Multiscale numerical study of turbulent flow and bubble entrainment in the surf zone

Wave breaking in the surf zone entrains large volumes of air bubbles into the water column. These bubbles are involved in intense interactions with mean flow and turbulence, producing a complex two-phase bubbly flow field. Many studies have revealed that the turbulent bubbly flow under surf zone breaking waves is characterized by large-scale, organized flow structures, which play a significant role on the bubble entrainment and transport. On the other hand, it is well known that the presence of bubbles can suppress liquid phase turbulence and alter the local vorticity field. Therefore, it is necessary to describe the dynamics of breaking waves as a two-phase flow with air bubbles of appropriate size distribution. In this thesis, a polydisperse two-fluid model is developed to study the bubble plume dynamics and void fraction evolution as well as large-scale coherent structures and their interactions with dispersed bubbles under surf-zone breaking waves. The bubble entrainment model is formulated by linearly correlating bubble entrainment rate with turbulent dissipation rate. The model is validated against laboratory measurements of oscillatory bubble plume as well as turbulence and void fraction evolution under surf zone breaking waves.;In order to better understand the turbulent bubbly flow field under breaking waves, both 2D and 3D simulations are performed. In 2D simulations, it is found that the void fraction has a linear growth and exponential decay in time. The vertical distribution of void fraction can be well described by an exponential function of the distance to the free surface. At the early stage of wave breaking, the bubble plume follows the propagating breaking wave crest. At the later stage of wave breaking, the bubble plume travels slower than the breaking wave, indicating that bubbles are spread behind the wave crest. In the 3D simulation, large-scale turbulent coherent structures, such as obliquely descending eddies and downburst of turbulent fluid, are captured by the model. These coherent structures play an important role in turbulent kinetic energy (TKE) and momentum transport as well as bubble entrainment. High TKE and Reynolds stress are located at regions with strong downward velocities, which are found at the outer part of vortices. However, high void fraction is not only located at the outer core of vortices, but also found at the center of vortices due to the preferential accumulation of bubbles by the vorticity field. As the vortices move downward, bubbles are transported to the lower part of the water column. Therefore, the turbulent coherent structures tend to transport bubbles more deeply into the water column. Both 2D and 3D simulations show that the presence of bubbles suppress liquid phase turbulence and enstrophy. The mechanisms of the generation of obliquely descending eddies are also investigated. It was found that the obliquely descending eddies evolve from vertical vortices, which are initially generated due to bending of the primary spanwise vortices. The downburst of turbulent fluid plays a significant role in vortex evolution processes.;A Non-Hydrostatic WAVE model NHWAVE is developed to simulate dispersive surface wave processes, wave breaking, surf-zone turbulence and wave-driven circulation. A bubble transport model based on the mixture theory is incorporated into NHWAVE to study the bubble generation and transport in the large-scale surf zone. The model is applied to investigate the rip current systems and bubble transport in a rip current experiment (RCEX), which was conducted in the Sandy City beach, CA. The wave height distribution and rip currents are well reproduced by the model. The bubble transport is dominated by the vorticity field inside the surf zone. Bubbles can be transported to the outer surf zone by rip currents.