NUMERICAL STUDY ON WAVE BOTTOM BOUNDARY LAYER OVER RIPPLES

摘要

Vortex ripple is widely formed in the coastal region. The vortex ripple enhance the separation of flow and turbulence, it plays a vital role in various coastal process, such as sediment transport, the wave attenuation and the mass transport due to wave. Nevertheless, the flow structure and sediment movement above vortex ripple has not been investigated owing to the complexity of the phenomena. Du Toit & Sleath (1981) used the Laser-Doppler to measure the flow over a ripple bed, the similar work was conducted by Sato S.J. et al. (1986), very detailed measurements of velocity closed to the bed over fixed smooth and rough bed are carried out using LAD. In resent years, most effort has been focused on obtaining a numerical solution of oscillatory flow over ripple, the first work was described by Lounguet-Higgins (1981), who used the discrete vortex model. Vittorri & Blondeaux (1991) treat a laminar flow situation by solving the vorticity transportation equations. Sato S.J. et al. (1986), Aydin (1987) and Tsujimoto et al (1991) used a two equation turbulent model k — ε to study the flow around the ripple and the suspended sediment concentration distribution. Recently, Fredsoe, J et al.(1999) have conducted experiments and numerical simulation by k — ω model to study the flow structure around the vortex ripples under wave action. In the present study, a numerical simulation system based on LES method is developed for analyzing flow structure and dynamic of vortex, the sub-grid-scale turbulent stress is evaluated by the Smagorinsky model (1963). The simplify marker and cell method (SMAC) is used to solve the basic equations in the the curvilinear coordinator system, and a detailed vortex dynamics is discussed.