A numerical study of aeration characteristics of a plunging solitary wave on a slope

In this paper, the characteristics of aerated flow under a plunging solitary wave on a 1:20 sloping beach are investigated numerically. The numerical model solves the Reynolds-averaged Navier-Stokes equations for mean flow. The turbulence is described by the k - e model, in which the turbulence production and dissipation modified by entrained air bubbles are considered by an additional term. A transient equation is solved for air bubble transportation. The numerical model is validated by comparing the air bubble concentration, mean flow velocities, and turbulent kinetic energy against experimental data, demonstrating its capability for simulating transient aerated flows under breaking waves. The validated model is further applied to reveal the detailed interaction of the entrained air bubbles and the turbulent free surface flows during the wave breaking process. Plunging breaking wave consists of four stages, namely, the wave front steepening, the initiation of overturning, the transitional stage, and the quasi-steady bore propagation stage. The results reveal that the overturning and breaking wave front is the main source for turbulence generation and air entrainment in the initiation and transitional stage of breaking wave, respectively. The entrained air bubbles are mainly transported backward and downward by turbulence structures and forming distinct bubble vortex rollers near the bottom. The distribution of air bubble concentration shows a linear correlation to the distribution of turbulence quantities in the initial and transitional stage of breaking wave, demonstrating the important role of local turbulent structures on air entrainment and transportation.