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Turbulence spectra in the atmospheric boundary layer over the sea Pond, Stephen


The work carried out for this thesis forms part of a program to study air-sea interaction processes which has been under way at the Institute of Oceanography of the University of British Columbia during the past four years. Measurements of the spectra of fluctuations of the velocity, temperature, and quantities related to the total rate of dissipation of kinetic energy were made in the air just above the sea surface, Particular attention was focused on the high wave number region of the spectra for which there are predictions for the spectral forms from the theory of turbulence. The measurements of the velocity fluctuations were made with hot-wire anemometers of the constant current type. The temperature fluctuations were measured with a platinum resistance thermometer consisting of a platinum wire 2.5 microns in diameter and about 2mm long. The fluctuations of the quantities related to the dissipation rate were obtained from the velocity fluctuations using an analog computer to perform the necessary operations. The spectral analyses were performed by analog techniques. The measured velocity spectra, when normalized in the required was showed excellent agreement with the measurements of other investigators who worked in different flow fields. The experimental evidence now available provides very strong support for the Kolmogoroff contention that there exists a universal form to the high wave number part of the spectrum of high Reynolds number turbulence. This universal form might be expected to be affected by the observed intermittency of the turbulence. However, such effects, if they exist, must be smaller than the observed experimental scatter of 10 to 15%. The spectrum of the temperature fluctuations shows good agreement with the predictions, obtained from arguments similar to the Kolmogoroff ones for the velocity spectrum, over the rather limited wave number range which it was possible to investigate. The wave number spectra of the fluctuations of the quantities related to the dissipation rate agree with the predictions of Novikov and Stewart (19 64) that these spectra should have a power law form in the inertial subrange with the power of the wave number lying between 0 and -1. One of the requirements for the validity of their prediction is that the flatness factor for the velocity fluctuations and their spatial derivatives should be very much larger than the flatness factor for a Gaussian random variable. The few measurements which were made of flatness factors showed that this requirement is very well satisfied.

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