Acoustical studies of breaking surface waves in the open ocean




Ding, Li

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The work presented in this thesis consists of two parts: development and application of a novel passive acoustical approach for field measurement of breaking surface waves, and interpretation of the resulting observations in terms of wave field information so as to improve the understanding of wave breaking. The development of the acoustical approach has been motivated by the difficulties inherent in measurement of breaking waves. This approach makes use of an array of four broadband hydrophones which is able to track individual breaking waves by passive detection of the naturally generated sound of wave breaking. The Generalized Cross Correlation method is used to determine time differences of acoustic signals from breaking waves arriving at the array, allowing the breaking waves to be located with the given array geometry. Observations of breaking waves were made by means of this technique during the Surface Wave Processes Program (SWAPP). The spatial and temporal statistics of breaking waves, including breaking wave density, travel velocity, lifetime of breaking and spacing, are obtained from the observations. Statistical models are developed to assess, and where appropriate, correct for any bias resulting from limitations of the measurement approach. The breaking wave statistics provide important information about the physical process of wave breaking and its distribution in different wave fields. It is found that wave breaking in the open ocean occurs at a scale substantially smaller than the scale associated with the dominant wind wave component in the wave spectrum. Numerical simulation of breaking wave statistics and comparison with the observations demonstrates that the scale of breaking can be predicted from the directional wave spectrum by a linear model with a single breaking threshold. These results will provide input to comprehensive models of wave dissipation. Acoustical radiation properties of individual breaking waves are a further aspect of wave breaking that has been observed with the aforementioned technique. Investigation of the sound radiated from breaking waves reveals information both on the nature of the sound generation mechanism by breaking and the dimension of breaking waves. Statistical analysis of the acoustic source intensity associated with wave breaking suggests that the source intensity can be related to the breaking scale and wave energy dissipation, thus implying that surface wave dissipation could be remotely measured by using ambient sound.



Acoustic surface waves, Signal processing