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Impact of coherent turbulence on wind turbine aeroelastic response and its simulation, awea wind power 2005, denver, co

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

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

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  • 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
  • 3. May 18, 2005 WindPower 2005 - Denver, Colorado 3 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
  • 4. May 18, 2005 WindPower 2005 - Denver, Colorado 4 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.
  • 5. May 18, 2005 WindPower 2005 - Denver, Colorado 5 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
  • 6. May 18, 2005 WindPower 2005 - Denver, Colorado 6 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
  • 7. May 18, 2005 WindPower 2005 - Denver, Colorado 7 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
  • 8. May 18, 2005 WindPower 2005 - Denver, Colorado 8 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
  • 9. May 18, 2005 WindPower 2005 - Denver, Colorado 9 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
  • 10. May 18, 2005 WindPower 2005 - Denver, Colorado 10 Simulating Coherent Turbulence Excitation
  • 11. May 18, 2005 WindPower 2005 - Denver, Colorado 11 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
  • 12. May 18, 2005 WindPower 2005 - Denver, Colorado 12 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
  • 13. May 18, 2005 WindPower 2005 - Denver, Colorado 13 Simulation Example TurbSim NWTC Spectral Model at ART Turbine Hub Height 3 coherent structures added to more homogeneous background turbulent wind field
  • 14. May 18, 2005 WindPower 2005 - Denver, Colorado 14 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) ? ?
  • 15. May 18, 2005 WindPower 2005 - Denver, Colorado 15 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!
  • 17. May 18, 2005 WindPower 2005 - Denver, Colorado 17