Abstract

In this study, an air entrainment model based on the turbulence production and laboratory measured air bubble size spectrum is incorporated into a two-dimensional numerical model (NEWFLUME) to investigate the bubble plume dynamics under breaking processes. The model solves the Reynolds equations for turbulent flow fields and employs the volume-of-fluid (VOF) method to track free surfaces. The integral properties of bubble plume are investigated and compared with the experimental data. The void fraction distributions are examined and the relationship between the void fractions and turbulence intensity is discussed.

Introduction

In the surf zone, wave breaking is important physical process which generates high intensity of turbulence and entrains a large volume of air bubbles. Numerical investigation of turbulent flow dynamics under breaking waves is still of great challenges, especially in the high intensity air bubble region, mainly due to the complex physical process of air entrainment during wave breaking.

Experimental investigations of air entrainment and evolution under breaking waves have been widely conducted for many years. Deane and Stokes (2002) measured the bubble size distribution under the laboratory-scale breaking waves. They found that the bubble size density is closely related to the bubble size, bubbles larger and smaller than 1 mm have bubble size density proportional to the bubble radius with power-law of -10/3 and -3/2 respentively. Cox and Shin (2003) carried out laboratory studies on void fraction distributions in the region close to wave breaking point. The results showed that the maximum ensemble-averaged void fractions are up to 15-20% in spilling breaking and a linear relationship exists between the void fraction and turbulence intensity. This linear dependence was also found by Mori and Kakuno (2008) in experimental measurement. Blenkinsopp and Chaplin (2007) investigated the bubble plume and splash-up properties in laboratory. The results suggested that air entrainment and splash generation account for at least 6.5%~14% of the total energy dissipation. Lim et al. (2015) measured the velocities and void fraction under an unsteady plunging breaking in laboratory. They found that the increase of turbulence level and vorticity level are strongly related with the increase of void fractions.

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