| Air cushion craft is amphibious and has been widely used for military purposes.Due to the increasing attention of hovercraft,it is particularly important to ensure the reliability and safety to guarantee operational capability.The full lift hovercraft is different from the conventional drainage ship,with special structures such as air bag apron and air cushion,and the traditional method of hull vibration calculation is no longer suitable for hovercraft.Therefore,it is of great significance to accurately assess the natural frequencies of hovercraft.The main purpose of this paper is to study the airbag bending stiffness algorithm and the equivalent modeling method and the vertical stiffness algorithm of air cushion,as well as the influence of air cushion stiffness and additional mass on the body vibration of the air cushion vessel under two operating conditions.This article mainly includes five parts,and the main contents of each part are as follows:1)The background and significance of this study are introduced,and then the current status of the research on the vibration of the hovercraft,the air bag apron material,the mechanical properties of the air bag in the inflatable state and the additional mass algorithm caused by the structural vibration are analyzed in turn.2)A series of theories,including the complex bending theory of the straight beam,the theory of the vertical beam vibration considering the axial force,the theory of thin plate vibration considering the middle force and the theory of the natural frequency and vibration mode of the ship’s total vibration,are introduced in the calculation of the stiffness of the air bag and the total vibration of the air cushion ship.3)The processing methods of some nonlinear problems in the numerical simulation of the airbag stiffness are analyzed,including the determination of the constitutive model of the air bag composite material,the simulation of the cord layer and the high pressure gas.Then the air bag model is established,and the influence of the pressure in the air bag,the compressibility of the gas,the parameters of the air bag force layer(the cord angle,the number of cord lines,the distance of the cord and the distance of the cord layer)on the bending stiffness are analyzed.Finally,the equivalent modeling of the flexural stiffness of the air bag is studied.The influence of the pressure direction on the flexural stiffness of the inflatable air bag is analyzed.The theoretical calculation formula of the equivalent elastic modulus of the air bag model is derived.At the same time,the bending stiffness of the four different models is calculated by the numerical method,and the comparison is made with the theoretical results.4)The lifting principle of the hovercraft is introduced.Then the vertical stiffness algorithm of the air cushion is studied.Based on the static equilibrium equation and the fan’s characteristic equation,the theoretical calculation formula of the vertical stiffness of the air cushion is derived by using the characteristics of the flow continuity.Finally,the relationship between the displacement of hovercraft and the vertical stiffness of air cushion is analyzed by programming in MATLAB.5)The model of a simple large air cushion ship is established,and the stiffness equivalent modeling of air bag and air cushion is made.The overall natural frequency of the simple model in landing navigation and water surface cushion is analyzed,and the frequency reserve evaluation for wet mode is completed. |
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