Presentation discussing the impact of organized or coherent turbulent on wind turbine aeroelastic response and its simulation.

1. 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

2. 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

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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

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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.

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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

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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

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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

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Turbulence Contains a Spectrum Of
Eddy Sizes and Intensities
Frequency, f (Hz)
0.0001 0.001 0.01 0.1 1 10
Turbulentkineticenergy,TKE(f)(m2
/s2
)
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
Turbulentkineticenergy,TKE(f)(m2
/s2
)
0.0001
0.0010
0.0100
0.1000
1.0000
0.0001
0.0010
0.0100
0.1000
1.0000
1 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
Turbulentkineticenergy,TKE(f)(m2
/s2
)
0.0001
0.0010
0.0100
0.1000
1.0000
0.0001
0.0010
0.0100
0.1000
1.0000
1 hour 1 min 1 sec
Distribution of Turbulence-Derived
Electrical Energy At Output of Generator
Parasitic Energy Needed To Be
Dissipated by Turbine Structure

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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

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Colorado
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Simulating Coherent Turbulence
Excitation

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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

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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

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Simulation Example
TurbSim NWTC Spectral Model
at ART Turbine Hub Height
3 coherent structures
added to more homogeneous
background turbulent wind field

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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
2 – neutral, stable
NWTC
(complex terrain) 9
2 – unstable
2 – neutral, stable
GP_LLJ
(future) ? ?

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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

16. Thanks for your attention!

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