Laboratory measurements of the sound generated by breaking waves

Abstract

in partial fulfillment of the requirements for the Degree of Doctor of Philosophy in Oceanographic EngineeringBreaking waves dissipate energy, transfer momentum from the wind to surface currents and breaking enhances the transfer of gas and mass across the air-sea interface. Breaking waves are believed to be the dominant source of sea surface sound at frequencies greater than 500 Hz and the presence of breaking waves on the ocean surface has been shown to enhance the scattering of microwave radiation. Previous studies have shown that breaking waves can be detected by measuring the microwave backscatter and acoustic radiation from breaking waves. However, these techniques have not yet proven effective for studying the dynamics of breaking. The primary motivation for the research presented in this thesis was to determine whether measurements of the sound generated by breaking waves could be used to quantitatively study the dynamics of the breaking process.Laboratory measurements of the microwave backscatter and acoustic radiation from two-dimensional breaking waves are described in Chapter 2. The major findings of this Chapter are: 1) the mean square acoustic pressure and backscattered microwave power correlate with the wave slope and dissipation for waves of moderate slope, 2) the mean square acoustic pressure and backscattered microwave power correlate strongly with each other, and 3) the amount of acoustic energy radiated by an individual breaking event scaled with the amount of mechanical energy dissipated by breaking. The observed correlations with the mean square acoustic pressure are only relevant for frequencies greater than 2200 Hz because lower frequencies were below the first acoustic cut-off frequency of the wave channel.In order to study the lower frequency sound generated by breaking waves another series of two-dimensional breaking experiments was conducted. Sound at frequencies as low as 20 Hz was observed and the mean square acoustic pressure in the frequency band from 20 Hz-l kHz correlated strongly with the wave slope and dissipation. A characteristic low frequency signal was observed immediately following the impact of the plunging wave crest. The origin of this low frequency signal was found to be the pulsating cylinders of air which are entrained by the plunging waves. The pulsation frequency correlated with both the wave slope and dissipation. Following the characteristic constant frequency signal, approximately 0.25 s after the initial impact of 2 the plunging crest, another low frequency signal was typically observed. These signals were generally lower in frequency initially and then increased in frequency as time progressed.To determine if three-dimensional effects were important in the sound generation process and to measure the sound beneath larger breaking waves a series of experiments was conducted in a large multi-paddle wave basin. Three-dimensional breaking waves were generated and the sound produced by breaking was measured in the frequency range from 10 Hz to 20 kHz. 'The observed...

Micro-seismic measurements of cliff motion under wave impact and implications for the development of near-horizontal shore platforms

“…By contrast the amplitude of the frequency peak near 100 Hz did not change substantially with tidal stage and is also substantially reduced in the cliff-top spectra. It is known that breaking (plunging) waves contribute to the ambient sound field at frequencies as low as 50 Hz (Vagel, 1989;Loewen, 1991). This may be an important contributor, given that the strong topographic break at the edge of the shore platform forces breaking at a similar location at all tidal stages.…”

Ambient noise and surface wave dissipation in the ocean

“…By studying the evolution of the bubble cloud generated by the breaking wave, they formd that a significant proportion of the dissipated energy (up to SO%) is due to work done by the liquid in entraining air against buoyancy. Using data from similar experiments, Loewen (1991) [see also Loewen and Melville (1992)] showed that the energy dissipated by a breaking event is proportional to the initial volume of air entrained (Figure 1.8). This is consistent with the Loewen & Melville (1991b) dipole sound source model which suggested that for similar bubble size distributions, the total acoustic energy radiated is proportional to the volume of air entrained.…”

“…Ea was scaled by the square of the product of the center wavenumber of the surface wave packet kc and the depth of the wave tank h (Loewen & Melville, 1991a). (Loewen, 1991). Plant (1982).…”

“…Recently, Loewen & Melville (1992) [see also Loewen (1991)] observed low frequency peaks of the order of tens of Hertz in the noise spectrum of two-dimensional and three-dimensional plunging waves in the laboratory. By using the measurements of void fraction of breaking wave bubble clouds (Lamarre & Melville, 1992) and the theory of collective bubble cloud oscillations by Lu et al (1990), they found that the low frequency peak in the spectrum of pressure fluctuations picked up by a hydrophone matches the resonant frequency of collectively oscillating bubble clouds predicted by Lu et al (1990) [see Figure 1.6].…”

Thermal Infrared Multipath Reflection from Breaking Waves Observed at Large Incidence Angles

“…4. Spilling and plunging breaking waves were created with a programmable wavemaker following the focused wave packet methods described by [18] and [19]. Wave trains of varying frequency were combined to produce a single breaking wave approximately 0.1 m high.…”