Simulation Of Nonlinear Wave Impacting On Suspended Structure

Wave force is a very important design load on ocean structures. The impact of wave can cause local failure or instability of structures and the negative pressure induced by the separation of wave from structures will also do harm to durability of concrete structures. The impact mechanism of waves refers to many factors including strong non-linearity, temporal effect, viscidity, turbulence as well as mixing of water and air. The whole process is so complex that it is still one of the most difficult problems in the field of coastal engineering mechanics.A wave model of two phase flow is formulated for the whole impact process of waves on structures. The VOF method is used to track the free surface and the sources are added to the momentum equation to absorb reflection of wave on boundaries. The impact process is simulated and several related phenomenon are analyzed, such as wave force dynamics, the air movement and its distribution.The impacting process of wave on the structures can be divided into three stages, the propagation of local impact pressure, the appearance and propagation of negative pressure, the appearance and propagation of secondary wavelet. The uplift force gets its peak value at the first stage, and then rapidly decreases from positive to negative at the second stage. At the third stage the fluctuation of uplift force occurs, and the pressure beneath the structures disappears when the wavelet is separated in the negative pressure area.When wave impacts on the structures, a part of air is ejected from the upper boundary and another part of air moves in a shape of wedge with the wave beneath the structures. The wedge -shaped air moves obviously faster than ambient water. A large angle between the movement directions of them exists. As the moving water entrains air continuously, the free water surface beneath structures protuberates at the tail of the wedge-shaped air.The air closed by wave under structures has some characteristics as follows. Firstly, the closed air forms in the shape of bubbles which adhere to the bottom of structures and move with the wave. Secondly, the air bubbles have different sizes and positions, but most of them gather in the middle and just a few in two ends.