Experimental study of cavity characteristic induced by vertical water entry impact of a projectile with a 90° cone-shaped head at different velocities
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摘要: 用高速摄像拍摄了90°锥头弹丸低速入水的空泡形态演变过程，全面讨论了不同入水冲击速度下空泡的闭合方式及其演变过程，分析了空泡闭合时间、闭合点水深和弹头空泡长度随入水速度的变化规律以及不同水深位置空泡直径的变化规律；研究了水幕闭合和近液面空泡收缩上升所形成的射流现象及其相互耦合作用过程，探讨了空泡深闭合后其壁面波动规律。结果表明：随着入水速度的增加，空泡分别发生准静态闭合、浅闭合、深闭合和表面闭合，每种闭合方式对应的一个速度区间；弹头产生空泡的临界入水速度为0.657 m/s；不同水深位置的空泡直径呈现非线性变化；随着水深的增加空泡扩张初速增大，空泡最大直径减小，扩张段缩短，收缩段延长；同一时刻水深越大空泡扩张收缩的加速度也越高；水幕闭合后会产生向上和向下两股射流，向下射流速度较大时会对弹丸运动产生影响；近液面空泡收缩上升时会产生强度正比于空泡体积大小和闭合点水深的射流，并与上两股射流相互耦合形成一股更强的向上射流；空泡深闭合后长度缩短和产生的向下射流使弹丸受力发生改变，弹丸速度因受力产生的变化带动了流体质点速度的波动，进而导致空泡壁面发生波动，壁面波动遵循空泡截面独立扩张原理。Abstract: Experimental studies of the vertical water entry of projectile with 90° cone-shaped head were conducted by using high speed camera. The pinch-off types and evolutionary process of the cavity were comprehensively discussed at different water entry impact velocities. The variations of cavitation closure time, water depth at the closure point and length of the warhead cavitation with water entry velocities, as well as the cavitation diameter at different water depth positions were analyzed. The jet phenomenon caused by the closure of the water curtain and contraction-rising process of the cavity near undisturbed free surface were studied, as well as the coupled effect between them. The cavity wall fluctuation occurring after the deep seal was discussed. The results show that with the increase of water entry velocity, the quasi-static closure, shallow closure, deep closure and surface closure of the cavitation occurs respectively, and each closure mode corresponds to a velocity range; the critical water entry velocity of forming cavitation is 0.657 m/s. The diameter of the cavitation presents a nonlinear increase along with the water depth. The initial cavity expansion velocity increases, the maximum diameter of the cavity decreases, the expansion section shortens, the contraction section lengthens, and the acceleration of the expansion and contraction of the cavity increases along with the increase of the water depth at the same time. When the water curtain closes, there will be upward and downward jets, and when the downward jet velocity is relatively large, the projectile motion will be affected. The strength of a water jet induced by longitudinal upward contraction of the cavity near free surface is proportional to the volume of cavity and pinch-off depth. A large strength upward water jet is induced by the coupling of the above all water jet. The longitudinal contraction of the cavity around the projectile and the impact effect of downward jet on projectile would cause the force change of the projectile after deep seal. The fluctuation of the projectile's velocity because of its force change brings the velocity change of fluid and then leads to the fluctuation of the cavity wall. The fluctuation of cavity wall follows the principle of independent expansion of cavitation section.
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图 7 弹头空泡长度随入水速度u0的变化
Figure 7. Change of the cavity length of warhead with the initial velocity (u0)
图 8 不同入水速度(u0)情况下弹头空泡
Figure 8. Cavities of warhead at different initial velocities (u0)
图 9 空泡闭合时间(tp)和闭合点水深(Hp)随入水速度(u0)的变化
Figure 9. Change of cavity closing time (tp) and closing depth (Hp) with initial velocity (u0)
图 10 空泡闭合点水深(Hp)、空泡总长度(La)和两者之比(Hp/La)随入水速度u0的变化
Figure 10. Change of cavity closing depth (Hp), cavity length (La) and their ratio (Hp/La) with initial velocity (u0)
图 11 不同水深(Hf)处空泡直径(Dc)变化
Figure 11. Change of cavity diameter (Dc) at different depths (Hf)
图 12 不同水深位置空泡径向速度
Figure 12. Radial velocity of cavities at different underwater depths
图 13 不同水深位置空泡径向加速度曲线
Figure 13. Radial acceleration of cavities at different underwater depths
图 15 射流对弹丸及空泡的影响(u0=4.20 m/s)
Figure 15. Effect of jets on projectile and cavity (u0=4.20 m/s)
图 16 惯性参考系中空泡的壁面波动(u0=2.80 m/s)
Figure 16. Fluctuation of the cavity wall in static coordinate(u0=2.80 m/s)
图 17 弹体坐标系中空泡的壁面波动(u0=2.80 m/s)
Figure 17. Fluctuation of the cavity wall in the body coordinate (u0=2.80 m/s)
图 18 空泡波动阶段弹丸速度随时间的变化
Figure 18. Change of projectile velocity with time at the stage of cavity fluctuation
图 19 两种速度情况下的空泡波动图
Figure 19. Fluctuation of the cavity at two initial projectile velocities
表 1 不同水深位置空泡特性参数
Table 1. Characteristic parameters of cavity at different underwater depths
Hf/mm tr/ms tm/ms ta/ms Te/ms Ts/ms Dm/mm 3.5 2.7 63.0 82.0 60.3 19.0 32.0 20 8.3 42.0 61.3 33.7 19.3 21.5 40 14.7 28.0 50.3 13.3 22.3 16.5 59 22.0 31.5 48.7 9.5 17.2 15.0 
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