Air entrainment, splash and energy dissipation in breaking waves

This thesis presents detailed laboratory measurements of the time-varying distribution of void fractions and bubble sizes within the dense bubble plumes generated immediately beneath breaking waves in a 2D laboratory wave flume. The measurements were made using a novel optical fibre probe technique, which was shown to perform well under violent, aerated flow conditions and produced results to a resolution not achieved by previous studies. The results obtained from these experiments demonstrated that the properties of the two-phase flow generated by breaking waves evolve rapidly as simple functions of time and behave in a similar manner for all of the breaker types under examination. The measurements indicated that the energy expended in entraining air accounted for a minimum 4% to 9% of the total energy dissipated during breaking, and that the contribution of the air entrainment mechanism was greater as the breaker type changed from spilling to plunging. In addition, it was estimated that the generation of splash contributes between 2.5% to 5% of the total dissipation. Additional measurements in artificial and natural seawater demonstrated that entrained bubble plumes evolve in an almost identical manner to those in freshwater, although small differences were observed in the distribution of bubble sizes. This is a significant result as the vast majority of laboratory studies are completed using freshwater and these measurements confirm the general applicability of freshwater model studies of violent, air entraining flows in many situations. To further assess the applicability of laboratory measurements of aerating flows to field conditions, an analysis of the effect of scale was completed using a Lagrangian bubble tracking model and it was shown that the post breaking evolution of entrained bubble plumes is subject to significant scale effects.