Investigations On The Hydrodynamic Characteristics Of The Low-speed Water-entry Process Of Hollow Projectiles

The hollow projectile is a new type of projectile with a through-hole structure.Due to its less mass than solid projectiles with the same diameter,it has a higher muzzle velocity and lower resistance with the same charge mass and more advantages than solid projectiles.Since the related research is still in the early stage,many special transient multiphase flow phenomena are not known clearly in the water-entry process with different initial velocities,such as cavity closure,through-hole jet,etc.These are more complicated than that in the entry process of solid projectiles.Therefore,the low-speed vertical and oblique water-entry cavity and motion characteristics of hollow projectiles are investigated experimentally and numerically in this paper.The work focus on multiphase flow characteristics of the entry cavity and the hydrodynamic characteristics of the projectile,which is of great significance to design and improve the new entry hollow projectiles.The main research contents and achievements are shown as follows:The vertical water-entry process of hollow projectiles is investigated experimentally with different apertures and initial velocities.The results show that the through-hole jet induced by the hollow structure makes the entry cavity pinch-off in a new circle line.Three closure patterns can be found based on the different closure characteristics.They transform into each other with the change of entry velocities and apertures.At the top of the through-hole jet,there is a bubble formed by the splash closure inside the through-hole.The jet speed increases with the increasing entry velocities and the decreasing apertures,which rises faster in the early stage and declines faster in the later stage.But the changes of entry velocities and apertures have few effects on the overall jet shapes.Two types of motion parameters variations of the projectile are found in a short period with the various water-entry velocities.When the entry speed is low,the resistance rises first and declines then,and the projectile velocity has a continuing upward trend in the later stage.But when the entry speed is high,the impact resistance is greater and the drag coefficient rises sharply after a slight decline,and the projectile velocity decreases significantly in the later stage.The apertures variation has litter effects on the variation trend and values of the motion parameters.Coupling the RANS model and motion equations with a single degree of freedom,the vertical water entry of the hollow projectiles is investigated numerically.The results show that the entry cavity is attached to both the inner and outer walls of hollow projectiles.The top bubble of the through-hole jet is formed after the collision of the inside splash at the central axis,and the bubble becomes smaller with the decreasing entry velocities and apertures.The transition of the three cavity closure patterns is mainly related to the pressure difference between the inside and outside of the splash.The surrounding splash converges faster when the entry velocities increase or the apertures decrease and the surface closure occurs more easily.Besides,the simulations indicate that the high-speed flow region is mainly formed when the air is entrained into the cavity and is extruded by the cavity shrinkage.Its evolution characteristics are different with the change of the cavity closure.The high-pressure region locates at the front of the projectile head and the merging place of the wake flow and the low-pressure zone mainly appears inside the attached cavity.Pressure fluctuations are found around the inner wall due to the continuous shedding bubble from the internal cavity.They occur more visibly with the increasing water entry velocities.The drag coefficient has saltations both in the impact point and the cavity closure point,which lead to the transient decrease and increase of the projectile velocity respectively.The oblique water entry of hollow projectiles is investigated experimentally under different entry conditions.The results show that the asymmetrical evolution of the water entry cavity and splash jets around the free surface are found in the oblique water entry.The decreasing entry angle can delay the transition of the cavity closure with the increasing entry velocities and the surface closure appears firstly with the same entry velocities when the entry angle is large enough.The cavity closure time and shape are affected by the apertures in the oblique water entry and the closure time is delayed and the cavity size becomes larger with the decreasing apertures.The entry velocities mainly affect the shrinkage process and the offset position of the cavity shape.The outlet direction of the through-hole jet can be altered due to the different guidance of the inner wall.It is affected distinctly when the aperture and the entry angle are small,and it tends to be opposite to the water entry direction.The entry trajectory of the projectile is similar to a parabola under the gravity and hydrodynamic resistance.The object’s posture continues to rotate clockwise after entering the water.The projectile velocities all rise first and fall then in the early stage,but rebound again.Coupling the RANS model and motion equations with six degrees of freedom,the low-velocity oblique water entry is investigated numerically.The results show that the cavities attached to both the inner and outer wall of the hollow projectile are asymmetrical noticeably.The external cavity on the back flow region of the left wall is smaller and pinches off earlier than the right one.And the left wall both is bedewed in a short time after the impact and the cavity closure.Two formation patterns on the right internal cavity are found under different entry conditions.One is formed by the impact,the other is formed by expelling the water near the right side when the left cavity moves right along the front and back inner sides.A little concave cavity in the top of the through-hole jet is still found and formed by the seal between the left splash and the right inner wall when the left splash tip coils right.When the projectile rotates clockwise,the body tail and the inner wall strike the surrounding flow and the through-hole jet alternately leading to the tail-slap of the projectile in rotation.Besides,the simulations show that the anticlockwise rotation of hollow projectiles is found with the decreasing entry velocities and angle.The aperture variations mainly affect the cavity size and the rotation rate of the body but have little effect on the overall cavity evolution of the projectile.