LARGE EDDY SIMULATION FOR PLUNGING BREAKER WAVE

As wave propagates into shallow water, the shoaling effect leads to increaseof wave height, and at a certain position, the wave will be breaking. The breaking wave is powerfulagents for generating turbulence, which plays an important role in most of the fluid dynamicalprocesses in the surf zone, so a proper numerical model for describing the turbulent effect isneeded urgently. A numerical model is set up to simulate the wave breaking process, which consistsof a free surface model using the surface marker method and the vertical two-dimensional model thatsolves the flow equations. The turbulence is described by Large Eddy Simulation (LES) method wherethe larger turbulent features are simulated by solving the flow equations, and the small-scaleturbulence that is represented by a sub-grid model. A dynamic eddy viscosity sub-grid scale stressmodel has been used for the present simulation. The large eddy simulation model, which we presentedin this paper, can be used to study the propagation of a solitary wave in constant water depth andthe shoaling of a non-breaking solitary wave on a beach. To track free-surface movements, The TUMMACmethod is employed. By applying the model to wave breaking problem in the surf zone, we found thatthese model results compared very well with experimental data. In addition, this model is able toreproduce the complicated flow phenomena, especially the plunging breaker.

Experimental Study on the Bed Shear Stress Under Breaking Waves

The object of present study is to investigate the bed shear stress on a slope under regular breaking waves by a novel instrument named Micro-Electro-Mechanical System （MEMS） flexible hot-film shear stress sensor. The sensors were calibrated before application, and then a wave flume experiment was conducted to study the bed shear stress for the case of regular waves spilling and plunging on a 1 ： 15 smooth PVC slope. The experiment shows that the sensor is feasible for the measurement of the bed shear stress under breaking waves. For regular incident waves, the bed shear stress is mainly periodic in both outside and inside the breaking point. The fluctuations of the bed shear stress increase significantly after waves breaking due to the turbulence and vortexes generated by breaking waves. For plunging breaker, the extreme value of the mean maximum bed shear stress appears after the plunging point, and the more violent the wave breaks, the more dramatic increase of the maximum bed shear stress will occur. For spilling breaker, the increase of the maximum bed shear stress along the slope is gradual compared with the plunging breaker. At last, an empirical equation about the relationship between the maximum bed shear stress and the surf similarity parameter is given, which can be used to estimate the maximum bed shear stress under breaking waves in practice.