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Experimental study on the coherent structure of turbulent open-channel flow using visualization and picture processing

Published online by Cambridge University Press:  21 April 2006

Tadashi Utami  and
Tetsuo Ueno
Tadashi Utami
Affiliation:
Ujigawa Hydraulic Laboratory, Disaster Prevention Research Institute. Kyoto University, Yoko-Ohji, Fushimi-Ku, Kyoto, 612 Japan
Tetsuo Ueno
Affiliation:
Ujigawa Hydraulic Laboratory, Disaster Prevention Research Institute. Kyoto University, Yoko-Ohji, Fushimi-Ku, Kyoto, 612 Japan
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Abstract

Coherent structures of turbulent open-channel flow in the wall region of a channel bed were examined quantitatively using experimental data obtained by flow visualization. Successive pictures of flow patterns in two horizontal cross-sections at different levels near the channel bed were taken, and then were digitized and analysed by a computer.

This method of flow visualization and picture processing enabled us to calculate the distributions of the three components of the velocity vectors. The distributions of velocities, streamlines, two-dimensional divergence and three components of vorticity could be calculated and are displayed as graphical output. In our numerical analyses, the idea of a two-dimensional correlation coefficient is introduced, through which the degree of similarity of turbulence structures can be better estimated than with the usual one-dimensional coefficient. Use of the data was based on the premise that the essential element in a turbulence structure is vortex motion.

We propose a conceptual model of turbulence structure in which the elementary unit of coherent structure in the buffer layer is presumed to be a horseshoe vortex and in which the characteristics of the multiple structure of turbulence are shown with respect to the scale, arrangement and generating process of horseshoe vortices and longitudinal vortices. Our model clearly explains the generating mechanism and mutual relations of low-speed regions, high-speed regions, ejections, sweeps and localized free-shear layers.

Type
Research Article
Information
Journal of Fluid Mechanics , Volume 174 , January 1987 , pp. 399 - 440
Copyright
© 1987 Cambridge University Press

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