Similitude of Air Bubble Entrainment and Dispersion in Vertical Circular Plunging Jet Flows. An Experimental Study with Freshwater, Salty Freshwater and Seawater

Air bubble entrainment at plunging jet takes place when the jet impact velocity exceeds a critical velocity Ve which is a function of the jet turbulence. Several researchers proposed an analogy between plunging jet flows and plunging breakers. Most works considered steady plunging jet results obtained with freshwater and there have been suggestions that air entrainment at breaking waves in the sea might be an entirely different process. This study investigates scale effects affecting air entrainment and bubble dispersion at vertical circular plunging jets. Three scale models were used and detailed air-water measurements were performed systematically for identical Froude numbers. For one scale, three water solutions were tested. All experimental results are presented in Appendices B and C. In freshwater, the results highlight significant scale effects when We1 = rw*V1 2*d1/s < 103 or V1/ur < 10. Identical experiments were performed with freshwater, salty freshwater and seawater. The results show lesser air entrainment in seawater plunging jets for identical inflow conditions. Results with saltwater are intermediate between freshwater and seawater results. Bubble chord times tch were also measured. The data were analysed in terms of pseudo-bubble chord length chab = V1*tch, which was found to overestimate real bubble chords by 10 to 30%. The data show that pseudo-bubble chord sizes range from less than 0.5 mm to more than 10 mm. The average pseudo-chord sizes are between 4 and 6 mm for all water solutions. Comparatively, however, more fine bubbles were detected in seawater than in freshwater. Fine bubbles (less than 0.5 mm) have a slower rise velocity and their underwater residence time is larger in seawater. They give a visual, misleading appearance to the air-water flow suggesting that very fine bubbles are predominantly entrained in seawater plunging jets. Overall the study contributes to a better understanding of air entrainment and bubble dispersion in vertical circular plunging jets. Differences in air entrainment mechanisms between freshwater and seawater are described. Scales effects are believed to occurs in laboratory wave flumes when the wave height at breaking is less than 0.25 to 0.35 m. Experimental results in seawater and saltwater suggest further that classical dimensional analysis of air entrainment is incomplete, and that further physical, chemical and biological fluid properties must be taken into account