Measurements Of Eddy Dissipation Rate In The Atmosphere

Overview

The direct measurement of the dissipation rate of turbulent kinetic energy in the atmosphere is an intricate task given the complexity of the instrument package needed, the difficulty of calibrating hot film probes necessary for the measurements, and other logistical constraints. Yet, the dissipation rate is a key parameter used in characterizing atmospheric turbulence, hence indirect measurements are used for evaluating it for such applications as wind turbine placement and the issuance of Clear Air Turbulence ...
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Overview

The direct measurement of the dissipation rate of turbulent kinetic energy in the atmosphere is an intricate task given the complexity of the instrument package needed, the difficulty of calibrating hot film probes necessary for the measurements, and other logistical constraints. Yet, the dissipation rate is a key parameter used in characterizing atmospheric turbulence, hence indirect measurements are used for evaluating it for such applications as wind turbine placement and the issuance of Clear Air Turbulence warnings. One of the major technical challenges for direct dissipation measurements is the variability of mean winds that preclude the application of hot film probes from flow and dissipation rate measurements. As such, probes require mean winds to be perpendicular to or inclined at a small angle to the probe wire, which is difficult to achieve under variable wind directions. To avert this problem a special instrumentation platform was developed, which tracks the wind speed and direction using a sonic anemometer and automatically directs the hot film system to be aligned with the mean wind. Two x-wire hot film probes were used for the measurement of all three velocity components. A calibration facility was also developed to calibrate the two probes used in the system. A standard polynomial fitting technique was applied to generate calibration curves based on 77 calibration points where voltage, angle, and mean velocity are known. This calibration method also served as a reference when comparing the novel, neural network based calibration approach based on in-situ input from a sonic anemometer. The hot film system was deployed at a field facility located in Phoenix, Arizona, and measurements were made during the day and night over a period spanning from March 29, 2008 to April 10, 2008. Skewness factors measured by hot films were consistent for stable, neutral, and convective atmospheric boundary layer conditions with an approximate value of -0.4 while those measured by sonic were almost 50 percent greater for the stable and neutral cases and 50 percent lower for the convective case. Measured dissipation rates converged only for the convective case. It is suggested that due to sharp velocity jumps inherent in the sonic signal, spurious spectra is produced close to the f to the -5/3 power form but which does not have dynamics requisite of turbulent flow. It was shown that a neural network of hot films, which is based on inputs from in-situ velocity data acquired from the sonic, is feasible and has the potential to greatly improve the logistics of using hot films in outdoor environments.
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Product Details

  • ISBN-13: 9781243420114
  • Publisher: BiblioLabsII
  • Publication date: 9/2/2011
  • Pages: 104
  • Product dimensions: 7.44 (w) x 9.69 (h) x 0.22 (d)

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