A Study Of Breaking Wave Flow Field In Front Of The Vertical Breakwater And Wave Force

The breakwater, which is one kind of the important coastal structures, is used to protect the harbour from wave intrusion and ensures the water smooth. As a barrier to protect the harbor from the intrusion of the offshore wave, the safety of the breakwater in bad weather condition has been widely studied by the scholars. If the wave height, water depth, mound foundation and the sea bed slope satisfy certain conditions, the waves (breaking or broken wave) which have great impactive and destructive power to the vertical breakwater would occur. The strong wave load can cause the instability and damage of the structure. The characteristic of the wave load has close relationship with the flow field. So the study of breaking and broken wave profiles and the characteristics of flow field has important significance. With PIV system and numerical model, the breaking and broken waves are studied experimentally and numerically in the case of regular waves in this paper:(1) A two-dimensional numerical model based on the Navier-Stokes equations and computational Lagrangian-Eulerian advection remap-volume of fluid (CLEAR-VOF) method is developed to simulate water wave problems. In this numerical model, large eddy simulation method is used to describe the turbulent effect and finite element method is adopted to discretize the governing equations. By using this numerical model, second order stokes waves are well simulated; the absorbing wave generation theory is verified by the simulation of standing wave; the dam-collapse flow is simulated with unstructured grids, and the capability of the numerical model combined with unstructured grids is verified; the breaking and broken waves in front of a vertical breakwater are simulated and the results have good agreement with the experimental data.(2) By using PIV system, the flow fields and wave pressures of breaking wave are studied:the breaking wave profiles and the characteristics of flow velocity are analyzed under different mound and relative wave height conditions; the breaking wave impcat processes are analyzed; the vertical distributions of the horizontal velocity at the instant of impact are given; the vortexes at the toe of the vertical breakwater during the impact process are found; the effects of relative wave height on wave pressures and forces are discussed. Results show, three breaker types, Flip-through, Well-developed plunging breaker and Turbulent bore, are found in the PIV experiment; at the instant of impact, the maximum horizontal velocity occurs near the wave surface in the case of medium-height mound foundation, and under the wave surface in the case of high mound foundation; when d1/d=0.26and H/d1increases to1.2, the breaking wave force decreases with the increase of relative wave height.By using the numerical model, more groups calculation for breaking wave problems are carried out:the breaking wave profiles under different mound, relative wave height and relative berm width conditions are analyzed; the variations of the characteristic horizontal velocities and the characteristic vertical velocities along the vertical breakwater are studied; the effects of relative berm width on breaking wave force are discussed; and the relation between peak wave pressures and the horizontal velocities of the water particles at the still water level is given. Results show, the relative wave height has different effects on the dimensionless VH with the change of relative water depth, the dimensionless VH decreases with the increase of relative water depth and increases with the increase of relative berm width under certain relative wave height condition; the relative wave height has different effects on the dimensionless VV with the change of relative wave height, the dimensionless VV increases with the increase of relative water depth or relative berm width under certain relative wave height condition; as the relative wave height varies from0.8to1.1, the maxium wave force occurs between b/L,=0.1～0.2, and with certain water depth, the relative berm width for the maxium wave force decreases with the increase of relative wave height; the relation of the peak wave pressures and the horizontal velocities at the still water level can be described as Ppeak≈0.75ρv2.(3) By using PIV system, the flow fields of broken waves are experimentally studied. Based on the experimental results, the broken waves are further calculated. The results show, the broken wave induced by the traveling-breaking wave breaks more intensely than the broken wave induced by the normal-traveling wave; bigger wave force could occur during the impact of the broken wave induced by the traveling-breaking wave.