Browsing by Author "Derakhti, Morteza"
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Item Bubble entrainment and liquid-bubble interaction under unsteady breaking waves(University of Delaware, 2013) Derakhti, MortezaWave 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.Item LES and σ-coordinate RANS simulations of laboratory surface wave breaking(University of Delaware, 2016) Derakhti, MortezaThis dissertation presents a three-dimensional (3D) numerical study of the turbulent bubbly flow in surface breaking waves, from steepness-limited unsteady breaking in deep water to depth-limited breaking in the surf zone. Because of available computational resources, the whole range of the relevant scales can not be resolved in a single high resolution framework. Instead, two different frameworks are chosen to study the relevant physics from small scales through field scales. In the first framework, a Volume-of-Fluid (VOF) based Eulerian-Eulerian polydisperse two- fluid model (Ma et al. 2011, Derakhti & Kirby 2014b) is used to study breaking-induced energy dissipation (chapter 2), bubble entrainment and liquid-bubble interaction (Derakhti & Kirby 2014b) in unsteady whitecaps as well as large-scale turbulent coherent structures and their interaction with dispersed bubbles in the surf zone (chapter 3). A 3D nonhydrostatic wave-resolving -coordinate framework is chosen as the lower-resolution framework. We derive a new set of equations, in conservative form, describing the kinematics and dynamics of continuous and dispersed phases in a multiphase mixture in a surface- and terrain-following -coordinate system, together with exact surface and bottom boundary conditions for the velocity and dynamic pressure fields as well as a Neumann-type boundary condition for scalar fluxes (chapter 4). The model capability and accuracy to reproduce the evolution of the free surface, velocity and vorticity fields and breaking-induced dissipation under regular and irregular breaking waves from surf zone to deep water is examined in detail (chapter 5).