Ocean surface gravity waves propagates from offshore to shoreline and are often regarded as a single phase flow using potential flow theories or the Navier-Stokes equation. Generally, the single phase flow approach to the ocean wave is successful for simulating wave transformation in the coastal area. However, waves steepen and break owing to bottom bathymetric effects in the nearshore. The wave breaking creates dense plumes of bubbles, and dissipates the energy and momentum. An accurate estimation of bubble size and population distributions in the surf zone is important for understanding two-phase flow characteristics, solving engineering problems and environmental mechanisms of the coastal area (, ). Recent photographic studies have illustrated the disintegration of entrapped air cavities divided into bubbles (, ,). However, there are unexplained aspects of the phenomena, such as enhanced bubble populations in salt rather than freshwater (, ), scale effects of void and bubble size distribution in the laboratory experiments, and the relation between void fraction and turbulence.
Measurements of bubble size and population distributions by in-situ instruments (, ), video or photographic instruments (, ), laser instruments (, ) and acoustic measurements (, ) have been conducted for several purpose. The bubble size measurements using lasers show high accuracy but are impossible to use in the presence of high void fractions owing to technical limitations. Acoustic measurements of bubble are useful in deep-water but have limitation for very shallow water region owing to the multi-reflection of sound beams between the sea surface and the bottom. Therefore, conventional optical or resistivity type void probe are useful for surf zone waves (, ). () compared four methods to measure bubble size and void fraction and concluded that electrical conductivity probes should be used for the high void fractions.
Understanding air entrainment and bubble distribution induced by wind wave breaking has long history of the research and is advanced compared to the surf zone.
The air bubble distributions under the wind wave breaking in the ocean
surface layer is summarized by (), theoretically.
() discussed bubble formation and dependence on the turbulent
energy dissipation rate of fluids and proposed a 
power-law scaling with
bubble diameter.
() proposed 
power-law scaling based on the
discussion of bubble fragmentation and bubble spectrum owing to strong
turbulent shear flow.
() also proposed a 
power-law scaling based on the
dimensional analysis of wind wave breaking.
Despite the fruitful knowledge of air entrainment for wind-wave breaking, a few studies of air entrainment and of surf zone wave breaking exist. () reported that the entrapped air of breaking wave gives influence to the wave impact owing to their greater compressibility compared with pure water. () found a little difference in the bubble populations beneath mechanically generated surface waves in saltwater and freshwater. The compressibility owing to air-water mixture decreases the velocity of sound and is being used to estimate large-scale prototype impacts, since the usual Froude scaling is unlikely to be corrected for engineering problems. Therefore, the connection between the air-mixture, the bubble distribution and the wave breaking induced turbulence is essential to understand the gas-liquid interaction in the surf zone. () investigated the dependence of void fraction on the turbulent intensity in the bore region of surf zone waves. The wave breaking induced bubbles in the surf zone are split by the strong local turbulent shear induced by breaking waves at the bubble scale (i.e. , ). The authors demonstrated on the dependence of void fraction on wave scale using different scales of wave flume following the Froude similarity law (, ). However, the bubble characteristics and connections between bubble characteristics and wave breaking are not well known owing to the lack of understanding of phenomena in the surf zone. The detail information of the two-phase flow characteristics is required for mathematical modeling.
The purpose of this study is to investigate the characteristics of void fractions, bubble distributions and turbulent properties in the surf zone waves using visualization technique. First, the automatic bubble shape recognition and moving velocity technique is developed. Second, the analysis of the experimental data of void fractions, bubble spectra of the surf zone waves, the power scaling law of bubble population spectra will be discussed.