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Research of Dynamic Distortion on a Ship Based on the Theory of Hydroelasticity

Author: ZuoPei
Tutor: TangWenHui
School: National University of Defense Science and Technology
Course: Mechanics
Keywords: Hydroelasticity Dynamic distortion Hydrodynamic coefficient Mode analysis
CLC: U661.1
Type: Master's thesis
Year: 2011
Downloads: 18
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Abstract

Ocean going ships have served mankind for thousands of years. Yet the underwater body patterns of most modern ships remain in slender shapes. To achieve the reliable analysis and rational description of structural distortions of a slender ship in rough seas has continuously been one of the key techniques for ensuring the safety of the sailing ship and the technologies in measurement and orientation. In investigating this problem, we have to deal with a coupled dynamic system composed by the slender ship structure and the surrounding fluid. Based on the hydroelasticity, this paper does research on the structural dynamic distortions in waves. The main contents and results is as follows:(1) The principal coordinate of dry ship is regarded as generalized coordinates and the equations of symmetrical vibration and antisymmetrical vibration are set up by the theory of hydroelasticity. Then, we solve it and get the dynamic solutions in time-domain. To solve the hydroelasticity equations requires calculating generalized wave excitation and the coefficient matrix including generalized mass, structural damping, stiffness matrices and generalized added inertia, hydrodynamic damping, fluid restoring matrices.(2) The building of beam model is based on the slender structural characteristic. The vibration analysis is made to get the commanding equations, and the dry mode is calculated by transfer matrix method. Then the results of generalized mass, structural damping and stiffness matrices are worked out. The results are good enough to regard the supposition of beam model as reasonable.(3) The conditional equations are obtained by potential flow theory. Based on the characteristic of green function, the velocity-potential of any points in drainage area could be expressed by the value of the velocity-potential on the boundary and the grads of it on the boundary. The drainage area boundary is meshed by simple green function and the value of the velocity-potential on the boundary is worked out through solving the meshing equations. From Bernoulli equation and the definition of added mass and damping coefficient, the hydrodynamic coefficient can be given. The general property is expressed by the contrast and analysis of the calculation results about the wigley hull.(4) The expression of the fluid force is written by strip theory and the generalized fluid force is exported based on the principal of mode germination. Then the whole hydroelasticity equations are got. Finally, the hydroelasticity equations are solved in time-domain and the change with time is figured. The magnitude of the range about the principal coordinates on elastic mode decides the contribution which every elastic mode makes to total distortion and the range of low frequency mode is much larger than it of high frequency mode. The range of every principal coordinates is related to the encountering frequency, and every principal coordinates has a max value. Therefore, the resonance is the characteristic of vibration. Not only the wave frequency influences the vibration, but also the speed and the course do it. For vertical oscillation, the distortion is harmonic no matter the speed is zero. For horizontal oscillation, the distortion is inharmonic when the speed is zero and the distortion is when the speed is not zero. For torsion, the distortion is inharmonic no matter the speed is zero.

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