Prediction Method Of Natural Characteristic Of Ship Hull Vibration And Propeller Induced Fluctuating Pressure

Ship is an important tool for water transport and ocean development. During the voyage, ship will be encountered with the external excitations from main engine, propellers and wave loads which can cause different degrees of vibration and even ship structural damage. The excessive vibration may lead to structural fatigue failure, affecting the health of human-beings and the normal use of instrument and equipment. To avoid the harmful vibration, the natural frequency of ship structure and propeller-induced fluctuating pressure are required to make accurate prediction.This paper aims to do research on prediction of natural characteristic of ship hull vibration and propeller induced fluctuating pressure, including:1) The equipment modeling method for ship’s local vibration is studied and lumped mass model, rod model and space structure model are respectively used to simulate the equipment of ship structure. Through the analysis of several common models, a new practical model is proposed based on rod model using rigidity equivalent method. Deal with the influence of equipment on ship’s local vibration according to the solution to conduct the influence of superstructure on ship’s overall vibration and provide the calculation formula of influence coefficient of equipment on natural frequency of ship’s local vibration. Numerical examples show that natural frequency of ship’s local vibration can be accurately calculated using the equivalent model and the calculation formula of influence coefficient given by this paper, which is of great practical significance.2) The prediction for natural frequency of superstructure’s overall longitudinal vibration is made systematically. The finite element modeling method of superstructure’s overall vibration is investigated and the influence of various models, boundary conditions, added mass and loading conditions on the natural frequency of superstructure’s overall vibration is discussed in this paper. Some meaningful conclusions for finite element modeling of superstructure’s overall vibration are summarized and a practical computing model for the calculation of natural frequency is also proposed. Considering more influences, the prediction formula for natural frequency of superstructure’s overall longitudinal vibration is derived. The picking algorithm of equivalent stiffness coefficient for calculating the natural frequency of superstructure with rigid-body rotation vibration is studied, drawing the curve of equivalent rigid coefficient. The formula for calculating the natural frequency of shear vibration is amended, and the correction coefficient formula of bending deformation is proposed. Intelligent method possesses many characteristics such as strong ability of identification and it can also handle the uncertain and nonlinear problems well. Therefore intelligent method can be used to predict the natural frequency of ship vibration. Compared with neural network and other intelligent methods, Support Vector Machine(SVM) which has many characteristics such as small samples study, global optimization and better generalization can handle small samples, nonlinear and other practical problems well. Natural frequency of superstructure’s overall longitudinal vibration is predicted by SVM. The proposed method in this paper is proved to be accurate and feasible, which provides a new idea for the prediction of natural frequency of superstructure’s overall longitudinal vibration.3) In terms of the prediction of natural frequency of ship’s vertical vibration, a new empirical formula is given. Based on the Timoshenko beam theory, the formula is derived for a uniform beam with free-free boundary condition by introducing shear coefficient to correcting the hypothesis of shear uniform distribution. Referring to large numbers of measured data of natural frequency of ship hull’s overall vertical vibration, a new formula for calculating the natural frequency of ship hull’s overall vertical vibration is obtained by statistical analysis based on the formula for a uniform beam with free-free boundary condition, offering a new method for predicting the natural frequency of ship hull’s overall vertical vibration. Predict the natural frequency of ship hull’s overall vertical vibration using SVM. Considering that the parameters which influence the natural frequency of ship hull’s vertical vibration are much more, a grey relation model between ship’s main parameters and natural frequency is established by grey relational analysis. Determine the influence of the parameters on natural frequency of ship hull’s vertical vibration by computing the grey correlation degree of each parameter and the parameters with greater correlation degree are used as input data in SVM. The predictions in this paper are proved to be accurate and valid, which provides a new method for the prediction of natural frequency of ship hull’s overall vertical vibration.4) Fluctuating pressure induced by propeller is predicted using CFD method. In this paper, propeller-induced fluctuating pressure on the hull surface is calculated using the analysis of two-way fluid-structure interaction based on Workbench platform. The pressure curve on the hull surface over time can be achieved directly by overall modeling with the hull and propeller. Then propeller-induced fluctuating pressure of stern can be gotten by Fourier analysis and make comparisons between the propeller-induced fluctuating pressure of stern calculated by CFD method and the experimental results. The two results are very close which indicates the method proposed in this paper can accurately predict propeller-induced fluctuating pressure on the hull surface.Vibration prediction of ship involves several interdisciplinary research including structural dynamics, elastic mechanics, fluid mechanics and so on, which is of great importance in theoretical and practical research. This paper does systematic research on vibration prediction of ship, which provides a new method for prediction of natural frequency of ship vibration.