Free-Surface Turbulent Fluctuations and Air-Water Flow Measurements in Hydraulics Jumps with Small Inflow Froude Numbers

Chachereau, Yann and Chanson, Hubert (2010) Free-Surface Turbulent Fluctuations and Air-Water Flow Measurements in Hydraulics Jumps with Small Inflow Froude Numbers. Hydraulic Model Report CH Series CH78/10, School of Civil Engineering, The University of Queensland.

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Author Chachereau, Yann
Chanson, Hubert
Title Free-Surface Turbulent Fluctuations and Air-Water Flow Measurements in Hydraulics Jumps with Small Inflow Froude Numbers
School, Department or Centre School of Civil Engineering
Institution The University of Queensland
Open Access Status Other
Report Number CH78/10
Series Hydraulic Model Report CH Series
Publication date 2010
Start page 1
End page 133
Total pages 133
Publisher School of Civil Engineering, The University of Queensland
Editor Hubert Chanson
Language eng
Subject 09 Engineering
0905 Civil Engineering
090509 Water Resources Engineering
Formatted Abstract/Summary
The transition from supercritical to subcritical open channel flow is characterised by a strong dissipative mechanism called a hydraulic jump. A hydraulic jump is extremely turbulent and is associated with the development of large-scale turbulence, surface waves and spray, energy dissipation and air entrainment. The jump is a region of rapidly-varied flow and the large-scale turbulence region is usually called the "roller". In the present study, some new experiments were conducted in hydraulic jumps with inflow Froude numbers between 2.4 and 5.1. A number of experimental techniques were used. Some dynamic free surface measurements were performed with acoustic displacement meters to record the mean and turbulent surface profiles, characteristic frequencies and integral length and time scales. The two-phase flow measurements were conducted with a dual-tip conductivity probe. Further some measurements of free surface fluctuations and two-phase properties were conducted simultaneously.
The free surface fluctuations were investigated for inflow Froude numbers between 2.4 and 5.1. The shape of the time-averaged free surface profiles was well defined and the longitudinal profiles were in agreement with visual and photographical observations, as well as previous studies. The turbulent fluctuation profiles exhibited a peak of maximum intensity in the first half of the hydraulic jump roller. The amplitude of this peak increased with increasing Froude number. The free surface fluctuations exhibited some characteristic frequencies typically below 3 Hz. Some simultaneous free-surface measurements at a series of two closely located points yielded the free surface length and time scales of free-surface fluctuations. The maximum cross-correlation coefficient between the free surface fluctuations at two different locations decreased exponentially with increasing distance between the sensors, while the integral length and time scales increased with increasing longitudinal distances from the jump toe. The simultaneous measurements of instantaneous void fraction and free surface fluctuations exhibited distinct features in the different regions of the roller: a positive correlation in the shear layer region, and a negative correlation in the free surface region. The acoustic displacement meters yielded a time-averaged free-surface elevation that corresponded to the upper free-surface where the void fraction increased rapidly from 0.20 to 0.90, and the quantitative values were close to the equivalent clear-water depth.
The air-water flow properties were investigated for Fr1 = 3.1 to 5.1. The vertical profiles of void fraction 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 denoted Cmax and Fmax respectively. Other air-water flow characteristics were documented including the distributions of interfacial velocity, turbulence intensity and integral turbulent time scales. Both the turbulence levels and the air-water turbulent integral time scales exhibited an increase with increasing distance to the bed. 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.
For a Froude number Fr1 = 5.1, the present results were compared with earlier data obtained with the same Froude number but smaller Reynolds numbers. The comparative analysis encompassed 25,000 < Re < 125,000. The results showed that the Froude similitude was not satisfied in a hydraulic jump for Fr1 = 5.1 within the range of Reynolds numbers. The void fraction data obtained with Re < 40,000 could not be scaled up to larger Reynolds numbers. The bubble count rate data, turbulence properties and bubble chord data exhibited some monotonic trends with increasing Reynolds numbers implying that the results could not be extrapolated to large-size prototype structures without significant scale effects.
Keyword Hydraulic jumps
Free-surface fluctuations
Turbulence
Air bubble entrainment
Air-water flow properties
Signal processing
Physical modelling
Scale effects
Additional Notes The full bibliographic details are: CHACHEREAU, Y., and CHANSON, H., (2010). "Free-Surface Turbulent Fluctuations and Air-Water Flow Measurements in Hydraulics Jumps with Small Inflow Froude Numbers." Hydraulic Model Report No. CH78/10, School of Civil Engineering, The University of Queensland, Brisbane, Australia, 133 pages (ISBN 9781742720036).

Document type: Department Technical Report
Collection: School of Civil Engineering Publications
 
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Created: Wed, 18 Aug 2010, 10:09:36 EST by Hubert Chanson on behalf of School of Civil Engineering