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Influence of Turbulent Flow Structure on Particle Motion over a Rough Bed Consisting of Hemispherical Roughness Elements

Author(s): Kazumasa Matsumoto; Taka-Aki Okamoto; Michio Sanjou; Tetsuya Oishi

Linked Author(s): Michio Sanjou

Keywords: Turbulent flow; Particle motion; Flow over a rough wall; PIV

Abstract: Turbulent flow and its influence on transport phenomena over a rough surface have been investigated in a wide range of fields in engineering. For example, when a flood occurs in natural rivers, fine sediments and seeds of plants are conveyed by flows and captured between river bed gravels, which leads to sediment deposition and development of riverine vegetation. Therefore, it is important to explore the characteristics of flow over a rough bed, including both mean and turbulent flow, and their impact on motions of particles in flow. In this study, two sets of laboratory experiments were carried out to clarify the mechanism of mass and momentum transport in a gravel bed river. First, PIV measurements were carried out for flow fields over a rough wall, and their results were analyzed to reveal the structure of mean and turbulent flow. Hemispherical roughness elements were placed in a staggered array on a bottom of an experimental flume to mimic a natural river bed composed of gravels with large grain size. Results of PIV revealed coherent structure of turbulent flow near a rough wall. Next, to understand the process of mass transport over a rough wall, seeds of a real plant were added in the flume and their motions were observed. Behavior of the particles in the flow was photographed by two video cameras installed above and to the side of the channel, and some of their trajectories were tracked in three dimensions by analyzing the taken images. From several examples of the trajectories, some typical patterns of particle behavior were observed. Results of the observation suggest that coherent structure of turbulent flows such as sweep and ejection in the vicinity of roughness may determine behavior of particles such as upward and downward motion.


Year: 2022

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