Multiphase Flow Study On The Water Entry Of Projectiles In Consideration Of Surface Wettability

The water entry problem is a complex multiphase hydrodynamic problem involving solid,liquid,gas three-phase interaction in a very short time.This problem is directly related to many engineering applicationss and natural phenomena such as anti-submarine missiles,spacecraft reclamation on water,seaplane landing,ship slamming,water walking creature and diving.In recent years,with the deepening of the research into the water entry problem,some factors neglected by the former scholars gradually get attention again.In this paper,the influence of surface wettability on the water entry cavity,multiphase flow field and hydrodynamics of sphere and axially symmetric cylinders is investigated by means of numerical simulation and experiment method.The research contents and achievements are as follows:Based on the theory of mean equilibrium flow,the basic mathematical models and numerical methods for water entry simulation in consideration of surface wettability are established.The CSF(Continuum Surface Force)model is coupled with the VOF(Volume of Fluid)multiphase flow model,the surface tension is introduced into the Navier-Srokes equation by the UDF(User Defined Function)through the secondary development of Fluent software,the dynamic contact angle model is introduced to modify the gas-liquid interface angle at the three-phase line.Through comparison with the experimental results,verify the establishment of mathematical models and numerical methods are effective.Based on the numerical method established above,the influence of surface wettability on the water entry cavity is investigated.Particular attention is focus on the developing of the water entry cavity,multiphase flow field and hydrodynamic force during vertical water entry of sphere with different surface wetting properties.The influence of the surface wettability on the pinch-off time and the pinch-off position is analyzed.The relationship between the critical water entry velocity and surface wettability is presented,and reveals the formation mechanism of water entry cavity.Through the numerical simulation of the vertical water entry of the sphere with different relative densities,the typical cavity shape differences between the hydrophilic spheres and hydrophobic spheres are studied,and the influence of the hydrophilic and hydrophobic surfaces on the hydrodynamic and kinematic characteristics of the spheres are obtained.Based on the numerical method established above,the three dimensional numerical simulation of the vertical water entry of sphere with asymmetric surface wettability is studied.Thedeveloping of the asymmetric water entry cavity created by the vertical water entry of half hydrophobic-half hydrophilic sphere is investigated,and the corresponding asymmetric multiphase flow field,hydrodynamic fordce and the bend trajectory are analyzed.The influence of asymmetric difference of surface wettability on the development of the asymmetric water entry cavity,the hydrodynamic force acting on the sphere and its trajectory is studied.Through the numerical simulation of the vertical water entry of the half hydrophobic-half hydrophilic sphere with different relative densities,the typical asymmetric cavity shape,and the hydrodynamic and kinematic characteristics of the spheres are obtained,the formation mechanism of the asymmetric water entry cavity is analyzed.A vertical water entry experiment system was established,and a launcher is designed to realize the rotation and clamping of the moving body at the same time.Experimental study on the vertical water entry of different surface wetting cylinders was carried out.The influence of surface wettability on the development of water entry cavity of the cylinder with different nose shape is studied.During the vertical water entry of cylinder,two times of cavity pinch off occur,the pinch off time and the pinch off location are studied.The asymmetric water entry cavities created by the vertical water entry of cylinder with asymmetric wetting surface were investigated,the cylinbder trajectory and the attack angle variety is studied with different cylinder and impact speed,the formation mechanism of the asymmetric water entry cavity creater by cylinder is analyzed.Finally,the numerical investigation of vertical water entry of cylinder in congsideration of wettability was carried out.The numerical method is validated again,and the multiphase flow field,hydrodynamic force acting on the cylinder is analyzed.