Study of transportation of fine sediments by flowing water

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Bubbly flow measurements in hydraulic jumps with small inflow Froude numbers

The transition from supercritical to subcritical open channel flow is characterised by a strong dissipative mechanism called a hydraulic jump. A hydraulic jump is turbulent and associated with the development of large-scale turbulence and air entrainment. In the present study, some new physical experiments were conducted to characterise the bubbly flow region of hydraulic jumps with relatively small Froude numbers (2.4 < Fr(1) < 5.1) and relatively large Reynolds numbers (6.6 x 10(4) < Re < 1.3 x 10(5)). The shape of the time-averaged free-surface profiles was well defined and the longitudinal profiles were in agreement with visual observations. The turbulent free-surface fluctuation profiles exhibited a peak of maximum intensity in the first half of the hydraulic jump roller, and the fluctuations exhibited some characteristic frequencies typically below 3 Hz. The air-water flow properties showed two characteristic regions: the shear layer region in the lower part of the flow and an upper free-surface region above. The air-water shear layer region was characterised by local maxima in terms of void fraction and bubble count rate. Other air-water flow characteristics were documented including the distributions of interfacial velocity and turbulence intensity. The probability distribution functions (PDF) of bubble chord time showed that the bubble chord times exhibited a broad spectrum, with a majority of bubble chord times between 0.5 and 2 ms. An analysis of the longitudinal air-water structure highlighted a significant proportion of bubbles travelling within a cluster structure. (C) 2011 Elsevier Ltd. All rights reserved

Two-phase flow measurements in turbulent hydraulic jumps

Hydraulic jumps are commonly experienced in industrial applications and manufacturing processes, aswell in rivers and canals. The hydraulic jump is the sudden transition from a high-velocity open channel flow to a subcritical flow motion. Despite nearly two centuries of studies, there is still a lack of knowledge on the two-phase flow properties of the turbulent shear layer and the roller. New series of experimental measurements were conducted in hydraulic jumps with Froude numbers between 5 and 8.5, and inflow Reynolds numbers between 38,000 and 62,000. The two-phase flow measurements included some vertical profiles of void fraction, bubble count rate, interfacial velocity and bubble chords. In the air–water shear region, the void fraction and bubble count rate distributions exhibited marked peaks, with the maximum of void fraction always above the location of the maximum bubble count rate. The dimensionless turbulent diffusivity coefficient was estimated. The dimensionless distributions of interfacial velocity compared favourably with some wall-jet flow equations. The data analysis showed that the mean bubble chord in the turbulent shear layer was between 1 mm and 6 mm. The probability distribution functions (PDF) of bubble chord time illustrated a broad spectrum with predominance to small bubbles compared to the mean