CONCLUSIONS

The two-phase flow measurement array mounted wave gages, a dual-tip void probe (DVP) and an Acoustic Doppler Velocimeter (ADV) were constructed. Using the two-phase flow measurement array, the characteristics of void fractions, air bubble distributions, and flow velocities were measured for surf zone breaking waves in the laboratory. In addition, two different scale of experiments were conducted to measure void fraction and bubble size spectra. The results of this study can be summarized as follows.

1)

The peaks of void fraction appears 0.1 to 0.2 wavelength onshore from the breaking point.

2)

The significant scale effects of void fraction is observed from the mid- and small-scale experiments. Following the Froude similarity law, the void fraction of the mid-scale experiments is 2-4 times larger than small-scale experiments.

3)

Despite the scale effects on void fraction, the shape of bubble size spectra for large scale bubble (chord length is larger than 0.1 mm) is independent of the experimental scale.

4)

The relationship between time-averaged void fraction and total kinetic energy shows linear dependence.

The unsteady characteristics of two-phase flow in the surf zone is important, although the time-averaged characteristics were analyzed in this study. These are further study to be investigated.

Acknowledgments The authors express grateful thanks to Dr. Daniel T. Cox for many helpful suggestions and also greatly appreciate Mr.Yoshihisa Ohnishi for his help in experiments. This research was funded by the Ministry of Education, Science, Sports and Culture, Japan, through Grant-in-Aid for No.17760409 and also acknowledges the support of the Sumitomo Foundation No.053493.