Modeling of Surfzone Bubbles Using a Multiphase VOF Model

Figure. Bubbles in different sizes in the surfzone water (model result). Click to see an annimation.

RECENT  RESULTS (animations and figures)

Bubbles in the surfzone (gif 1.0M)  AVI format(2.3M)

Bubble concentration  (gif 1.0M)  AVI format  (1.5M)

OSU experiment (gif 1.2M ) AVI format (1.2M)

Plunging jet (gif 1.9M) AVI format (3.9M)

Bubbles trapped in vortex (jpg)

 Buoyancy induced circulation (5 bubble sizes)

The ability to make optically-based observations in the surf-zone is strongly influenced by the presence of suspended sediment particles and of air bubbles, both of which are present due to the action of breaking waves. Wave breaking is instrumental in injecting large volumes of air into the water column. This air volume subsequently evolves into a distribution of bubble sizes that interact with the fluid turbulence and are advected by the organized flow. The bubble population in the surf-zone is intensified due to the greater intensity of breaking processes, leading to increase in turbulence intensity and associated energy dissipation. The bubble sizes are also affected by the densely sedimented flows that could alter the relationship between turbulent perturbing forces and surface tension-based restoring forces leading to the determination of critical bubble diameters. Models for the distribution of bubble populations in the surfzone are rare, and, where they exist, are based on a simplified view of the circulation process of interest without involving detailed processes of bubble injection, interaction and evolution (for example, Vagle et al., 2005). The problem of bubble injection and initial distribution in the water column is happening on the time scale of the individual waves. A prediction of the distribution of bubbles over depth due to a breaking event depends on a good representation of the fluid velocity field at the wave-resolving time scale.

In the present study, we formulated a general, multiphase model representing water, multi-component sediment and multi-component bubble populations. The multiphase model incorporates not only the interaction between fluid and sediment phase and fluid and bubble phase, but between sediment and bubble phases as well. A vertical 2-D Reynolds-averaged Navier-Stokes solver with a k-e  turbulence closure is modified for the wave phase calculation. Formulations associated with Lagrangian tracer have been extended to consider entrained bubbles and sediment as a second and a third phase.

 FORMULATIONS (SEE ICCE 2008 ABSTRACT)