engleski

Determination of turbulence averaging interval by Fourier analysis

Lengths of averaging intervals applicable for investigation of intermitted turbulence during the nighttime stable, wintertime conditions are determined by two methods, both based on Fourier analysis. Wind data were measured by five ultrasonic anemometers with a sampling frequency of 5 Hz. Anemometers were placed at a mast located in a suburban, industrial area of Kutina, Croatia. Corresponding measurement heights were 20, 32, 40, 55 and 62 m above the ground, respectively. The mast is located above the grassy surface, surrounded by approximately 20 m high trees. The closest trees are about 20 – 30 meters far from the mast. The first method for determination of the length of the averaging interval is based on a cummulative integral of cospectrum of perturbances ogive) of the wind speed component and sonic temperature. The averaging interval is taken as a value at which ogive converges to a constant value. Second method assumes the length of averaging interval as an inverse of frequency at which a minimum in the velocity spectrum occurs. Variations of the lengths of averaging intervals with the height obtained by both methods are inspected and the differences between results obtained by the two methods are discussed.

turbulence; ultrasonic anemometer; 5 Hz

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