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Relevant criteria for testing the quality of turbulence models

Relevant criteria for testing the quality of turbulence models. Sten Frandsen Hans E. Jørgensen, John Dalsgaard Sørensen Risø/DTU and AUC EWEC2007 Session DS2 – paper 56 – May 10, 2007. Structure. Aerodynamics. Response. Atmospheric turbulence. Problem.

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Relevant criteria for testing the quality of turbulence models

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  1. Relevant criteria for testing the quality of turbulence models Sten Frandsen Hans E. Jørgensen, John Dalsgaard Sørensen Risø/DTU and AUC EWEC2007 Session DS2 – paper 56 – May 10, 2007

  2. Structure Aerodynamics Response Atmospheric turbulence Problem • A simulator of atmospheric turbulence is a crucial part of any wind turbine response simulation code • The IEC standard’s turbulence models for aeroelastic codes give divergent results • Extrapolation of loads for identification of lifetime extreme is presently a big issue, but we have a chain of models: • the result is not better than what is put in (turbulence): • Thus, IEC61400-1 gives specific models • it should rather specify how turbulence models shall/must work to emulate the atmospheric flow.

  3. Mission • Devise requirements that will make sure that – when followed – different turbulence models will yield the same response with any particular aerodynamic/structural model • Candidate characteristics to be evaluated: • scale of turbulence • gust factor • (extreme) distribution of gusts • coherence function factor

  4. How is turbulence characteristics identified? • By full scale measurements

  5. The Risø/DTU wind turbine test site – overview • 5 Wind Turbines • 5 Wind Turbine Masts (70-116m) • 2 Obstruction Lighting masts (165m) • Meteorological Mast (116m)

  6. Instrumentation • Wind shear and turbulence • Sonic anemometers in 6 levels • Wind speed in 7 levels • Wind direction in 3 levels • Absolute temperature in 2 levels • Temperature difference in 5 levels • Also, a 160 m tower with measuring levels at 160, 100 and 60 m

  7. Scale of turbulence IEC61400-1 - Kaimal: Fitting data to Kaimal:

  8. Scale of turbulence – height-dependency • In average IEC • fits quite well • COV is 50% Scale of turbulence as function of height. Wind speed at H=100m: 10m/s<U<12m/s. The “error bars” represent the standard deviation of the observations.

  9. Scale of turbulence – wind speed and wake dependency Luunder wake conditions

  10. Gust factor kp Cartwright+Longuet-Higgins (1956): For Kaimal spectrum, the up-crossing frequency is: Tis the time duration of each considered period – typical 10min fCis the structural cut-off frequency – the rotor/blade filter frequency

  11. Gust factor kp – pre-averaging time • Most often, a 3-sec “running” average of recorded wind speed is performed, assuming that this time averaging represents the spatial averaging of the structure. • By integrating the coherence function of the wind, the “rotor filter” may be found Comparison of the filters: This way it is found that the wind speed data should be pre-average over 5 to 20 sec to emulate the rotor filter

  12. Gust factor kp – pre-averaging time Model works qualitatively but estimates10% too high Gust factor as function of pre-averaging time, measurement height 100m and wind speed 10m/s<U<12m/s.

  13. Gust factor kp– height-dependency Gust factor as function of height for different wind speeds. Sinc-filter frequency is 1/3 Hz. Gust factor as function of height for different wind speeds. Sinc-filter frequency is 1/10 Hz. IEC61400-1: For relevant averaging times, kp is set 25% too high

  14. wake Gust factor kp – wake effects Gust factor only marginally different under wake conditions

  15. Distribution of extreme gusts Return period for Gust factor based on 10 sec averaging data for 80m height (50 largest data). Density function for Gust factor based on 10 sec averaging data for 80m height (201 data points). Gumbel distribution fits the data well, especially at higher kp values

  16. Coherence of turbulence In near-future: experimental determination of b: • Directly from wind speed measurements • Indirectly from tower bending moments Also previous work will be included IEC coherence: where b = 12, but should rather be of order 5 to fit Mann model

  17. Conclusion • IEC scale of turbulence qualitatively good though 10% too high • Scale of turbulence is function of wind speed, for heights relevant for MW wts • IEC gust factor approx. 25% too high – approx. 1 std • Distribution of gust factor should be Gumbel • Proposal for amendment to IEC61400-1: • specify that selected turbulence model shall yield (output) a flow field where • mean and std. of Lu ,kp and b are within given limits • distribution of (extreme) gusts must be Gumbel, with given values of scale and shape parameters

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