Numerical Study of Plunging Wave in Deep Water

Abstract

Wave breaking, a common occurrence in the oceans, plays an important role in air-sea interactions including the transfer of energy and mass across the air-sea interface, turbulent mixing in the surface layer of the ocean. It is a highly non-linear, intermittent 3D phenomenon involving two-phase flows and turbulent mixing. Due to the complexity of the breaking process, studies of wave breaking (both experimental and numerical) are limited by the methodologies available and there remain significant gaps in fully understanding the mechanics of wave breaking. In this thesis, a detailed numerical study of wave breaking has been carried to examine the local physics of the wave plunging process, with an emphasis on the mechanics of the plunging jet, air entrapment, subsequent breakdown of the entrapped air, vertical sprays and turbulent mixing. An enhanced Smoothed Particle Hydrodynamics (SPH) methodology has been developed for the numerical study. The key controlling parameters of the SPH model are carefully selected through calibration and sensitivity studies to minimize errors at the air-water interface. At the solid boundaries, an enhanced ¿ghost particle¿ method is developed to improve the consistency of the flow field near the boundaries. Within the fluid domains, flow regularization techniques including the velocity correction and the 1st order density re-initialization are applied. These methods are modified to account for large differences in the density and pressure gradient across the air-water interface and the conservation of momentum. The SPH code is also coded to run in a parallel computing cluster, hence increasing the computational speed and resolution. As the simulation is still compute-intense, even with parallel computation, a multi-scale nesting approach is also developed to reduce the overall computational cost. An experimentally simulated plunging wave (Kway 2000), generated through wave-wave interactions, is simulated in this study and this forms the basis for the detailed studies of the wave plunging process. Both the air and water layers are modelled in the simulation, hence permitting a more accurate description of the air-water interaction. The study has been conducted with very high temporal and spatial resolutions. This is necessary in order to pick up the finer details that have been observed in the experiments but not captured in the past numerical simulations. The numerical results of the 2D plunging wave in deep water obtained in this study compare well with the experimental results of Kway (2000). These include details of the plunging jet, jet impingement, air entrapment, disturbances on the surface of the entrapped air tube, vertical jet ahead of the plunging jet, upward water sprays, and collapse of the entrapped air tube. The numerical results have also helped to elucidate finer details of the wave breaking process. These include the bifurcation of the flow field relative to the crest velocity, especially on the wave front near the crest, circulations coupled to the air entrapment process, the air tube ¿rolling¿ forward, vertical jet collapsing in conjunction with air ¿squirting¿ out from the entrapped air pocket generating the characteristic vertical water spray, distributions of pressure, acceleration and vorticity in the vicinity of the plunging crest, and the dissipation of wave energy associated with the plunging.