Numerical study of large-scale turbulence and bubble entrainment under surfzone breaking waves

Large-scale turbulence such as obliquely descending eddies (Nadaoka et al., 1989) and downburst (Kubo and Sunamura, 2001) plays an important role in the bubble entrainment and transport under surfzone breaking waves. The present study employs a three-dimensional surface capturing large eddy simulation coupled with a two-phase bubbly flow model (Ma et al., 2010) to investigate the detailed interactions between coherent vortical structures and bubble transport in a laboratory scale breaking wave. We first use the vortex core representation technique (Jeong and Hussain, 1995) to investigate the occurrence of coherent vortical structures and to quantify the primary direction of the vortices. The obliquely descending eddies behind breaking wave crest can be recognized as shown in figure 1. After wave breaking, spanwise vortices are dominant near the water surface, while obliquely descending eddies become dominant inside the water column with angles tilting toward the x-axis to be around 25 and 205 degree. Through the analysis of vorticity equation, it is shown that vortex turning and stretching play an important role in the evolution processes from spanwise to obliquely descending eddies. To illustrate the correlation of coherent vortices with bubble transport, we then employ the quadrant (QD) method (Kim and Moin, 1986) to study the relationship between vortical structures and turbulent transport. It is found that the vertical transport of bubbles is greatly enhanced by the presence of coherent vortices. Coherent vortical structures in the breaking wave at 0.3T (T is wave period). The vortical structures are identified by the isosurface of λ2=-0.5