Two-phase flow characteristics of a breaking tidal bore roller: microscopic properties

A tidal bore is an unsteady rapidly-varied open channel flow characterised by a rise in water surface elevation in estuarine zones, under spring tidal conditions. After formation, the bore is traditionally analysed as a hydraulic jump in translation and its leading edge is characterised by a breaking roller for Fr1 > 1.3-1.5. The roller is a key flow feature characterised by intense turbulence and air bubble entrainment. Detailed unsteady air-water flow measurements were conducted in a breaking bore propagating in a large-size channel, using an array of three dual-tip phase detection probes and an ultra-high-speed video camera. The data showed a relatively steep roller, with a short and dynamic bubbly flow region. Air entrainment took place in the form of air entrapment at the roller toe, air-water exchange across the roller 'free-surface', spray and splashing with dynamic water drop ejection and re-attachment, roll up and roll down of water 'tongues' engulfing air pockets. The roller free-surface profile and characteristics were comparable to observations in stationary hydraulic jumps and steady breaker, for similar flow conditions. Within the roller, the amount of entrained air was quantitatively small for Fr1 = 2.2. The number of air bubbles was limited, with between 5 to 20 bubbles per phase-detection probe sensor detected at each vertical elevation. The entrained air bubble chord lengths spanned over several orders of magnitude, with a large proportion of clustered bubbles. The air-water interface detections presented of lot of data scatter and randomness, linked to the three-dimensional nature of the air-water roller motion and evidenced of the in-homogeneity of the turbulent air-water mixture