Advective Diffusion of Air Bubbles in Hydraulic Jumps with Large Froude Numbers: an Experimental Study

Chanson, Hubert (2009) Advective Diffusion of Air Bubbles in Hydraulic Jumps with Large Froude Numbers: an Experimental Study. Hydraulic Model Report CH Series CH75/09, Civil Engineering, The University of Queensland.

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Author Chanson, Hubert
Title Advective Diffusion of Air Bubbles in Hydraulic Jumps with Large Froude Numbers: an Experimental Study
School, Department or Centre Civil Engineering
Institution The University of Queensland
Report Number CH75/09
Series Hydraulic Model Report CH Series
Publication date 2009
Start page 1
End page 89
Total pages 89
Publisher School of Civil Engineering, The University of Queensland
Language eng
Subject 09 Engineering
0905 Civil Engineering
090509 Water Resources Engineering
Abstract/Summary A hydraulic jump is a rapid transition from a high-velocity open channel flow to a slower fluvial motion. It is commonly experienced in streams and rivers, in industrial channels and during manufacturing processes. Herein new detailed air-water flow characteristics were measured in the developing shear layer of hydraulic jumps with partially-developed inflow. The measurements were conducted in a relatively large-size facility with large Froude numbers (5.1 < Fr1 < 11.2). In the developing shear layer, the distributions of void fractions were modelled by an advective diffusion equation. The experimental data demonstrated a close agreement with the theoretical developments, and the air bubble diffusivity was observed to be independent of the Froude and Reynolds numbers although increasing linearly with the distance from the jump toe. The experimental observations highlighted a strong air entrainment rate as well as some spray and splashing above the roller. The measurements of jump toe fluctuations were close to earlier studies. The new data showed that the jump toe oscillation frequency was equal to the production rate of large-scale vortical structures in the developing shear layer, and the average convection speed of the large coherent structures was in average Vej/V1 = 0.32. The void fraction distributions presented a local maximum in the air-water shear layer and its value decreased quasi-exponentially with increasing distance from the jump toe. The shear zone was also characterised by a maximum in bubble count rate. The depth-averaged void fraction data demonstrated a large amount of entrained air as well as a rapid de-aeration of the jump roller. The velocity profiles followed closely some wall jet equations, and the air-water turbulent properties indicated some increasing turbulence levels with increasing distance from the bed. The bubble chord time distributions exhibited a broad range of entrained bubble chord times spreading over several orders of magnitudes. A detailed analysis of the longitudinal structure of the air and water chords suggested a significant proportion of bubble clustering in the developing shear region, especially immediately downstream of the jump toe. The data showed further that, in the shear layer, there was no preferential bubble chord time in the cluster structures. Overall the study highlighted that the convection of air in the mixing zone was an advective diffusion process, although there was some rapid flow de-aeration for all Froude numbers. The technical report is supported by a digital appendix (Appendix D) containing three movies available at the University of Queensland institutional open access repository UQeSpace {http://espace.library.uq.edu.au/}.
Keyword Hydraulic jumps
Air bubble entrainment
Advective diffusion
Physical modelling
Air-water flow measurements
Large-scale vortical structures
Turbulence
Additional Notes The full bibliographic details are: CHANSON, H. (2009). "Advective Diffusion of Air Bubbles in Hydraulic Jumps with Large Froude Numbers: an Experimental Study." Hydraulic Model Report No. CH75/09, School of Civil Engineering, The University of Queensland, Brisbane, Australia, 89 pages (ISBN 9781864999730). The technical report is supported by a digital appendix (Appendix D) containing three movies available at the University of Queensland institutional open access repository UQeSpace {http://espace.library.uq.edu.au/}.

 
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Created: Tue, 24 Nov 2009, 08:15:25 EST by Hubert Chanson on behalf of School of Civil Engineering