Last modified: 2019-06-05
Abstract
Wave breaking is a prominent research subject in coastal and marine engineering. It is a two phase flow phenomenon involving air and water, and it strongly influences the air–sea interaction by enhancing mass, momentum and energy transfer between the phases. It thereby limits the wave steepness, generating vorticity and turbulence, enhancing wave energy dissipation, entraining air and white water formation. Despite a considerable number of experimental, numerical and theoretical studies and field observations have been carried out to investigate the process, the wave breaking mechanism is not completely understood. A comprehensive examination of breaking wave properties is inevitable to understand the mechanism of wave breaking and thus the description of the breaking process. Wave breaking over a submerged reef primarily depends on the tidal level and the characteristics of the incident waves. Moreover, an accurate description of waves breaking over submerged structures has always been a central issue in estimation of hydrodynamic loads on marine structures.
Numerous studies have attempted to explain the wave evolution and breaking process and their characteristics. In the case of submerged terrain and reefs, wave breaking is strongly influenced by the local environmental parameters, such as water depth (d) and sea bed slope (m). This has been studied in laboratory experiments by Blenkinsopp and Chaplin (2008). In order to research the nonlinear phenomenon, several surface wave theories have been investigated and proposed to resolve the wave breaking issues. Meanwhile, many theories were put forward to describe the wave breaking, and most studies in the field of submerged breakwater structures have only focused on the prediction of the reflection and transmission characteristics of waves for a given environmental condition. Ting and Kim (1994) investigated the wave transformation over a submerged structure and concluded that potential theory cannot be applied to model the flow process such as flow separation and energy dissipation. However, the breaking process and generation and dissipation of vortices are created by rotational flow (Takikawa et al., 1997). Numerical modeling of wave breaking becomes challenging due to the intricacy in describing the physical processes involved such air-sea interaction, vorticity generation, overturning motion and the air entrainment. Hence, a straightforward approach to describing the breaking process numerically is applied to solve the fundamental fluid dynamic equations with CFD (Computational Fluid Dynamics) method. Chella et al. (2015) did the investigation about the characteristics and profile asymmetry properties of wave breaking over an impermeable submerged reef by the CFD method, and the capture of free surface is conducted by the level set method. The numerical result showed great correlation to the experimental results which is just the contribution of consideration of vortex and viscous.
In this paper, a viscous flow solver (naoe-FOAM-SJTU) which is developed and based on the popular open source toolbox OpenFOAM is presented. The solver is adopted to study the effect of wave steepness and relative crest submergence on wave breaking properties over submerged reef. Wave steepness and relative crest submergence will be considered separately and the asymmetry properties and cavity properties that induced in the process of wave over-turning will be investigated in detail.