Bubble entrainment and liquid-bubble interaction under unsteady breaking waves
Date
2013
Authors
Derakhti, Morteza
Journal Title
Journal ISSN
Volume Title
Publisher
University of Delaware
Abstract
Wave breaking is a highly dissipative process, and also a source of turbulence
in the ocean surface layer. It entrains a large volume of air in bubbles that rapidly
evolves into a distribution of bubble sizes which interacts with liquid turbulence and
organized motions. The liquid-bubble interaction, especially in the complex two-phase
bubbly
ow under breaking waves, is still poorly understood. In the present study,
we perform a large-eddy simulation (LES) using a Navier-Stokes solver extended to
incorporate entrained bubble populations, using an Eulerian-Eulerian formulation for
a polydisperse bubble phase. The volume of
uid (VOF) method is used for free surface
tracking. We consider an isolated unsteady deep water breaking event imposed
by a focussed wave packet. The bubble-induced dissipation and momentum transfer
between two phases are considered. The model is shown to predict free surface evolution,
mean and turbulent velocities and integral properties of the entrained dispersed
bubbles (hereafter bubble plume) fairly well. We investigate bubble plume kinematics
and dynamics, turbulence modulation by dispersed bubbles as well as shear- and
bubble-induced dissipation, both in spilling and plunging breakers. We nd that the
total bubble-induced dissipation accounts for more than 50% of the total dissipation
in the breaking region. The averaged dissipation rate per unit length of breaking crest
is usually written as b g1c5, where c is the phase speed of the breaking wave. The
breaking parameter, b, has been poorly constrained by experiments and eld measurements.
We examine the time dependent evolution of b for both constant-steepness and
constant-amplitude wave packets. The scaling law for the averaged breaking parameter
is obtained. The exact two-phase transport equation for turbulent kinetic energy
(TKE) is compared to the conventional single phase transport equation, and it is found
that the former over predicts the total SGS dissipation and turbulence production by mean shear during active breaking. All of the simulations are repeated without the
inclusion of dispersed bubble phase, and it is shown that the integrated TKE in the
breaking region is damped by the dispersed bubbles about 20% for the large plunging
breaker to 50% for the spilling breakers. In the plunging breakers, TKE is damped
slightly or even enhanced during the initial stage of active breaking. In addition, we
examine the nonlinear interaction of di erent components in a wave packet. Phase
locking between spectral components is observed in the breaking region, and explained
by calculating the wavelet bispectrum.