Experimental Study Of Wave Propagation And Breaking On Lens Topography And Research On Wave Numerical Model

Wave propagation and wave breaking on the topography is an important research topic of ocean engineering.The existing wave breaking indices are usually based on unidirectional regular and irregular waves and two-dimensional topographies,which ignore the variation along the width direction.Since the actual topographies are complex and the sea waves are multidirectional waves,the study of propagation and wave breaking of multidirectional waves on 3D topography has an important significance.Besides,the numerical simulation of wave breaking phenomenon is an important subject.The premature wave breaking phenomenon occurred when simulating the wave breaking process on the slope by the two-phase flow model if the aspect ratio of the grids is not equal to 1.Therefore,it is necessary to analyze the premature problem and improve the two-phase flow model to avoid the premature wave breaking.In this thesis,physical experiment was conducted to investigate the wave propagation on the lens topography.Two different water levels are adopted in the experiment to distinguish the positions of wave breaking.At the low water level,wave breaking point is on the slope since the water level and the top surface of the topography are at the same height.At the high water level,waves break on the platform at the top of the topography and some waves break again in the following propagation.The investigation shows that the non-uniform variation of the topography in width direction has an important effect on the wave propagation.The wave heights of different types of waves on lens topography are larger than that on two-dimensional topography because of the wave focusing effect of lens topography.When the waves break on the slope,the wave nonlinear parameters and wave height distributions on lens topography are close to the two-dimensional topography results.This is because of the weak focusing effect due to the short propagation distance.When the waves break on the platform of the topography,the asymmetry of wave profile is larger on the lens topography.The skewness and kurtosis on the lens topography are larger when wave period is relatively short,while the opposite when wave period is relatively long.Besides,the wave heights of normal incident waves with relatively long wave period are smaller than oblique and multidirectional waves,due to the wave energy dissipation on the slope.At the same time,the breaker indices of lens topography and two-dimensional topography are compared based on wave types and wave breaking positions.The results show that the 3D topography has an effect on the wave breaker indices.The breaker indices on lens topography are larger than that on two-dimensional topography,due to the wave focusing effect.Classified by different wave conditions,appropriate breaker indices on lens topography are selected and fitted by the experimental data,and the wave breaker indices are given for the present lens topography.Further,to provide a basis to simulate the physical waves in the numerical model,a three-dimensional numerical wave tank was developed.The moving boundary method is adopted in this model to simulate the movement of wavemaker and generate water waves.To expand the effective wave area,the commonly used L-type wave maker system in physical wave basin is used in the numerical model.The movements of wave-maker are replicated by smooth boundaries.An effective method is established to prevent interruption between the adjacent wave makers at the corner.The movements of boundary mesh points can be calculated by theoretical formulas to mimic the movements of wave-maker,while the movements of internal mesh points can balance the boundary mesh deformation.Two-dimensional regular and irregular waves are simulated,with the capability to trigger the active wave absorption algorithm.Oblique regular waves and multidirectional random waves are simulated,yielding a good agreement with theoretical solutions and verifying the effectiveness of the numerical model.To solve the premature breaking wave problem in numerical simulation with different aspect grids,a coupled volume-of-fluid and Level Set method was proposed for interface calculation.Both the advantages of mass conservation by VOF method and the accurate calculation of normal vector by Level Set method are maintained in this method.A geometrical VOF method-isoAdvector method was adopted in this work for the advection of volume fraction field,with a surface reconstruction algorithm added by calculating the contour surfaces.Level Set function was calculated by an iterative geometrical method,which is similar to the VOSET method.The dam break problem and Zalesak disk problem were used to verify the validity of the numerical method.Then the method was applied to simulate the process of wave propagation and wave breaking on topography.The results were compared with the experimental results to verify that it can effectively simulate the wave breaking process on the topography.The simulation results are identical to each other with grids of different aspect ratios,which indicate the premature breaking wave problem is solved effectively.It provides an effective measure for the simulation of 3D breaking wave.