The High Frequency Fluid Structure Interaction Problems With Free Surface Effect

There are many reasons for the happening of the high frequency fluid structure interaction phenomenon. For example, the underwater moving structure is subjected to the transient load, and the underwater structure suddenly begins moving or stops moving, and the structure drops into water freely. These are all common high-frequency fluid structure interaction phenomenon. And there are some other high-frequency fluid structure interaction phenomenon not often seen. But they have the similar mechanism with the common ones. For example, the missile leaves the launch tube and the intense impact happens leading the high frequency fluid structure phenomenon, or the missile goes out of water and the load on the missile changes rapidly leading to the high frequency fluid structure phenomenon. Some high frequency fluid structure problems may be accompanied with the free surface effect sunch as the structure going out of or entering into water. When the high frequency fluid structure interaction phenomenon happen, the underwater moving structure may be subjected to invironmental loads such as the wave or the ocean current simultaneously. The scope of the research in the paper is about high frequency fluid structure interaction phenomenon of the high speed structure near the free surface. The problems about the missile going out of and entering into the water are taken as example.In the paper it emphatically researches on the effect of the fluid structure interaction, the free surface, the slamming, and the environmental loads based on the basic law of the movement of the missile to give out the hydrodynamic characteristics of the missile which interacts with the free surface while going out of water.Firstly, the development of the missile going out of water and related problems are reviewed from the aspects of the experiment techniques and the numerical methods. It introduces the basic phenomenon involved with the missile going out of the water, including the fluid structure interactions, the spikes, the slamming, going out of water in wave and so on. It emphatically summarizes the development of the numerical simulation methods of the fluid structure problems. And it reviews and discusses the developing process and the applicability of the numerical methods about the fluid structure interaction problems. At last, the slamming phenomenon of the water entry problems and the movements of underwater structures in wave are summarized. On the basis of the above, it puts forward some problems unresolved presently to give the instructions for the late researches in the paper.The missile is the kind of the structures which navigate in water accompanying high-frequency elastic vibration. Its fluid structure interaction problems behave special characteristics. So, it's very important choosing proper methods to research on the coupling problems between the missile and the water. In the paper, it puts forward a fluid structure interaction method which introduces non-linearity of the missile movement into the DAA calculation according to the movement characteristics of the missile on the basis of the DAA2 method. The method is called NDAA for the moment because it considers non-linearity of the missile movement. The method is a kind of blending techniques considering partial non-linearity. In the meanwhile, the influence of non-linearity of movement on the dynamic behavior of the coupling movement between the missile and the water is studied taking an elastic sphere as an example to give the boundary condition for non-linearity not neglectable.The upwards movement of the missile can make free surface emerging spike while the missile going out of water and the load characteristics will change simultaneously. In the paper it builds the NDAA model considering the free surface effect. The influence of the compressibility on the momentum equation and the continuity equation is studied to analyze the consistency of the governing equation and the free surface boundary condition of the NDAA model. On the basis of the feasibility analysis, the spike phenomenon, the load characteristics and the mechanics of going out of water are analyzed using the model. At last, the movement of the missile going out of water is researched.The missile is elastic-plastic and the elastic vibration will accompany the missile's upwards move. So, in the paper it develops a kind of fluid structure interaction method called the NDAA model which adapts to the high frequency fluid structure interaction calculations. The missile suddenly moving is taken as an example to analyze the characteristics of the high frequency fluid structure phenomenon using the NDAA method. And the method is compared with the theoretical result without considering the compressibility to analyze the influence of the compressibility on the high frequency fluid structure interaction problems and the mechanisms of the happening of the high frequency phenomenon. The method is compared with the estimated result considering the compressibility to validate the rationality of the NDAA model. And the influencing factor for the calculation of the high frequency fluid structure interaction is also given.The environmental loads are the critical factors influencing the attitude of the missile, especially for the wave load. In the paper, the wave load is considered into the numerical calculation. The plane wave is taken as example to explore the coupling dynamic model between the wave load and the structure's elastic vibration and mainly simulate the change of the wave surface while the missile piercing through the water. In the meanwhile, the influence of the wave height and wave period on the attitude of the missile is researched. The fluid structure numerical model for the three-dimensional elastic structure entering into water is built to simulate the movement of the free surface while the cylinder dropping into water freely and the characteristics of the slamming force is researched. On the basis of the feasibility analysis and superiority analysis, the face-contact water entry problems are simulated using the method to analyze the influence of compressibility, compare the difference of the elastic body and the rigid body and analyze the influence of stiffness on the slamming force. At last, the semi-analytical result for the slamming force of the missile dropping into water is given to conveniently calculate the water entry problems of the missile.The numerical model for the coupling of the rigid movement and the elastic vibration of the revolving body is given. The numerical model is validated to analyze the rigid movement and the elastic vibration of the submarine. And according to the load characteristics of the missile going out of the launch tube, the influence of the rigid movement of the submarine on the missile's movement and load is studied.