Study of Bubble Size Distribution for Breaking Wave Propagates Over a Submerged Dike

Abstract

Breaking wave plays an important role in air-sea, wave-wave, wave-current, current- current interaction. Wave breaking not only disturbs communication of sound chie∞y because of the transient noise of the air-entrained bubbles. A useful analytical method, Gabor trans- formation, is applied due to its advantages over nonsymmetric and abrupt transition signals in breaking events. Numbers of bubbles and size distribution in a speciflc wave condition is analyzed in laboratory experiments. DOI: 10.2529/PIERS060906120020 Protective structures like sea walls, breakwaters, submerged dikes are popularly deployed in nearshore ocean engineering. Formation of propagating wave will deform due to the wind drafting force, to- pography variation and release the accumulated wave energy. Breaking of water waves plays an important factor in many ocean studies, such as air-sea interaction, remote sensing, wave dynamics, and gas transfer to the atmosphere. Breaking wave always results in severe destructions no matter if it happens on the wide open ocean or nearshore coast; vessels, mariners and creatures sufiered for that. Distortion of a periodic wave starts to collapse when the front of wave crest is onset to break beneath downstream, dubbed as an active phase (1). The collapse wave front will entrain air volume into the surface water layer in a very short time, say whitcapping process and which will produce dense plumes of difierent sizes air bubbles. Breaking attracts more and more attentions with its speciflcally characteristics. Various methods and techniques such as acoustic methods, photography method (5,7) and remote sensing method (4) are presented in order to detect bubble sizes at the breaking region. Bubble clouds are distributed underlying propagating waves, the mainly disturb source of the underwater communication (2). Estimation of the bubble size and numbers from the breaking event is normally very hard to obtain. Stokes (1993) used the photographic method to obtain the bubble size spectrum. As a result of fragmentation of turbulence, that dominates the production of larger bubble distribution. In terms of acoustic, Minnaert (1933) proved the noise produced by bubbles is related to the simple radial pulsation of bubbles and derived the resonance frequency of bubbles. Friedlander (1995) derived a representation lead for those signals with behavior of nonsymmetric and abrupt transitions, such as bubble emission noise. Application of Gabor coe-cient is adapted in this study and got satisfled results of the distribution of bubble size when wave propagates through a rectangular submerged dike. The resonant frequency of the bubble and the radius is expressed as !0 = 1 R0