Numerical Modelling of Breaking Waves under the Influence of Wind

Wave breaking plays an important role in air-sea interaction, surf zone dynamics, nearshore sediment transport, marine hydrodynamics, and wave-structure interaction. When the wind is blowing over water waves, it not only enhances the exchanges of heat, mass and momentum on the air-water interface, but also affects the wave breaking process. The objective of this thesis is to contribute to the understanding of breaking waves under the influence of wind. A two-phase flow model is presented to solve the flow in the air and water simultaneously. Two strategies for turbulence modelling, namely, the Reynolds-averaged Navier–Stokes (RANS) equations with the k−ǫ turbulence model and Large Eddy Simulation (LES) with the Smagorinsky subgrid-scale model, are employed to study two-dimensional (2D) and three-dimensional (3D) breaking waves, respectively. The governing equations are solved by the finite volume method in a Cartesian staggered grid and the partial cell treatment is implemented to deal with complex geometries. The SIMPLE or PISO algorithms are utilized for the pressure-velocity coupling and a backward finite difference discretization is used for the time derivative. The air-water interface is modelled by the interface capturing method via a high resolution VOF (Volume of Fluid) scheme. The numerical model is validated by simulating 2D overturning waves on a sloping beach and over a reef, and 3D solitary wave run-up on a conical island, in which good agreement between numerical results and experimental measurements is obtained. Moreover, the overturning jet and subsequent splash-up are captured in the computation. The numerical model is further employed to investigate 2D breaking solitary waves on a sloping beach, 2D periodic breaking waves (both spilling and plunging breakers) in the surf zone, and 3D overturning waves over a submerged conical island. Numerical results in the absence of wind are presented and compared with available experimental data, and then the effect of wind is included in the computation of breaking waves. The key findings of this thesis are that the wind can influence the kinematics and dynamics of breaking waves, as onshore winds assist the development of water particle velocities towards the critical wave phase speed, cause the wave to break earlier in a deeper water further off shore. There is recirculation of air flow above the wave crest in the absence of wind whereas air flow separation is observed in the presence of a sufficiently strong wind. In addition, the wind affects the shape of the overturning jet, generation of vorticity, and energy transformation and dissipation during wave breaking. This study has contributed to the characteristics of breaking waves, focusing on the period during wave overturning. The information gained in this study shed some light on wind effects on breaking waves, which have import implications for coastal engineering and air-sea interaction.

Name dc:type.qualificationname

Ph.D

Level dc:type.qualificationlevel

doctoral

Grantor dc:publisher.institution

University of Leeds

Year dc:date.issued

2010

Author dc:creator

• Xie, Zhihua

Advisors dc:contributor.advisor

• Wen, X.
• Ross, A.

Identifier

uk.bl.ethos.516039

OAI identifier oai:identifier

oai:etheses.whiterose.ac.uk:939