Water Entry Flow-Field Visualization of the ObliquePenetration of a High-Speed Projectile
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摘要: 为认识弹丸高速斜侵彻入水的气/水界面变形破碎、入水空泡和水中冲击波传播，利用可变发射角立式二级轻气炮发射高速弹丸，结合高速激光阴影和纹影流场显示，给出了高速弹丸斜侵彻入水流场的演化图像。结果表明：当弹丸速度在350 m/s附近时，弹丸尾部气流会越过弹丸头部产生冲击波，因为时间短且水惯性大，冲击波在气/水界面反射但不会影响弹丸姿态和气/水界面。弹丸斜侵彻在水中产生冲击波系，气/水界面发生形变和破碎(“碎片云”)，水中产生冲击波系和空化气泡区，难以识别气泡和“碎片云”的边界，不同头部构型弹丸会影响气泡和“碎片云”体积大小以及水下弹道稳定性。弹丸速度为1.8 km/s时，碎片云体积大于水下空泡体积，但流场结构和350 m/s情形相似。采用立式二级轻气炮和流场显示系统，为研究高速弹丸斜侵彻入水现象提供了新的途径。Abstract: To study the air/water interface deformation and breakup, the water bubble and the blast waves in water, we launched a high-speed projectile using a vertical second-stage gas gun, and visualized the flow-filed close to the air/water interface using the laser shadow and schlieren photography.The images show that the high pressure air downstream the projectile overtakes the projectile at the speed of about 350 m/s and generates blast waves in the air.In the meantime, the blast waves and cavitation bubbles are also generated in the water.The air blast wave reflects on the air/water interface but cannot deform it due to water's large inertia at such a short time.The projectile traveling is not disturbed although the reflected blast wave interacts with it.The droplets cloud from the broken interface is produced after the projectile water entry.It is hard to distinguish the border between the bubbles and the droplets clouds.For projectiles with different head shapes, the bubbles are obtained in different shapes and sizes but the projectile trajectory is seldom disturbed.Similar flow-field characteristics can be identified for a projectile at the speed of 1.8 km/s but with different size and shape of clouds and bubbles.The results demonstrate that such a vertical two-stage gas gun can provide a way for experiments of projectile water entry.
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Key words:
- two-stage gas gun /
- high-speed water entry /
- air-water interface /
- cavitation bubbles /
- blast waves
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图 1 立式二级轻气炮结构(1.高压储气罐; 2.气动球阀; 3.平衡活塞; 4.重活塞; 5.充气腔; 6.压缩管; 7.集气腔; 8.液压阻尼缸; 9.弹丸; 10.发射管; 11.弹托捕捉器; 12.实验舱)
Figure 1. Schematic of vertical second-stage gas gun (1.High pressure gasholder; 2.Pneumatic ball valve; 3.Balance piston; 4.Heavy piston; 5.Inflation chamber; 6.Compression tube; 7.Gas-collecting chamber; 8.Hydraulic damping cylinder; 9.Projectile; 10.Launching tube; 11.Sabot catcher; 12.Chamber)
图 2 弹托捕捉器(1.BOPP膜片; 2.泄气口; 3.弹丸; 4.弹托; 5.泄气舱; 6.泄气口; 7.发射管)
Figure 2. Sabot catcher (1.BOPP film; 2.Gas outlet; 3.Projectile; 4.Sabot; 5.Venting chamber; 6.Gas outlet; 7.Launching tube)
图 3 激光光源阴影光路(1.激光; 2.匀化器; 3.扩束镜; 4.凹球反射镜Ⅰ; 5, 6.观察窗; 7.凹球反射镜Ⅱ; 8.高速CCD相机; 9.弹托捕捉器; 10.发射管)
Figure 3. Schematic of laser shadowgraph system (1.Laser; 2.Homogenizer; 3.Beam expander; 4.Concave mirror Ⅰ; 5, 6.Observation window; 7.Concave mirror Ⅱ; 8.High-speedCCD camera; 9.Sabot catcher; 10.Launching tube)
图 5 弹丸入水前流场高速阴影照片
Figure 5. High-speed shadowgraphs of flow-field before projectile water entry
图 6 弹丸入水后流场阴影照片
Figure 6. High-speed shadowgraphs of flow-field after projectile water entry
图 7 弹丸入水前、后的纹影照片
Figure 7. Schlieren photographs before and after projectile water entry
表 1 实验工况和弹丸参数
Table 1. Experimental conditions and projectile parameters
Exp. No. Projectile type Mass/g v1/(m·s-1) v2/(m·s-1) η/% Case A Cone-cylindrical，60° 53.4 350.5 331.6 5.4 Case B Cone-cylindrical，90° 52.8 361.9 342.3 5.3 Case C Truncated cone-cylinder 52.6 356.4 329.9 7.4 
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