Analysis On The Self-vibration Of The Moving Structure Under Water

When the underwater structure vibrates, the vibration changes the condition around the structure and the fluid field acts on the structure inversely. So a coupling system composed of the structure and the fluid field around the structure is formed. In this paper the vibration character of the structure moving under the water has been investigated, and the emphasis is put on the influence of the speed on the self-vibration character of the structure.At first, a finite element dynamic equation of the structure vibration is developed with the displacement-pressure mode. And the finite element expressions of the added mass matrix, added damping matrix and added stiffness matrix are obtained. The added damping matrix and the added stiffness matrix are relevant to the moving speed of the structure. Comparing with the common dynamic equation of the structure, only some added matrixes are added in this equation. Moreover, the added stiffness matrix that is relevant to the hydrostatic pressure is considered in this paper. When the added stiffness matrix that is relevant to the hydrostatic pressure and the added mass matrix, the added damping matrix and the added stiffness matrix that is relevant to the hydrodynamic pressure are put into the dynamic equation of the structure, the vibration equation of the structure in the fluid-structure coupling system is changed into the equation of the common elastic vibrationthat can be easily solved.Then three testing programs are adopted in this paper. By comparing the added mass matrix from the program and the one from the Morison's formula about a regular structure, the deducing of the finite element formula is testified to be right. To testify the accuracy of program in computing the self-vibration character, another comparison is done between the natural frequencies from the program and the one from the theory to a simply supported square plate. The head and tail of the simplified model are divided with the triangular elements, and its body is divided with the quadrilateral elements. The self-vibration character of the model is obtained with the program. The first four bend natural frequencies and modes of the structure from the program are similar to those from the ANSYS analysis. It can be seen that the results from the program is reliable.At last, the proper fluid scope around the structure is chosen and divided with six-node wedge elements and eight-node brick elements. The self-vibration character of the structure under the water is computed with the finite element formula and the program. The effects of the moving speed and the depth of the water on the vibration character of the structure are discussed separately.