Investigation On The Hydroelastic Vibration Characteristics Of Two-dimensional Elastic Structures

Large-scale elastic structures,such as ships and offshore platforms,interact with water to form fluid-structure coupling vibration problems under external forces.The violent vibration leads to fatigue and failure of marine mechanical components and hull structures,threats the safety navigation of the ship,interferes with the normal operation of a variety of mechanical and electrical equipment,and affects the comfort of the staff living on board and the ship working efficiency,so it is necessary to predict the structures' hydroelastic vibration characteristics in the preliminary design stage to avoid adverse effects.Ships and marine structures are mainly made up of beams and plates,so,in this thesis,the vibration characteristics of two-dimensional plates and stiffened plates in contact with water are investigated through theoretical analysis,numerical computation,experiments and engineering applications.In the theoretical analysis section,through solving the boundary value problem of flow field potential on the assumption that the fluid is non-viscous and non-compressible,the expression of flow field potential is derived,containing the plate's unknown transverse vibration displacement function.Then,the mode shape function of the structure in contact with water is approximated by the linear superposition of that in air,and the vibration characteristic equation of the system is derived using energy method.In the numerical computation section,The vibration characteristics of the structure in air and water are compared,and it shows that the two mode shapes are almost the same,but the frequencies in water decrease a lot because of added mass,and the influence varies with vibration orders and boundary conditions.Then,non-dimensional parameter analysis is conducted,and the results show the sizes of water and structures and boundary conditions have impact on added mass,and the modes of stiffened plates in water may have some transitions compared to that in air.In the engineering applications section,the parameters affecting non-dimensional virtual added mass incremental are fitted based on least square method,and the fast prediction formulas are proposed to predict two dimensional structures' vibration characteristics in contact with water,and the wall effects are taken into account in the formulas.In the experiment section,the modal identification method based on strain frequency response function is deduced.Then,strain measurement is applied to structure's wet modal tests,and the comparison between experiment and fast prediction is made.The results show that,applying strain measurement to structure's displacement modal identification is feasible,and the natural frequencies' relative errors are within 5%,so the fast prediction formulas proposed in this thesis are precise enough,and they can be used in engineering preliminary designs.