ABSTRACT:

In this study, the distributions of the flow fields induced by nonlinear water waves near a natural asymmetric rippled bed are investigated experimentally and numerically. The wave field solver is based on the two dimensional Reynolds Averaged Navier-Stokes (RANS) equations with a k-ε closure. A piston-type wave maker is set up in the computational domain to produce incident cnoidal waves and the free surface is traced through the PLIC-VOF (Piecewise Linear Interface Construction-Volume of Fluid). Verified by the laboratory tests, the numerical model was used to predict the flow structure in the asymmetric rippled bed under the action of the cnoidal wave, and a complete process of vortex formation, evolvement and disappearance was observed.

INTRODUCTION

In the coastal zone where the water is relatively shallow, a ubiquitous phenomenon is the formation of ripples in the sand. The engineering importance is that the appearance and the evolution of these small bed ripples arises in connection with the transport of sand and the damping of waves in nearshore regions. Because the presence of sand ripples usually causes the separation of the bottom boundary layer and the generation of large vortex structures, the sediment transport is strongly modified not only quantitatively, but also qualitatively. Due to the outlined engineering importance, seabed ripples have been deeply studied both theoretically and experimentally (Sleath,1984; Nielsen,1992; Blondeaux et al., 2000; Faraci and Foti, 2001; Hansen,1994; Trouw et al., 2000). Recently, the research has tended to focus on oscillating bed and oscillatory flow (e.g., Barr and Slinn, 2004; Blondeaux and Vittori, 1991; Hara and Mei, 1990; Jiang et al., 2004; Longuet-Higgins,1981; Ranasoma and Sleath,1994; Sleath and Wallbridge, 2002;Scandura et al., 2000; Fredsøe et al., 1999; Voropayev et al., 1999; Toit and Sleath, 1981).

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