Tadashi Utami

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

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.

MEASUREMENT OF FIOOD FLOW DISCHARGE USING PICTURE PROCESSING OF AERIAL PHOTOGRAPHS

Aerial photographs of 1998 flood flow in the Tone River were analyzed by cross-correlation method to obtain velocity vectors over the flow surface. Characteristic features of flow downstream of bridge piers were examined by using the data. It was pointed out the discharge measurement by float estimates larger discharge than real one. Features of velocity distribution and of the cross section profile of river bed were examined to get the guide-line on choosing the appropriate cross-section for discharge measurement.

TRIAL OF SEDIMENT DISCHARGE MEASUREMENTS UNDER FLOODS IN THE HII RIVER

The river-bed configurations were measured by a echo-sounder attached to a cross-type float under floods in the Hii River which is the typical sand-bedded river in Shimane Prefecture. At the same time, time series of photographs were taken of the flow patterns on the flow surface, and the vertical distributions of the velocity and turbidity were also measured.In this paper, by using the data, moving velocities of dunes were obtained at every stage of the floods, and approximate estimations of discharge rate of bed load and suspended load were tried.

Picture Analysis of Aerial Photographs of Flood Flows

A pair of aerial photographs of the flood flow in the Agano River on Sept. 25, 1966 was analyzed by the cross-correlation technique to obtain velocity vectors over the area of 1700 × 1000 m of the water surface. About 340 × 400 velocity vectors were obtained at every 5.0 m in streamwise and 2.5 m in spanwise direction.The interval and arrangement of the longitudinal low-speed band and high-speed band appearing alternately in the spanwise direction over the water surface were analyzed and two kinds of spanwise scales of large scale turbulence were found; one is about twice water depth and another is 4 5 times.

Picture Processing of Aerial Photographs of a Flood Flow by the Cross-Correlation Technique

A pair of aerial photographs of flood flow of Ishikari River were analysed by picture processing to obtain two-dimensional distribution of velocity vectors of the flood surface. Streamline patterns, vorticity and divergence were calculated using the obtained velocity data and the structure of the vortices and its relation with river sedimentation were elucidated.Strong upwellings (boils) in the vortices and strong downflows around the edge of the vortices were shown to be the main cause of the formation of the core of scroll bars.

Investigation of the Coherent Structure of Turbulent Open Channel Flow by Two-Dimensional Pattern Recognition

The horizontal cross-sections of the uniform open channel flow was visualized and twodimensional velocity distribution in each cross-section was obtained by picture processing.Two-dimensional pattern recognition method was introduced into the obtained velocity data. Two-dimensional patterns of vorticity distributions was proposed to detect the typical arrangement of vortex motions.Ensemble averaged pattern of two-dimensional distribution of velocity vectors, x- and zcomponents of velocity vectors, vorticity and two-dimensional divergence around the detected points were shown. Based on these patterns, mechanism of sweeps and ejections, and onedimensional methods to detect these events were examined.