Ocean surface gravity wave propagation from offshore to shoreline is 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 due to the bottom bathymetric effects in the near shore. The wave breaking create dense plumes of bubbles, and dissipates energy and momentum. An accurate estimate 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 (Donelan et al., 2002). Recent photographic studies have illustrated the disintegration of entrapped air cavities divided into bubbles (Deane and Stokes, 1999; Deane and Stokes, 2002). However, there are unexplained aspects of the problem, such as enhanced bubble populations in salt rather than freshwater (Thorpe, 1982), 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, in-situ measurements (Chanson et al., 2002), video or photographic measurements (Leifer and de Leeuw, 2002), laser measurements (Mori, 2002) and acoustic measurements (Deane, 1997) have been conducted. The bubble size measurements using lasers show high accuracy but are impossible to use in the presence of high void fractions due to instrument limitations. Acoustic measurements of bubble are useful in deep-water but have limitation for very shallow water region due to the multi-reflection of sound beams. Therefore, conventional optical or resistivity type void probe are useful for surf zone waves. Vagle and Farmer (1998) compared four methods to measure bubble size and void fraction and showed that electrical conductivity probes should be used for the high void fractions.
Understanding air entrainment and bubble distribution for wind waves is
advanced compared to the surf zone because it is directly connected to
gas transfer at the air-sea interface.
The air bubble distributions under the wind wave breaking in the ocean
surface layer had been summarized by Thorpe (1982), theoretically.
Baldy (1993) discussed bubble formation and dependence on the turbulent
dissipation rate of fluids and proposed a 
power-law scaling with
bubble diameter.
Garrett et al. (2000) proposed 
power-law scaling based on the
discussion of bubble fragmentation and bubble spectrum due to strong
turbulent shear flow.
Mori (2003) 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, few quantitative studies of air entrainment and of surf zone wave breaking exist. Peregrine (2003) reported that the entrapped air of breaking wave influence the effect of wave impact, due to their greater compressibility compared with pure water. Loewen et al. (1996) found little difference in the bubble populations beneath mechanically generated surface waves in saltwater and freshwater. The compressibility due 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 correct for engineering problems. Therefore, the connection between air-mixture, bubble distribution and wave breaking induced turbulence is essential to understand the gas-liquid interaction in the surf zone. Cox and Shin (2003) reported on the dependence of void fraction on turbulent intensity in the bore region of surf zone waves. The wave breaking induced bubbles in the surf zone are split by strong local turbulent shear induced by breaking waves at the bubble scale (i.e. Deane and Stokes, 1999). However, qualitative and quantitative bubble characteristics in the surf zone and connections between bubble characteristics and wave breaking are not well known due to the lack of detail measurements. This detail information of 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 experimentally. First, a series of synchronized measurements of void fractions, bubble distributions, mean and turbulent components of flow above the mean water level in the surf zone are carried out using acoustic Doppler velocimetry (ADV) and dual-tip resistivity void probe (DVP). Second, the analysis of the experimental data of void fractions, bubble spectra and turbulent properties of the surf zone waves, on onshore-offshore, vertical and scale dependence of entrained air will be discussed.