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Extreme gust measurements - are Dines or cup anemometers the answer?

Extreme gust measurements - are Dines or cup anemometers the answer?. www.cawcr.gov.au. Bob Cechet (Geoscience Australia) John Ginger (James Cook University) John Holmes (JDH Consulting) Jeff Kepert (CAWCR). What is a Dines Anemometer?.

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Extreme gust measurements - are Dines or cup anemometers the answer?

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  1. Extreme gust measurements - are Dines or cup anemometers the answer? www.cawcr.gov.au Bob Cechet (Geoscience Australia) John Ginger (James Cook University) John Holmes (JDH Consulting) Jeff Kepert (CAWCR)

  2. The Centre for Australian Weather and Climate ResearchA partnership between CSIRO and the Bureau of Meteorology

  3. The Centre for Australian Weather and Climate ResearchA partnership between CSIRO and the Bureau of Meteorology

  4. What is a Dines Anemometer? • The “head” is a large-diameter pitot tube, mounted on a vane. Dines anemometer head, Townsville Airport. Photo JCU/CTS. The Centre for Australian Weather and Climate ResearchA partnership between CSIRO and the Bureau of Meteorology

  5. How does it record the wind? Inside the tank. The float. • Tubing carries the pressure signal from the head to the interior of an open-bottomed float (the manometer). • Increased pressure expels water from the float and causes it to rise. Dines float chamber and chart recorder. The Centre for Australian Weather and Climate ResearchA partnership between CSIRO and the Bureau of Meteorology

  6. The Centre for Australian Weather and Climate ResearchA partnership between CSIRO and the Bureau of Meteorology

  7. Why is the Dines important? • How does the float respond to gusts? • Are there resonant frequencies? • Can the float bob up and down? • Australian record wind gusts measured by Dines anemometers: • Cyclone Tracy, Dec 24 1974, 217 km/hr. • Cyclone Trixie, Feb 19 1975, 246 km/hr. • Cyclone Vance, March 23 1999, 267 km/hr. • The Vance measurement had a co-located cup anemometer that measured ~35 km/hr lower. Can we trust the Dines? The Centre for Australian Weather and Climate ResearchA partnership between CSIRO and the Bureau of Meteorology

  8. What are the project aims? • Modelling of the transient response of the float chamber when forced by gusty winds (me). • Measurements of a float chamber forced by varying winds (John Ginger, JCU CTS) • Comparison of Dines and cup anemometer climatologies (Bob Cechet, GA) • Transfer functions (John Holmes) The Centre for Australian Weather and Climate ResearchA partnership between CSIRO and the Bureau of Meteorology

  9. The Centre for Australian Weather and Climate ResearchA partnership between CSIRO and the Bureau of Meteorology

  10. Modelling the Transient Response of the Dines Anemometer www.cawcr.gov.au Jeffrey D. Kepert Head, High Impact Weather Research Weather and Environmental Prediction Program Southern Hemisphere Extreme Winds Workshop, Aug 4, 2010

  11. Modelling: Simplify the geometry Area A Piston x1 Trapped air c(t) -xe xe x2 0 The Centre for Australian Weather and Climate ResearchA partnership between CSIRO and the Bureau of Meteorology

  12. Modelling: The equations. F = ma for the float and water (with linear damping), plus Boyle’s law for the trapped air. Air Float Water The Centre for Australian Weather and Climate ResearchA partnership between CSIRO and the Bureau of Meteorology

  13. Linear solutions Equilibrium water position • Equilibrium solution: • Seeking coupled, linearised solutions of the form: • yields: Equilibrium float position Low frequency, float and water in phase High frequency, float and water out of phase The Centre for Australian Weather and Climate ResearchA partnership between CSIRO and the Bureau of Meteorology

  14. Numerical solutions (no friction) Float position Actual mean float position Expected mean float position Mean and instantaneous water position • In-phase and out-of-phase oscillations present. • Positive bias in mean wind speed (over-speeding) • Trapped air acts as a nonlinear spring. The Centre for Australian Weather and Climate ResearchA partnership between CSIRO and the Bureau of Meteorology

  15. 5. Power Spectrum • Power spectrum (float, water) • Linear frequencies dominate • Numerous harmonics and interharmonics (nonlinear, but possibly not too much) The Centre for Australian Weather and Climate ResearchA partnership between CSIRO and the Bureau of Meteorology

  16. Forced, damped response – resonances! Positive bias in gusts at resonant frequencies Amplitude Resonances and overspeeding occur near linear frequencies. Negative bias in gusts at other frequencies Phase difference Frequency The Centre for Australian Weather and Climate ResearchA partnership between CSIRO and the Bureau of Meteorology

  17. Observations! Amplitude and phase of float Lab. measurements (Borges 1968) Amplitude Amplitude Phase Frequency Phase Frequency The Centre for Australian Weather and Climate ResearchA partnership between CSIRO and the Bureau of Meteorology

  18. Real geometry The Centre for Australian Weather and Climate ResearchA partnership between CSIRO and the Bureau of Meteorology

  19. Standard Dines anemometer resonances • Power spectral density (log scale) of float motion as a function of mean wind speed, standard Dines anemometer. • Oscillation at ~0.5 Hz is “in-phase”, slight wind-speed dependence • Oscillation at 1 – 3 Hz is “out-of-phase”, marked wind-speed dependence The Centre for Australian Weather and Climate ResearchA partnership between CSIRO and the Bureau of Meteorology

  20. … and more observations! CTS Lab. measurements (Henderson et al., 2010) White noise forcing Ratio forcing : response Spectral power (kPa2/Hz) Anemometer response Frequency (Hz) The Centre for Australian Weather and Climate ResearchA partnership between CSIRO and the Bureau of Meteorology

  21. High-speed Dines resonances • Power spectral density (log scale) of float motion as a function of mean wind speed, high-speed Dines anemometer. • Oscillation at ~0.3 Hz is “in-phase” • Oscillation at 1.5 – 4 Hz is “out-of-phase” The Centre for Australian Weather and Climate ResearchA partnership between CSIRO and the Bureau of Meteorology

  22. Conclusions • Dines anemometer has two resonances • low frequency (~0.5 Hz), water and float in phase • high frequency, (1 – 3 Hz) water and float out of phase • System is nonlinear but not strongly so • Excellent agreement between model and observations • Acknowledgements: Dept of Climate Change funding, JCU Cyclone Testing Station, Jeff Callaghan, Bob Cechet, Dave Edwards, John Ginger, Bruce Harper, David Henderson, John Holmes, Paul Leigh, Craig Miller, and Ian Muirhead. The Centre for Australian Weather and Climate ResearchA partnership between CSIRO and the Bureau of Meteorology

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