Development Of Boundary Element Water-wave Model And Its Application In Environment-friendly Breakwaters

Since China put forward the strategic policy of “promoting the construction of ecological civilization”and “building a maritime power”,environment-friendly breakwaters have gradually been applied in marine engineering.It is of great significance to investigate its hydrodynamic performance under wave action and provide suitable structural parameters for engineering design.Theoretical analysis is one of the most important methods to study the hydrodynamic performance of structures,which can reveal the interaction laws between waves and structures in a comprehensive and fast manner,and provide a reference for the preliminary design of structures.However,analytical methods are only applicable for regular shapes,and it is difficult to solve analytically for environment-friendly breakwaters with complex shapes.To study the hydrodynamic performance of environment-friendly breakwaters,a semi-analytical water-wave model in frequency domain based on boundary element method(BEM)is proposed.Semi-analytic solutions for wave interactions with arbitrary rigid structures and flexible membranes have been implemented,which addresses the difficulty of decoupling the motion of arc-membranes from the flow field in existing boundary element models and extends the scope of applications of analytical models.Model experiments and computational fluid dynamics(CFD)simulations further verified the correctness of the boundary element model and the effectiveness of the breakwater.In this paper,we propose to study the hydrodynamic performance of two environmentally friendly breakwaters,namely a vertical wall breakwater with culvert and a floating fluid-filled membrane breakwater,using BEM as the main method,supplemented by model experiments and CFD simulations to obtain suitable structural parameters for engineering applications.The main works are as follows:(1)Development and validation of a BEM-based semi-analytical water-wave model in the frequency domain.The eigenfunction expansion method was used to construct the wave inlet and outlet boundary conditions of the boundary element model,and a generalized boundary element model that can be used to solve the wave problem was developed by combining the boundary element method with fluid boundary conditions.BEM has been used to solve wave interactions in two-dimensional flexible membranes,rigid structures,permeable structures,and floating structures of arbitrary shape and number,thus extending the application of existing BEM to the problem of water waves.A general solution equation is constructed,where the boundary conditions are specified and the rest of the computation is implemented automatically,effectively reducing the modeling time.Model accuracy is verified by comparing it with examples of analytical solutions,such as submerged permeable breakwaters and floating bodies,which sets the stage for subsequent analysis of the interaction between waves and environmentally friendly breakwaters.(2)Improvement and validation of a mass conserved CFD numerical wave tank based on viscous flow.It is shown that the traditional velocity boundary conditions will lead to mass nonconserved phenomenon in numerical wave tank and the relaxation method will cause the problems of interface fragmentation and virtual wave train.In view of the above shortcomings,a zonal modified relaxation method is proposed,which improves the mass conservation and wave dissipation performance of the numerical wave tank by adding equivalent countercurrent velocity boundary conditions to the wave-making boundary and introducing spatio-temporal corrections to the relaxation area.The improved CFD numerical wave tank shows better accuracy and reliability in verifying wave-induced mass transport and spilling wave breaking.(3)Hydrodynamic performance and design optimization of a vertical wall breakwater with culvert.Solution of the interaction between waves and vertical wall breakwater with horizontal culvert and the analytical expression for the oscillatory flow in culvert are obtained,and the influence of the culvert and foundation on the wave dissipation,wave force and water exchange characteristics(oscillatory flow)of the structure is analyzed.Through model experiments and CFD numerical simulations,the effectiveness of the analytical solution is verified,and the exchange mechanism and the corresponding nonlinear characteristics of the water inside and outside the culvert are revealed.In order to further promote water exchange,based on vertical wall breakwater with inclined culvert,an inclined culvert breakwater with better hydrodynamic performance is put forward.The difference between the hydrodynamic performance of inclined and horizontal culverts was investigated using BEM,and the influence of the inclined angle of the culvert and the permeable rubble mound foundation on the hydrodynamic performance was emphatically analyzed,revealing the mechanism of water exchange in inclined culverts through CFD simulations.On this basis,the evaluation formulae considering the hydrodynamic performance of breakwaters is put forward,and culvert parameters suitable for engineering applications are obtained.(4)Hydrodynamic performance and design optimization of a floating fluid-filled membrane breakwater.The static boundary conditions of the floating fluid-filled membrane breakwater are derived,and the integral expression of the membrane dynamic tension and the boundary conditions of the curved membrane motion are derived based on the membrane intrinsic structure relationship in the column coordinate system.The above boundary conditions are applied to the boundary element model developed to achieve the first fully coupled solution of the membrane motion and the flow field.Based on BEM and model experiments,the analysis focuses on the resonant mechanism of the structure in different modes and the influence of the structure density,filling ratio and radius on its hydrodynamic performance.The interactions between the resonance response of the structure and the transmission coefficient,wave force and dynamic tension of the membrane is obtained,and parameters such as the optimum density and filling ratio of the structure are put forward,which provides reference for further improving the wave attenuation effect and stability of the structure in specific frequency bands in engineering practice.