ABSTRACT

An interface-preserving level-set numerical method has been incorporated into a Reynolds-Averaged Navier-Stokes (RANS) method for the simulation of wave runup and greenwater effect on offshore structures. In this method, the free surface flows are modeled as immiscible air-water two-phase flows and the free surface itself is represented by the zero level-set function. Calculations were performed first for two- and three-dimensional problems for dam-breaking flow and free jet problems involving violent free surface motions. The level-set RANS method was then employed for the simulation of greenwater on offshore structures and slamming on platform deck. The numerical results clearly demonstrated the capability of the level-set method to deal with complex free surface flows involving breaking waves, water droplets and trapped air bubbles.

INTRODUCTION

Greenwater loads on an offshore platform occur when an incoming wave significantly exceeds the free board and water runs onto the deck. The primary difficulty in the simulation of this green water phenomena lies on how to track the air-water interface. Many methods have been proposed in predicting the interface between two different fluids. The interface-tracking methods follow the free surface motions and use boundary-fitted grids which are re-adjusted in each time step whenever the free surface moves. The interface-capturing methods do not define a sharp free surface boundary. Instead, the computation is performed on a fixed grid, which is extended beyond the free surface and the shape of this free surface is determined by cells that are partially filled. A variety of numerical methods in this interface-capturing approach have been developed over the past several decades. Three typical methods from this approach are Marker and Cell (MAC) scheme (Harlow and Welch, 1965), volume of fluid (VOF) scheme (Nichols et al., 1980; Hirt and Nichols, 1981) and the level set method (Osher and Sethian, 1988).

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