Fluid Dynamics And Experimental Investigation On The Cavitation Characteristics Of The Water-exit And-Entry Process Of Underwater Vehicle

The underwater launch of submarine weapons is a complex fluid,gas and solid coupling process,which is divided into three stages:exiting the launcher,moving in water and exiting water.The load suffered by the missile is constantly changing for the complex interactions between the missile,launcher,cavity and the free surface with the changing of the surrounding environment and the formation,development and collapse process of the surrounding cavity.Because of the complexity of the hydrodynamic problem of the underwater launch of submarine weapons,the mechanism of the basic mechanical problems in the complex fluid,gas and solid coupling process has not been fully achieved.In the process of the vehicle out of the water,the load of cavitation collapse may affect the sailing body movement,and lead to the structure damage,so the research of the load of cavitation collapse in the process of vehicle body out of water is of very important engineering significance.Therefore,the paper studies the load of high speed submarine vehicle with cavitation based on the numerical method and model experiment.Firstly,the basic physics involved in the penetrating process is introduced.An overview is conducted in detail on the theoretical method,experimental techniques and numerical simulations domestically and aboard in three aspects,i.e.,water-exit of the vehicle,cavitation loads and water-entry problem of the structure.On this basis,the main contents of this article are proposed.The different load forms are introduced during the process of the water exit of the vehicle.This paper mainly introduces the research progress of the numerical calculation method of the cavitation domestically and aboard,analyzes and summarizes the problems and shortcomings that existing in the research.The problems that need to be solved are put forward,which is the foundation for the subsequent work in this paper.On the basis of the potential flow theory,the paper establishes mathematical calculations of the cavity flow around underwater 3D vehicle.The problems of partial cavity and no cavity flow around underwater vehicle are researched.And its validity is demonstrated with the comparisons between the calculation results and experiments.The paper analyses the effects of different head shape and different cavitation number to the characteristics of flow around underwater vehicle.In the exceeding water process of underwater vehicles,the existing of trailing cavity will have distinct effects on the vehicles’ hydrodynamic characteristics.Recent researches mostly leave gravity effect out of consideration,while the gravity will affect trailing cavity characteristics and then affect the vehicles ’ hydrodynamic characteristics.So in this paper,a complex boundary model which taken the gravity into consideration is established based on potential flow theory which is proved to be effective by comparison the results with the data in existing papers.Then we calculate the trailing cavity forms of a cone and an underwater vehicle under different cavitation numbers and Froude numbers to study the rules of trailing cavity forms and hydrodynamic characteristics changing with cavitation number and Froude number.The water-exit process of the underwater vehicle is simulated considering the cavitation characteristics based on the CFD method.The validity of the calculation method is verified by the comparison between the numerical results and the steady and unsteady experimental data.The paper establishes a one degree numerical model of the cavitation water-exit process of underwater vehicle and makes a research on the generation,development and collapse process of the cavity.The effect of the initial velocities on the loads on the vehicle surface is investigated.Analysis on the vehicle trajectory is carried out to compare the differences between cavitation and un-cavitation condition through the establishment of the multi degree numerical model of the water-exit process of underwater vehicle.In the process of model test,the model will fall into the water every time,and the model structure will bear a great transient impact load.The load may cause a structural damage on the model,and even break the water tightness of the model,which may affect the subsequent collection of experimental data and the conduct of the rest experiment trials.After the experimental flying stage of the model,there is a cavitary structure at the end of the model;the elastic vibration should be taken into consideration in the water-entry process because the model is a thin shell structure;the compressibility of the fluid effects cannot be ignored with the high velocity of the water-entry of the model.On the basis of the Nonlinear Doubly Asymptotic Approximation(NDAA)considering the compressibility of the water,the paper deduces and establishes mathematical calculations of the water entry of the model structure.By comparing the results of numerical calculation with the experiment,the reasonability of the procedure was validated.This paper calculated the hydrodynamic loads and response of the structure falling into the water,analyzed the weak position of the structure,and made a research on the surface hydrodynamic loads laws in different drowning angle.The research results indicate that the loads of the structure have a non-linear relationship with the pitching angle and get the dangerous working angle.The underwater launch and motion of weapons are very complex mechanical behavior.Cavitation will be generated in the liquid surrounding the underwater vehicle due to its high speed,and the missile body will also be surrounded by the gas emitted from the launcher.The existence of the cavity during the whole process of water-exit has a great impact on the hydrodynamic characteristics,posture and trajectory and the structural strength of the underwater vehicles.The paper analyzed the whole process of the vertical water-exit of slender bodies based on high speed photography,in order to have a better understanding of the mechanism of the formation,development and collapse of the cavity.Besides,the effects of water depth and initial speed of the slender body on the motion of the cavity are analyzed.The relationship between the length and width of the cavity and a given single variable,i.e.,the velocity or the water depth,are obtained,thus providing references for study on the dynamics of cavity generated during the water-exit of slender bodies.By comparing the process of vertical and oblique water-exit of slender body,this paper has analyzed how the initial inclination angle makes an impact on the cavity.In view of the domestic and foreign representative marine engineering experimental tanks,this paper introduces the main experiment equipment,summarizes the development status of the technology in the field,which provides a reference for the whole construction scheme of the mesoscale depressurized tank.Then the basic theory of dimensional analysis is introduced,the similarity criterion in the dynamics experiment is derived,and the similarity criterion has been met and unmet in the present situation.The necessity and feasibility of the nuclei seeding system to make the nuclear in the water between the experiment and the real marine environment similar are inquired,and then the nuclei seeding system scheme is presented to meet the function in the conditions of different regions,wave,current and different emission cases.