaling parameters Kelvin-Helmholtz Instability (KHI) in a Stable Boundary Layer Turbulence-Induced Rotor Loading Characteristics Flux of Coherent Turbulent Energy Into Turbine Structure Overall Interpretation of Field Measurement Campaigns

• Simulating Coherent Turbulence Excitation

Conclusions from Field Measurements That Must be Addressed Overview of Simulating a Single Stochastic Inflow Realization Simulation Example of Inflow Containing Coherent Turbulent Structures Comparison of Number of Probabilistic Degrees of Freedom in Spectral Models

• Conclusions


Research Objectives

• To document the impacts of coherent turbulence on wind turbine structures

• To improve existing numerical inflow simulations to include coherent turbulent structures that induce loading events that will impact the longevity and operational reliability of turbine designs meeting the DOE Low-Wind Speed Turbine (LWST) Program goals

• To provide criteria important for site specific design and locating of LWST turbines


Research Approach

• Make simultaneous, detailed measurements of both the turbulent inflow and the corresponding turbine response!

• Interpret the results in terms of how various turbulent fluid dynamics parameters influence the response of the turbine (loads, fatigue, etc.)

• Let the turbine tell us what it does not like!

• Develop the ability to include these important characteristics in numerical inflow simulations used as inputs to the turbine design codes

• Adjust the turbulent inflow simulation to reflect site-specific characteristics or at least general site characteristics; i.e., complex vs homogeneous terrain, mountainous vs Great Plains, etc.


Conclusions from Measurements In San Gorgonio Pass Wind Farm and at NREL’s

National Wind Technology Center • Similar load sensitivities to vertical

stability (Ri) and vertical wind motions were found at both locations

• We found that the turbine loads were also responsive to a new inflow scaling parameter, Coherent Turbulent Kinetic Energy (CTKE) with greater levels of fatigue damage occurring with high values of this variable

• In both locations, the peak equivalent fatigue damage occurred at a slightly stable value of Ri in the vicinity of +0.02

• Clearly, based on both sets of measurements, coherent or organized turbulence played a major role in causing increased fatigue damage on wind turbine rotors

San Gorgonio Micon 65/13

NWTC 600 kW ART


Overall Interpretation of the Field Measurements

• The greatest fatigue damage occurs during the nighttime hours when the atmospheric boundary layer at the height of the turbine rotor is just slightly stable (0 < Ri < +0.05)

• Significant vertical wind shear was also present

• Both of these conditions are prerequisites for Kelvin-Helmholtz Instability or KHI

• The presence of KHI can be responsible for generating atmospheric motions called KH billows or waves which in turn generate coherent turbulence as they breakdown or decay


Using Wavelet Analysis to Observe Time-Frequency Variation of Blade Root Loads Induced by Coherent Turbulence from a Simulated KH Billow Breakdown

• Blade root flapwise load time series

• Scalogram showing dynamic stress levels as a function of time and frequency

• Time series of root loads in 7 frequency (detail) bands using the discrete wavelet transform

• Detail band frequency ranges roughly correspond to groups of modal frequencies including . . .

D9 (0.234 – 0.468 Hz) = 1-P, tower 1st bending mode

D5 (3.750 – 7.500 Hz) = blade bending/torsion/tower

D3 (15.00 – 30.00 Hz) = blade bending/torsion/tower

D6 (1.875 – 3.750 Hz) = blade, tower bending modes D7 (0.936 – 1.875 Hz) = blade 1st bending modes

D4 (7.500 – 15.00 Hz) = blade/tower interactions


Turbulence Contains a Spectrum Of Eddy Sizes and Intensities

Frequency, f (Hz)

0.0001 0.001 0.01 0.1 1 10

Turb

ulen

t kin

etic

ene

rgy,

TK

E(f)

(m2 /s

2 )

0.0001

0.0010

0.0100

0.1000

1.0000

0.0001

0.0010

0.0100

0.1000

1.0000

Turbulent Kinetic Energy TKE(f)

(m2/s2)

Distribution of Inflow Turbulent Energy with Frequency

Schematically . . .

