An experimental study of air entrainment by breaking waves
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LocationSan Diego, CA
Buzzards Bay, MA
Breaking waves charge the surface layer of the ocean with small air bubbles which play an important role in air-sea gas transfer and in underwater acoustics near the ocean surface. This work reports on a series of laboratory and field experiments on the measurement on air entrainment by breaking waves. The first part of this thesis addresses the measurement of high volumetric concentrations of air (0.3% to 100% void-fraction) found immediately beneath breaking waves. Instrumentation based on the change of electrical impedance of the bubbly mixture with change in void-fraction is developed. Laboratory measurements are conducted in a wave channel and in a large three-dimensional wave basin. Maps of the evolution of the void-fraction distribution in bubble plumes generated by various size breaking waves are presented. Moments of the void-fraction field are shown to scale with the initially enclosed air volume at breaking and the energy dissipated by breaking. A significant fraction (30 to 50%) of the energy dissipated by breaking is found to be expanded in entraining bubbles against their buoyancy. The results reveal that the bubble plumes experience rapid transformations within the first wave period after the onset of breaking. In particular, the plumes loose 95% of the initially entrained air volume during the fust wave period. Predictions of the low-frequency resonant oscillations of the bubble plumes from measurements of the void-fraction compare well with acoustic measurements. Measurements near the ocean surface show high void-fractions up to 24% immediately beneath breaking waves. These are several orders of magnitude greater than previously reported time-averaged measurements. The second part addresses the measurement of very low void-fractions. Instrumentation based on the propagation velocity of low-frequency acoustic pulses is developed. Simultaneous measurements of the sound-speed (and thus the void-fraction) at several depths are conducted during two field experiments. Time-series of sound-speed and attenuation show dramatic fluctuations over time periods on the order of minutes or less. These are attributed to the formation of bubble plumes or passage of bubble clouds. Frequent occurences of sound-speed anomalies greater than 1 OOrnls and attenuation greater than 30dB/m are observed for moderate wind conditions (8m/s). The signals at various depths are highly correlated and mostly coherent at frequencies below 0.05Hz. The time-averaged (20min) sound-speed profile is found to be significantly more pronounced and shallower than previously reported. Simultaneous measurements at several acoustic frequencies show that the sound-speed is non-dispersive below 20kHz for moderate wind conditions. Bubble size distributions are inferred from the soundspeed and attenuation measurements.
Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution May 1993
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