Analytical Study Of Wave Hydrodynamics For Structure With Lower Porous Thin Wall

In recent years,porous structures have received more and more attention due to their significant advantages in wave reduction and wave dissipation,and have been widely applied in port engineering,coastal protection engineering,and sea reclamation engineering.However,the existing theoretical research results and engineering practice cases are mostly concentrated in the field of coastal engineering.In the field of offshore engineering,it is still in the initial and exploratory stage.As with the wave load characteristics and oscillation response characteristics of porous structures in the field of offshore engineering,the studies associated with theoretical solution methodologies can not only gain a better understanding of the wave action mechanism for porous structures,but also be important to further promote the engineering application of porous structures.Therefore,based on the classical potential flow theory,this thesis takes box-shaped porous structure and porous cylinder as the research objects,an analytical calculation model for the interaction between porous structures and waves is established.The main research work and related results are as follows:(1)Under the framework of the classical potential flow theory,the processing methods for the boundary conditions of the porous thin wall is proposed,and the Laplace equation in the Cartesian coordinate system and the cylindrical coordinate system are theoretically deduced,on base of which,the solution for the issue associated with the interaction between porous box/porous cylinder and waves can be further derived and calculated.(2)Design of engineering quantitative experiment schemes for the porous-effect parameters of porous thin wall.The dimensionless porous-effect parameter is closely related to the type and manufacturing conditions of the porous structure.Based on the theoretical analysis of the interaction between the regular waves and the porous thin wall under the potential flow theory,an experimental scheme aiming at combining dimensionless porous-effect parameter with diameter and porosity of porous wall is proposed to measure the dimensionless porouseffect parameters,and the experimental research is implemented to verify the correctness of the theoretical analytical solution.(3)Analysis of the wave scattering characteristics of the box-shaped structure with lower porous thin wall.Using the separation variable method and the orthogonal characteristics of some functions in the corresponding integration interval,combined with the matching conditions of the adjacent boundaries of the fluid subdomains,the analytical solutions for the Laplace equations of the potential function in each fluid subdomain are established,and the influence mechanism of the porous-effect parameters on the wave exciting force(torque),wave transmission coefficients and the reflection coefficients is analyzed,meanwhile,the results are compared with the experimental data and the existing studies.The results show that the transmission coefficient is significantly more affected than the reflection coefficient in case of lower porous thin wall,and the porosity hardly affects the transmission coefficient under the condition of h/λ≥ 0.15.When the porous thin wall in the lee side is constant,G02=1.0,a total of 47%wave energy is dissipated corresponding to G01=0.7.The vertical force is slightly affected by the porous thin wall,and the horizontal force is decreasing with the increase of porous-effect parameter,the pitching torque is affected significantly most,especially for the case of small box-shaped structure,where the increase of porous-effect parameter from 1 to very large in the same way,the peak value of pitching torque can be decreased by 75%.(4)Analysis of the oscillating radiation characteristics of box-shaped structure with lower porous thin wall.Focus on the oscillation of box-shaped structure with lower porous thin wall in three degrees of freedom(pitch,heave,and roll),the variable separation method and the boundary matching characteristic function expansion method are comprehensively applied to develop the dimensionless theoretical analytical calculation model for the oscillating radiation potential function.Thereinafter,the oscillating added mass and damping coefficient is deduced for the box-shaped structure,and the model is verified by the existing research results.The results show that the surging added mass is increased with the adding of lower porous thin wall,and the peaking point will appear under the condition of G01=G02≤ 1.0,corresponding withλ/h=5π.The surging damping coefficient can be obviously decreased with the increase of porous-effect parameter,and the heaving added mass is significantly more affected by the porous-effect parameter than that of heaving damping coefficient.The dimensionless wavenumber for the pitching damping peak point coefficient is almost the same for different porous-effect parameter,that is k0h=1.8.In case of different porous thin wall,the dimensionless wavenumber for pitching damping peak point is around 1.8,k0h=1.8 approximately.(5)Analysis of the wave load characteristics of a cylinder with lower porous thin wall.The cylinder with lower porous thin wall is oscillating in three degrees of freedom(surging,heaving,and pitching)under the excitation of regular incident waves.The characteristic function expansion method is applied to derive the analytical solution for the potential of each fluid subdomain in the cylindrical coordinate system,based on which the calculation of the wave exciting force and hydrodynamic coefficients of the cylinder with lower porous thin wall is implemented,and the correctness of some analytical solutions are verified by the existing results.The results associated with wave load show that the heaving force is affected slightly by the porous-effect parameter,only in case of λ/a≥π,can the influence of porous-effect parameter to the horizontal force be observed,and the decrease tendency of pitching torque is obviously observed with the increasing of porous-effect parameter.As with the hydrodynamic characteristic,both the surging added mass and damping coefficient decrease with increasing porous-effect parameter,and the decrease value of added mass is basically not affected by the dimensionless wavenumber while the damping coefficient is decreased obviously within certain wavenumbers.The decrease of heaving added mass is observed with increasing porous-effect parameter while the variation for damping coefficient is on the contrary.The appearance of lower porous thin wall can cause the disappearing of peak point in the curve of pitching added mass.