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Statistics of coherent structures in turbulent fluid flow Loewen, Stuart Reid


This thesis compares power spectra produced from hot-film anemometery measurements in turbulent flow with those generated from the distribution of eddy sizes obtained from photographs of the same flow. The coherent structures were made visible by photographing the paths of aluminum tracers on the surface of a water filled towing tank. A parallel row of bars was used to generate the turbulent flow. Flow Reynold's numbers, based on bar spacing, of about 20,000 were studied. A simple analysis of the flow patterns photographed proved adequate to predict power spectra consistent with those measured using a hot-film anemometer and spectrum analyzer. The frequency range and shape of the eddy power spectra were similar to the hot-film ones but were consistently lower in magnitude. The integral length scales obtained from the power spectra agree within 30% and it was found that the average eddy size predicted the length scale obtained from the eddy produced power spectra. Successive photographs of the flow as well as the change in the eddy size distributions with distance from the grid showed evidence of eddy evolution. The results of this thesis suggest that new knowledge and understanding of the interaction of coherent structures in a turbulent fluid flow can be obtained from such flow visualization experiments. This study is part of a new model for turbulence in which the flow is described as a superposition of coherently rotating eddies and laminar flow. In this model we attempt to derive macroscopic properties of a turbulent flow from the interactions of eddies with the fluid, fluid flow and each other. The eddy size distribution can also be predicted from an understanding of the eddy interactions. The statistical description of turbulence and the often used k -power spectrum approach are reviewed and compared with our new model.

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