Study On Propagation Characteristics Of Interaction Between Wave And Multi-box Free Surface Structure

As a kind of protective facility for coastal and offshore construction,breakwater is mainly used to attenuate wave and reduce the impact of wave on coastal and offshore constructions.Breakwater can be divided into bottom mounted breakwater and free surface breakwater according to the location of breakwater main body.Compared with bottom mounted breakwater,free surface breakwater has gained widespread attention due to its advantages of environmental friendliness,low cost,rapid construction,easy disassembly and flexible movement.However,for longer wave,free surface breakwater is not effective.How to design a free surface breakwater to attenuate longer wave effectively is key and difficult point.In order to improve the attenuation effect of free surface breakwater on longer wave,this paper introduces the concept of wave Bragg resonance and the topography of rectangular trench to study the influence of free surface multi-box structure on wave propagation,especially the influence of transmission and reflection on longer wave.Based on potential theory,a two-dimensional weak nonlinear numerical wave tank(NWT)is established by Desingularized Boundary Integral Equation Method(DBIEM).Ramp function is adopted to decrease velocity mutation of inlet,damping layer is adopted to dissipate wave reflection at the end of tank,Mixed Eulerian-Lagrangian(MEL)method is utilized to track the instantaneous free surface,Fourth-order Adams-Bashforth-Moulton scheme is adopted to update wave elevation and the velocity potential of free surface,Chebyshev 5-pts.smoothing scheme is used to avoid the non-physical saw-tooth numerical problem of free surface during time marching,and cubic spline interpolation function is adopted to fit the curve of free surface.As for result analysis,transmission coefficient and reflection coefficient are adopted to express the transmission and reflection characteristics while wave propagating through free surface multi-box structure.Reflection coefficient is dealt with a reflection wave separation method with independent intellectual property rights.By comparing with the relative error of energy conservation equation of a double-box model,the accuracy of transmission coefficient and reflection coefficient is analyzed.Firstly,a second-order Stokes wave is made,three calculation parameters,including source point count,desingularization distance and time step,are varied respectively,and the calculated numerical results are compared with the analytical solutions.Considering the calculation accuracy and calculation efficiency,suggested values of the three calculation parameters are given for reference,which establishes basic for the following contents.Then,regular waves propagating through single free surface box models,rectangular trench bottom models and box-trench combination models are computed.The calculated transmission coefficients of single free surface box are compared with experimental results,and the calculated transmission coefficients of rectangular trench are compared with analytical results,and calculation accuracy is satisfactory.The influences of width and draft of single free surface box,width and depth of rectangular trench and width and draft of box-trench combination on the transmission and reflection coefficient are analyzed.Velocity vector distributions are showed for analyzing transmission and reflection properties because of different wavelengths.Increasing the width and draft of box,the width and depth of rectangular trench could decrease wave transmission and improve wave attenuation efficiency;the wave attenuation efficiency of the box-trench combination is generally better than single free surface box.Although the rectangular trench reduces the longer wave transmission,it is not obvious enough.The characteristics of reflection performance is opposite to transmission.Lastly,on the basis of the above,free surface multi-box and multi-box-trench combination is studied.The wave transmission and reflection characteristics are studied,while the migration characteristics of Bragg resonance point and the effect of rectangular trench bottom are especially noted.The influences of width,draft and spacing of free surface multi-box,width,draft and spacing of multi-box-trench combination,and the number of boxes on the transmission coefficient and reflection coefficient are analyzed.Velocity vector distributions of different wavelengths are analyzed.It is found that the Bragg resonance point of free surface multi-box and multi-box-trench combination is at the right of theoretical Bragg resonance point.Increasing the number of boxes could improve the efficiency of wave attenuation,and Bragg resonance point shifts to the left.Increasing the width of boxes could improve the efficiency of wave attenuation,and Bragg resonance point shifts to the right.Increasing the draft of boxes could improve the efficiency of wave attenuation,and Bragg resonance point shifts to the left.Bragg resonance point of free surface multi-box shifts to the left with the increase of the spacing,while Bragg resonance point of multi-box-trench combination shifts to the right with the increase of the spacing.Near Bragg resonance point,reflection is strengthened and transmission is weakened.The wavelength near the resonance point is close to box spacing,which could recommend one of breakthrough points to improve the effect of surface breakwater on wave attenuation on longer wave.The wave elimination efficiency of multi-box-trench combination is generally better than free surface multi-box,but it is not obvious enough too.