Study On The Fluid-Structure Coupling Load Characteristics Of The Water Entry Of The Projectiles With Different Nose Shapes

In the process of the water entry of the air-to-sea weapon,there are many mechanics and control problems such as the cross-media fluid-structure coupling mechanisms,evolution of the free interface,and corresponding flow control methods.During the water entry of the revolution bodies,the characteristics of the flow field and the dynamic response of the projectile are of great significance to master the controllability of the impact load,effective load reduction measures,and enhanced the controllability of the underwater trajectory.The thesis describes the research progress of the free surface evolutions,cavity characteristics,impact load and trajectory stability of the water entry problems.The investigations show that the scholars at home and abroad have made great contributions on the above series of issues.However,from the current theory and literature that has been published,there are still some imperfections in the research of scholars at home and abroad,which need to be further studied.This thesis first studies the cavity characteristics,transient impact loads and trajectory characteristics of the truncated cone nose projectile;then,the mechanical characteristics were examined of the water entry of the projectile with a spring buffer.Subsequently,we focus on the flow field and load characteristics of concave nose projectiles,which has not been studied thoroughly in the previous research.Finally,the cavity characteristics of the water entry of the sphere with the near-wall effect are investigated.Firstly,the experiments on oblique water entry of high-speed projectiles are performed to investigate cavity evolution and impact loads.The cavity and load characteristics are investigated using a high-speed camera and an acceleration sensor,respectively.We divide the cavity into two types.We also find that the transient impact load and the cavity characteristics are closely related,which has not been discussed in detail in previous studies.The quantitative relationship between the load peaks and the two dimensionless parameters is revealed.Then,in order to further reveal the characteristics of the water entry of the truncated cone projectiles,we report an experimental study of the underwater trajectories of truncated cone projectiles.The experimental results show that the nose parameters affect the trajectory stability significantly.The effects of the nose parameters on the kinematic of the projectiles and the interactions between the projectile and the cavity are studied systematically.Results present the characteristics of the displacement,attitude deflection,deviation and cavity.It is also found that the trajectory stability depends on the free surface evolutions.We quantify the relationship between the trajectory stability and two nose parameters systematically,and a phase diagram is obtained for a large parameter space.The effect of the spring buffer on the water entry load of the projectile are investigated.A projectile with a spring connects the main body and disk is designed.The force of the main body and the cavity characteristics are studied in the experiments.The effects of the spring stiffness,the disk mass,and the impact velocity on the axial force and impulse are extensively discussed.Compared with the case without buffer,the axial force of the main body is only mitigated in a small range of stiffness.When the stifness continues to increase,the axial force begins to fluctuate with a larger peak and a narrow pulse.It is found that the impulse of the main body of the projectile with buffer is lower that the projectile without buffer.Additionally,the increase of the disk mass can also mitigate the force peak and the impulse of the main body.Also,the coefficients of the axial force and the impulse of the main body are decreased with the increase of impact velocities.In the research of the water entry of the concave nose projectiles,the effects of concave depth and the impact velocity are examined.Firstly,the effects of concave depth on the free surface evolutions and cavity characteristics are given experimentally and numerically.In addition,compared with the flat nose projectile,the increase rate of the axial acceleration of the concave nose projectile in the early stage is slower,and the acceleration curve has a steep drop stage,and then enter the uniform reduction stage.As the concave depth increases,a ’two peaks’phenomenon on the axial acceleration curve appears.It is also found that the’two peaks’acceleration curve goes to ’one peak’ with increasing the impact velocity.In addition,the load peak of the concave projectile is greater than the flat nose projectile.Results also show that with increasing the concave depth the maximum axial acceleration of the concave projectile increases firstly,and then decreases.Additionally,the pressure distribution of the fluid domain and the bottom of the nose were given numerically.The above water entry studies are carried out in the free field.Finally,the near-wall effect are considered to study the cavity characteristics of the vertical water entry of the spheres.The characteristics of the non-axis symmetrical cavity with the near-wall effect is given for the first time.We also found the formation of the twins-vortex during the pinch-off of the cavity,and the mechanisms of the phenomenon are revealed.The influence of near-wall effect on the pinchoff location and time are revealed for vertical water entry of the sphere under the low Froud number.The results show that as the impact position approaches the rigid wall,the pinch-off location and time of the cavity is increased.Additionally,the pinch-off time τ=(?),where the k is related to the distance between the rigid wall and the impact position.Subsequently,the influence of the near-wall effects on the oblique Worthington Jet is analyzed.