Numerical Simulation of Local Scour with Free Surface and Automatic Mesh Deformation

Abstract

A numerical model for local scour with free surface and automatic mesh deformation is constructed and numerical simulation is carried out to compare with experimental results. The k2 model is used to simulate the turbulent flow field. Two interfaces (water and air, water and sediment) in the domain are captured with different approaches. The free surface of the flow is captured by Volume of Fluid (VOF) scheme which is a Eulerian approach. The water-sediment interface (bed) is captured with moving mesh method which is a Lagrangian approach. The flow field is coupled with sediment transport using a quasi-steady approach. Good results have been obtained using current model. The flow field is comparable with the experiment. Scour patterns are similar to the experimental data. INTRODUCTION In this paper, a new numerical model is proposed to simulate the scour process together with free surface and automatic mesh deformation. In scour problem, there are two sharp interfaces: water-air and water-sediment. The two interfaces are captured using different methods. The water-air interface is captured by Eulerian method, namely VOF method. The water-bottom interface is captured by Lagrangian method, namely moving mesh method. In numerical simulation of open channel flow, free water surface is usually replaced by a rigid lid. This is valid only if the free surface does not change too much along the channel. There are many surface tracking or capturing method available to simulate the free surface, e.g. marker and cell (MAC) method, volume of fluid method (VOF) method (Hirt and Nicholls 1981), level set method (Sethian 1996). In this paper, a high resolution VOF method proposed by Ubbink and Issa (1999) is chosen to track the free surface. The CICSAM (Compressive Interface Capturing Scheme for Arbitrary Meshes) scheme treat the whole domain as the mixture of two liquids. Volume fraction of each liquid is used as the weighting factor to get the mixture properties, such as density, viscosity.