The Study On Similitude Of Under Water Complex Elastic Shell-structure

This research was sponsored by Special Research Fund for the Doctoral Program of Higher Education. In this paper, the similitude theory is applied in the underwater acoustic engineering. The vibration and sound radiation of underwater complex elastic shell-structure are studied by the similitude model experiment. This kind of method not only can predict the character of vibration and sound radiation of underwater complex elastic shell-structure but also can give some helpful and useful information for the optimized design and noise control of the one.Using finite element method and similitude theory, the similar relationships and conditions of vibration , coupling vibration and sound radiation of the ring-stiffened cylindrical shell with hemi-spherical caps on the ends are presented in this paper. When the material of prototype and the fluid around it are same as ones of its scaled-down model, the similar ration of modal frequency between the underwater complex shell-structure and its scaled-down model is inversely proportional to the similar ratio of their geometric properties. And the modal type of prototype is as same as one of it scaled-down model. Meanwhile, their similar relationships can not be changed by the fluid loading. When the geometrical similar conditions between prototype and their scaled-down model are satisfied, the radiation efficiency and the directivity of sound pressure in far field for the prototype are equal to ones of its scaled-down model respectively.In this paper, the statistical energy analysis(SEA) and the similitude theory are applied to research the similitude of vibration , coupled vibration and sound radiation of underwater complex shell-structure, the similitude conditions and relationship with the statistical character are presented. When the material of prototype is the same as one of its model, and the geometrical similar conditions are satisfied, the radiation loss factor of prototype is equal to one of its model. Namely, the coupling loss factor between subsystems in the prototype is equal to the corresponding one of its model.The predicted project that the vibration, coupling vibration and soundradiation of prototype are predicted by ones of distorted scaled-down model are presented in this paper. The predicted curve and curve-fitting equation are derived by the predicted coefficient method of FEM for the first time. On the basis of these predicted curves and curve-fitting equation, the characters of vibration and sound radiation can be predicted easily.The reliability of similitude theory which is applied in the underwater acoustic engineering is analyzed at length. The non-similar function or parameter is defined in this paper. The self-similitude of the physical parameter is studied and the self-similitude section is established rightly. In the end, the scale effect and boundary effect in the similitude model experiment are researched. According to these results, the influence of scale effect and boundary effect on the predicted results of prototype can be avoided or be reduced.On the basis of these similar conditions, three experimental models are designed and made. The experimental results are good agreement with theoretical results, and the similar relationships are verified by this similar experiment between prototype and its scaled-down model.The similitude model experiment method not only can predict the character of vibration and sound radiation of the underwater big-scale shell-structure but also can give some helpful and useful information for the optimized design and noise control of the underwater big-scale shell-structure. So the study of this paper has very important significance for the appliance in the underwater acoustic engineering field.