Turbulent advances of a breaking bore: Preliminary physical experiments
Introduction
A sudden increase in flow depth in an open channel induces a positive surge [13], [3], [19]. In an estuary, a positive surge of tidal origin is called a tidal bore which may be generated by the early flood tide propagating upstream into a narrow funnelled channel under a large tidal range [34], [8]. After formation, the bore may be analysed as a hydraulic jump in translation [30], [20]. The shape of the surge front is a function of its Froude number Fr1 [25], [9]. For a rectangular channel, Fr1 equals:where V1 is the initial flow velocity positive downstream, is the mean bore celerity positive upstream, g is the gravity acceleration, d1 is the initial flow depth (Fig. 1A). For Fr1 > 1.4–1.6, the leading edge of the bore is characterised by a breaking roller. Fig. 1B presents a photograph of a large breaking tidal bore in China. The bore roller is characterised by a sudden increase in water depth, a highly turbulent flow with large-scale vortical structures, some kinetic energy dissipation, a two-phase air–water flow region and strong turbulence interactions with the free surface associated with splashes and droplet ejections.
In this contribution, a physical investigation was conducted in laboratory with a focus on the bore roller properties. Detailed measurements were performed in a relatively large facility. The observations included a series of video observations of propagating breaking bores to characterise the roller toe perimeter, the bore front celerity and their fluctuations, as well as some preliminary unsteady air entrainment measurements in the bore roller using a dual-tip phase-detection probe. It is the purpose of this contribution to study thoroughly the upstream roller propagation and its turbulent fluctuations.
Section snippets
Experimental facility and instrumentation
New experiments were conducted in a 19 m long 0.7 m wide tilting channel, made of glass sidewalls and smooth PVC bed (Fig. 2). The bed slope was horizontal (So = 0) and the channel ended with a free overfall. The initially steady flow was supplied by a constant head reservoir, delivered into an upstream intake channel and led to the glass sidewalled test section through a series of flow straighteners followed by a smooth bed and sidewall convergent. A fast-closing Tainter gate was located next to
Roller toe perimeter
In hydraulic jumps and breaking bores, the roller toe is a flow singularity where air is entrapped and vorticity is generated [14]. It is also called breaker foot [2] and corresponds to the position for base of a breaker, at the boundary between smooth and turbulent flow at the water surface, see the examples in Figs. 1B and 2. View in elevation, the roller toe formed a continuous line, herein called the roller toe perimeter (Fig. 1A). The shape of the roller toe perimeter and its evolution
Discussion: unsteady air bubble entrainment in the roller
The instantaneous void fraction c is defined as 0 in the water and 1 in the air. Fig. 7 presents the time variations of instantaneous void fraction c at different vertical elevation z/d1, where ttoe is the time of passage of the bore roller toe. In Fig. 7, the black lines correspond to the leading tip probe signal and the red lines to the trailing sensor signal. The data showed consistently that a substantial number of bubbles were entrapped between 1.25 < z/d1 < 1.5. No bubble was detected for z/d1
Conclusion
New experiments were conducted in a large flume to investigate breaking bores and the bore roller propagation. The results demonstrated several outcomes.
- (1)
The propagation of breaking bore roller toe was a highly turbulent process. Although the transverse shape of the roller toe perimeter was quasi two-dimensional on average, the toe perimeter shape fluctuated rapidly with transverse distance and time.
- (2)
The sidewalls had little effect on the upstream propagation of the breaking bore roller, within
Acknowledgments
The authors thank Dr Frédéric MURZYN (ESTACA Laval, France) and Dr Hubert BRANGER (University of Aix-Marseille, France) for their detailed review of the report and very valuable comments. They also thank Professor Pierre LUBIN (University of Bordeaux, France) for his personal involvement, contribution and comments to the research project. They thank further both reviewers for their constructive and helpful comments. The authors acknowledge the technical assistance of Jason VAN DER GEVEL and
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