Study On Trajectory Stability Mechanism Of Supercavitating Projectile In High-speed Water Entry With Small Incident Angle

The supercavitating projectile is a type of underwater weapon with broad potential military applications.It makes the body completely covered by cavity using the supercavitation drag reduction technology,which greatly reduces navigation resistance and efficiently destroys underwater torpedo,UUV,frog man,and other targets.The supercavitating projectile will transition from air to water when it is fired from the gun system.Particularly for projectiles fired from ship-based gun systems,the incident angle is frequently quite small,and various parts of the projectile will interact with the free surface in different ways,resulting in a serious impact disturbance to the projectile.The projectile is very likely to ricochet under these initial circumstances,which will significantly reduce underwater ballistic performance.Therefore,it is of great engineering significance to study the ballistic stability of supercavitating projectiles entering water at small incident angles.In this paper,the water entry of projectiles at small incident angle are studied using theoretical modeling,numerical simulation,and experimental research methods.The evolution of the multiphase flow field,the hydrodynamic force,and the ballistic stability of the water entry process of projectiles with different shapes under different initial conditions are discussed in depth.The main research contents and results are as follows:(1)A numerical simulation method has been developed to precisely simulate the flow field and motion characteristics of the projectile’s high-speed water entry process at small incident angle.A feasible solution strategy is proposed after examining the variables that affect calculation accuracy,such as the turbulence model,fluid compressibility,mesh size,and time step.The accuracy of the calculation method is then tested on a representative example.(2)Experimental and numerical simulation studies were conducted on ballistic characteristics of the projectile in the process of high-speed water entry at small incident angles.The ballistic characteristics of the projectile with different shapes under different initial conditions and the main factors influencing the instability phenomenon were obtained.The mechanism of the hydrodynamic forces on the stability of the projectiles’ water entry trajectory is analyzed from the viewpoint of the interaction between the cavity and the projectile.The mechanism of stability of projectiles entering water at small incident angles is revealed.The reasons for the projectiles whip in the process of water entry at small incident angles and its quantitative relationship with the initial parameters of water entry are explained theoretically.(3)Based on the stability of water entry at small incident angle,a comprehensive and systematic analysis of the hydrodynamic layout of the projectile is carried out,and the design guidelines of the half cone-segment angle,the head-neck ratio,the slenderness ratio and the counterweight parameters are obtained,which are favorable to the stability of water entry.The effectiveness and mechanics of the special tail configuration to increase the stability of the projectile in water is studied.The underwater tail-flapping motion of the projectile is simultaneously analyzed for its cavitating flow field and hydrodynamic force,and the influence of the initial disturbance on its characteristics is obtained.(4)The calculation model of the lift during the water entry process of the cone-cylinder combination projectile at small incident angles,the water entry ventilation model considering the effect of gas entrainment in water entry,and the planning force correction model considering the “secondary cavity ” caused by the projectile shoulder immersion,are established respectively,forming a six-degree-of-freedom trajectory prediction method which is applicable to the high speed water entry of the projectile at small incident angle.On this basis,the range of the main projectile shape parameters is presented with the aim of accounting for water entry stability and long range.Through further optimization of the hydrodynamic layout and trajectory,the preferred supercavitating projectile shape is proposed to realize the long range trajectory stability of the projectile in the process of water entry at very small incident angle.The study results in this paper can be used to direct the engineering application of the shape optimization design and the trajectory prediction for the water entry supercavitating projectiles at small incident angles.