An experimental study of air entrainment by breaking waves
An experimental study of air entrainment by breaking waves
Date
1993-05
Authors
Lamarre, Eric
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Location
San Diego, CA
Buzzards Bay, MA
Buzzards Bay, MA
DOI
10.1575/1912/5509
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Keywords
Acoustic surface waves
Air
Waves
Air
Waves
Abstract
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.
Description
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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Citation
Lamarre, E. (1993). An experimental study of air entrainment by breaking waves [Doctoral thesis, Massachusetts Institute of Technology and Woods Hole Oceanographic Institution]. Woods Hole Open Access Server. https://doi.org/10.1575/1912/5509