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