ABSTRACT:

The unsteady, two-dimensional Navier-Stokes equations and the exact free surface boundary conditions have been solved to simulate the flow separation and the vortex generation in water waves propagating over a submerged rectangular dike. The computed water surface elevations at different locations have been compared with experimental ones to verify the accuracy of the numerical method. The vorticity of the flow near the dike are calculated from the numerical velocity fields. Effects of the Keulegan-Carpenter number on the vortices is investigated. The wave forces on the dike are also determined. It was found that the wave force nearly reaches its highest value when the wave crest is above the dike.

INTRODUCTION

ha previous approaches, impenneable seawalls or revetments were very. often used to prevent beach erosion. However, the incident waves are reflected from the seaward faces of these structures and ultimately cause structural subsidence. In new techniques, the seaward surthces of the seawall are covered with riprap or made of step-faces to reduce the reflection. The offshore breakwater has also received great attention in the past few decades. Offshore breakwaters are used along shorelines to reduce the wave energy impinging opon beaches. A submerged dike has been used as one of the general types of offshore breakwater. When a wave propagates over a submerged dike, it breaks ocassionally as the wave crest meets the top of the dike. In addition, the flow separates from the structure and vortices are generated near the dike. A major effect of the vortices is aggravation of the erosion of the sea bed at the toe of the dike. Furthermore, the great rate of shear strain associated with the vortices can seriously damage the thbric of the submerged structure. Hence it is important for ocean engineers to understand the dynamics of vortex shedding from the submerged dike. Previous studies on water waves over a submerged obstacle have been mainly concerned with determining the reflection and transmission characteristics of water waves.

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