Frequency, f (Hz)

0.0001 0.001 0.01 0.1 1 10

Turb

ulen

t kin

etic

ene

rgy,

TK

E(f)

(m2 /s

2 )

0.0001

0.0010

0.0100

0.1000

1.0000

0.0001

0.0010

0.0100

0.1000

1.00001 hour 1 min 1 sec

Distribution of Inflow Turbulent Energy with Frequency

Frequency, f (Hz)

0.0001 0.001 0.01 0.1 1 10

Turb

ulen

t kin

etic

ene

rgy,

TKE

(f) (m

2 /s2 )

0.0001

0.0010

0.0100

0.1000

1.0000

0.0001

0.0010

0.0100

0.1000

1.00001 hour 1 min 1 sec

Distribution of Turbulence-Derived Electrical Energy At Output of Generator

Parasitic Energy Needed To Be Dissipated by Turbine Structure


Energy Flux from Coherent Turbulence (CTKE) to Blade Dynamic Pressure at 78% Span Under Three

Inflow Conditions Wavelet Continuous Transform Co-Scalograms of CTKE and qc

• Steady, High Shear (α = 1.825)

• Slightly stable (Ri = + 0.05)

• Steady, equilibrium flow conditions

• IEC Kaimal NTM (α = 0.2)

• Neutral stability (Ri = 0)

• Steady, equilibrium flow conditions

• Breaking KH Billow (αo = 1.825)

• Slightly stable (Ri = +0.05)

• Unsteady, non-equilibrium, flow conditions

CTKE Time Series

Dynamic Pressure, qc Time Series

May 18, 2005 WindPower 2005 - Denver, Colorado

10

Simulating Coherent Turbulence Excitation


Conclusions from Field Measurement Programs That Must Be Addressed in the Simulation of Inflow

Turbulence

• Large load excursions are generally associated with encountering organized or coherent turbulent elements in the inflow even when distinct “gusts” are not present

• Stably stratified inflows, associated with the nocturnal atmospheric boundary layer, are the primary source of coherent turbulent structures affecting wind turbines

• Coherent turbulent structures are generated by non-stationary and non-Gaussian processes that produce inhomogeneous flow elements that are correlated in both time and space (spatiotemporal) and are not adequately being reproduced by currently available inflow simulations which limit the number and severity of large load excursions generated by the design codes

• Coherent turbulent structures induce narrowband excitation of the turbine vibration mode shapes that can produce large load excursions through the superposition and raising the possibility of local dynamic amplification of stresses at the equivalent modal frequencies within the turbine structure


Generate coherent turbulent structures

Generate quasi-homogenous background turbulence field

Spectral Representation (Veers) Approach for Simulating a Single Realization of a Stochastic Turbulent Inflow for a Given

Turbine Operating Envelope Using the NREL TurbSim Code

Generate Time Series of U,V,W wind components

at Y-Z Grid Points with IEC Kaimal Spectral &

U-component Coherence Models

Choice of Turbulence Spectral Model . . .

• Smooth Terrain

• Wind Farm Related (3)

• NWTC (complex terrain)

To Generate Time Series of U,V,W wind components on Y-Z Grid

Randomly Create Spatiotemporal Coherent Structures as Scaled by Inflow Boundary Conditions and Requested Spectral Model

Hub Mean Wind Speed

Turbulence Level (A,B,C)

Random Seed

IEC Specifications

Hub Mean Wind Speed

Turbulence Level (u*)

Rotor Layer Stability (Ri)

Rotor Layer Shear Exponent

Optional User-defined Parameter Values

Random Seed

General & Site Specific


Simulation Example TurbSim NWTC Spectral Model

at ART Turbine Hub Height

3 coherent structures added to more homogeneous

background turbulent wind field


Comparison of Maximum Number of Probabilistic Degrees of Freedom of TurbSim Turbulence Spectral Models for a Given

Set of Inflow Boundary Conditions

Spectral Model

Max Stochastic Degrees of Freedom

Number of Spectral Peaks

per Stability Class

IEC Kaimal

1

1 (neutral)

Smooth Terrain

7

2 – unstable

1 – neutral, stable

Wind Farm

7

3 – unstable


NWTC (complex terrain)

9

2 – unstable


GP_LLJ (future)

?

?


Conclusions

• Purely Fourier-based inflow simulation techniques cannot adequately reproduce the transient, spatiotemporal velocity field associated with coherent turbulent structures

• Spatiotemporal turbulent structures exhibit strong transient features which in turn induce complex transient loads in wind turbine structures

• The encountering of patches of coherent turbulence by wind turbine blades can cause amplification of high frequency structural modes and perhaps increased local dynamic stresses in turbine components that are not being adequately modeled with current inflow simulations

• The TurbSim stochastic inflow simulator has been designed to provide such a capability for both general and site specific environments

Thanks for your attention!