the centre for australian weather and climate research a partnership between csiro and the bureau of...
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The Centre for Australian Weather and Climate ResearchA partnership between CSIRO and the Bureau of Meteorology
Extreme gust measurements - are Dines or cup anemometers the answer?
Bob Cechet (Geoscience Australia)
John Ginger (James Cook University)
John Holmes (JDH Consulting)
Jeff Kepert (CAWCR)
www.cawcr.gov.au
The Centre for Australian Weather and Climate Research A partnership between CSIRO and the Bureau of Meteorology
The Centre for Australian Weather and Climate Research A partnership between CSIRO and the Bureau of Meteorology
The Centre for Australian Weather and Climate Research A partnership between CSIRO and the Bureau of Meteorology
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 Research A partnership between CSIRO and the Bureau of Meteorology
How does it record the wind?
•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 float.
Inside the tank.
The Centre for Australian Weather and Climate Research A partnership between CSIRO and the Bureau of Meteorology
The Centre for Australian Weather and Climate Research A partnership between CSIRO and the Bureau of Meteorology
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 Research A partnership between CSIRO and the Bureau of Meteorology
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 Research A partnership between CSIRO and the Bureau of Meteorology
The Centre for Australian Weather and Climate ResearchA partnership between CSIRO and the Bureau of Meteorology
Modelling the Transient Response of the Dines Anemometer
Jeffrey D. KepertHead, High Impact Weather Research
Weather and Environmental Prediction Program Southern Hemisphere Extreme Winds Workshop, Aug 4, 2010
www.cawcr.gov.au
The Centre for Australian Weather and Climate Research A partnership between CSIRO and the Bureau of Meteorology
Modelling: Simplify the geometry
0
x1
Piston
-xeTrapped air c(t)
x2
xe
Area A
The Centre for Australian Weather and Climate Research A partnership between CSIRO and the Bureau of Meteorology
Modelling: The equations.
F = ma for the float and water (with linear damping), plus Boyle’s law for the trapped air.
Wa
ter
Flo
at
Air
The Centre for Australian Weather and Climate Research A partnership between CSIRO and the Bureau of Meteorology
Linear solutions
Equilibrium solution:
Seeking coupled, linearised solutions of the form:
yields:
Equilibrium water position
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 Research A partnership between CSIRO and the Bureau of Meteorology
Numerical solutions (no friction)
• In-phase and out-of-phase oscillations present.
• Positive bias in mean wind speed (over-speeding) • Trapped air acts as a nonlinear spring.
Mean and instantaneous water position
Expected mean float position
Actual mean float position
Float position
The Centre for Australian Weather and Climate Research A partnership between CSIRO and the Bureau of Meteorology
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 Research A partnership between CSIRO and the Bureau of Meteorology
Forced, damped response – resonances!
Resonances and overspeeding occur near linear frequencies.
Positive bias in gusts at resonant frequencies
Negative bias in gusts at other frequencies
Frequency
Ph
ase
diff
ere
nce
Am
plit
ud
e
The Centre for Australian Weather and Climate Research A partnership between CSIRO and the Bureau of Meteorology
Observations!
Amplitude and phase of float Lab. measurements (Borges 1968)
Am
plitu
de
Frequency
Pha
se
Am
plitu
de
Frequency
Pha
se
The Centre for Australian Weather and Climate Research A partnership between CSIRO and the Bureau of Meteorology
Real geometry
The Centre for Australian Weather and Climate Research A partnership between CSIRO and the Bureau of Meteorology
• 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
Standard Dines anemometer resonances
The Centre for Australian Weather and Climate Research A partnership between CSIRO and the Bureau of Meteorology
… and more observations!
CTS Lab. measurements (Henderson et al., 2010)
White noise forcing
Ratio forcing : response
Anemometer response
Frequency (Hz)
Spe
ctra
l pow
er (
kPa2 /
Hz)
The Centre for Australian Weather and Climate Research A partnership between CSIRO and the Bureau of Meteorology
• 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”
High-speed Dines resonances
The Centre for Australian Weather and Climate Research A partnership between CSIRO and the Bureau of Meteorology
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.