Experimental And Numerical Methods For Active Ventilated Cavity Flows Of Underwater Vehicles

The underwater launching progress consists of three stages: launching from platform,sailing in the water and piercing the water.Although the total time is short,the vehicle needs to pass through the interface between gas and water repeatedly,which leads to more violent forces.Meanwhile,cavitation usually occurs around the head of high speed underwater vehicle,and the cavity may shed randomly from vehicle and then collapses.As a consequence,random characteristics of hydrodynamic loads make against the better control performance of underwater vehicle.In order to improve the stability of underwater vehicles,the pressure-equalizing ventilated cavitating technology or Water Piercing Missile Launcher(WPML)is adopted to change the environment of underwater vehicles.The former can produce relatively stable and controllable cavities attached to underwater vehicle,which can eliminate the random characteristics of hydrodynamic load to achieve better control performance.The latter is a new concept launcher,which uses the exhaust gases to create a high speed underwater jet and forms a dry environment for vehicle traveling in the water.The advantages of the WPML are that one can reduce the influence of the exhaust gas on the underwater vehicle movement,and the pressurization system and the gas generator system can be eliminated simultaneously.In a word,the environment of underwater vehicles has been changed through ventilated cavities.So the experimental and numerical researches on the flow characteristics of ventilated cavities are studied.Underwater launching is a complex and transient gas-liquid-solid multiphase flow.Density and pressure are discontinuous at the interface due to the cavity flow.In order to accurately simulate the movement of interface,it is necessary to improve the accuracy of the numerical method.Therefore,on the basis of previous studies,the high order Discontinuous Galerkin method is introduced and then coupled with the ALE or CLSVOF method to capture the interface.Meanwhile,a simple method of constructing the surface boundary is proposed to develop a high order numerical boundary condition.A novel numerical method for ventilated cavities of the exhaust crevice is proposed based on ALE Discontinuous Galerkin method.The characteristics and mechanism of cavity growth are investigated,and then the effects of gas velocity and inflow velocity are analyzed.The experimental research on the characteristics of growth and coalescence of ventilated cavities are carried out.The influence of the ventilated rate and velocity of underwater vehicle on coalescence of ventilated cavities are discussed,and the criterion of coalescence is proposed.According to the physical phenomena of cavitation collapse,the collapse process has been divided into the cavity movement stage and the jet impact stage.The ALE Discontinuous Galerkin method is proposed for numerical simulation of cavity movement.The influence of initial pressure and thickness of cavity on dynamic characteristics are investigated.As for jet impact stage,a new numerical method for mixture jet is derived on the basis of the mass and momentum conservation.The water jets of different shapes impact on vehicle are studied.In addition,the gas-water jets impact on vehicle are simulated,and the influence of gas volume fraction is analyzed.Water Piercing Missile Launcher is essentially an annular jet of high pressure gas at the initial stage,which involves complex hydrodynamic problems such as gas-liquid multiphase flow,fluid compressibility,strong interaction between gas and liquid.The numerical simulation of high pressure gas jet flow is carried out based on coupled CLSVOF method,Ghost Fluid method and Discontinuous Galerkin method.After verifying the rationality of this method,the influences of three factors,namely jet velocity,gap ratio and total pressure,on characteristics of ventilated cavity are analyzed.