Research On Supercavitating Flow Characteristics Of Multiple Successively Fired Underwater Projectiles

The problem of multiple successively fired underwater projectiles comes from the background of sea war such as continuously firing underwater guns and anti-submarine missiles,underwater launched missiles,etc.The relevant research has strong engineering application interest.When multiple supercavitating projectiles are fired continuously at high frequency,the complex hydrodynamic characteristics are caused by interaction of the supercavitating flow fields formed by the projectiles,which makes the motion characteristics of the supercavitating projectiles disturbed.The supercavitating flow characteristics of multiple successively fired underwater projectiles are very different compared with that of a single underwater projectile.Due to complexity of the flow field,up to now,the problem has not been studied in detail.In this thesis,the numerical simulation and theoretical analysis are used to carry out a systematic and in-depth study on supercavitating flow characteristics of multiple successively fired underwater projectiles.This thesis focuses on the analysis of the interaction law between multiple supercavities.The main contents are as follows:(1)The numerical method of calculating the supercavitating flow of multiple successively fired underwater projectiles is established and its validity is verified.Based on solving RANS equations of the mixture phases,the VOF method,RNG k-? turbulence model and Schnerr-Sauer cavitation model are introduced to calculate the supercavitating flow of two successively fired projectiles near the free surface.The simulating results agree well with the experimental results from the literature,which verifies the reliability of this numerical model.Based on the above numerical simulation method,the coupling solution of projectiles motion equations and RANS equations is realized by using UDF and the dynamic grid technology.The numerical calculation method which can calculate flow fields and motion parameters in the process of horizontal motion of two or three successively fired underwater projectiles and two successively fired projectiles in water entry and exit is established.The simulating results agree well with the experimental results from the literature,showing that the numerical model is reliable.(2)The supercavitating flow characteristics in the horizontal motion of two or three successively fired underwater projectiles are studied.Some new phenomena of the unsteady supercavities evolution are discovered,and the evolution law of supercavities is obtained.The distribution of velocity field and pressure field of supercavities are given.The interaction law of supercavitating flow fields is analyzed,and the collapse mechanism of tail cavity is revealed.Combined with the evolution law of supercavities,the influences of supercavities' evolution process on fluid dynamics and motion of projectiles are analyzed.In addition,the evolution characteristics of supercavities,hydrodynamic characteristics and motion characteristics of two successively fired underwater projectiles under different initial cavitation numbers are obtained.The results show that the latter projectile can enter into the supercavity of the former projectile under the influence of the supercavity flow field of the former projectile,and finally the two projectiles collide with each other.(3)The research on the supercavitating flow characteristics of two successively fired projectiles near the free surface is carried on.The evolution characteristics and flow field distribution characteristics of supercavities of two projectiles near the free surface are obtained.The influence of free surface on the development of supercavities and the interaction between supercavities are analyzed.The wave excitation effect of two successively fired supercavitating projectiles is discussed.Besides,the influences of different water depths and cavitation numbers on the supercavitating flow characteristics are studied.The results show that the evolution process of supercavitation includes the independent development of supercavitation,the collapse of the supercavity tail,the deformation of supercavitation head,the fusion and separation of two supercavitation,and the collapse of the supercavity tail.When the projectile is very close to the free surface,the water layer above the tail of the supercavity is broken during the development of supercavitation,and the ventilated supercavity appears.(4)For the water entry problem of two successively fired projectiles,the evolution characteristics of supercavities,the flow fields changes during the water entry and the hydrodynamic characteristics of two projectiles are obtained.The interaction law of the cavitation flow fields is analyzed.Combined with the evolution law of supercavities,the influences of the evolution process of supercavities on the attenuation of water-entry velocity and the change of water-entry depth of two projectiles are analyzed.Besides,The influence of the distance between two projectiles on the water entry of two successively fired projectiles is studied.The results show that the impact load of the secondary projectile is smaller than that of the first projectile in the stage of impact into water.Two cavities induced by the first and secondary projectiles fuse into one supercavity,and the secondary projectile is eventually completely encapsulated by the supercavity.(5)For the water exit problem of two successively fired projectiles,the interference law of supercavitating flow field of the first-fired projectile on supercavitating flow field of the second-fired projectile is mainly studied.The evolution characteristics of supercavities,the flow fields changes of supercavities and the hydrodynamic characteristics of two projectiles are obtained.The influence of supercavities evolution process on the motion of two projectiles is analyzed.The results show that the supercavity induced by the first projectile will stay below the water surface when it meets the free liquid surface,while the secondary projectile flies into the air after it drops the water layer.Under the influence of supercavity peeled off by the first projectile,the secondary projectile can break away from its own supercavity,enter the supercavity of the first projectile,collapse of the supercavity,and generate new local supercavity.Based on the research results of this thesis,it can provide necessary support for the analysis of flow fields under more complex conditions,and it also can provide important data for the related engineering designs.