Experiment on tail-slapping motion characteristics for oblique water-entry of a projectile
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摘要: 基于高速摄影技术，开展多工况下弹丸倾斜入水实验，并利用自编程序，对实验图像进行像素点捕捉及数据提取与处理。通过分析弹丸倾斜入水空泡形成、发展及溃灭过程，得到尾拍过程中弹丸的空泡演化特性。此外，通过对比分析不同入水初速下空泡尺寸及弹丸速度与加速度的变化规律，总结出入水初速对弹丸空泡演化特性及入水运动特性的影响规律。结果表明：弹丸发生尾拍后，部分弹尾穿透原始空泡发生沾湿，同时自弹尾向后产生了新的尾拍空泡，尾拍空泡与原始空泡间贴合紧密；尾拍结束后，尾拍空泡在水中的位置基本不变，最终从弹丸原始空泡表面拉脱溃灭，而相同深度的原始空泡在尾拍产生射流影响下加速溃灭；随着入水初速的升高，尾拍空泡的尺寸及原始空泡的长度逐渐增大，尾拍过程中弹尾最大沾湿面积也逐渐增大；随着尾拍次数的增加，弹丸在每次尾拍过程中速度衰减幅值增大，同时弹丸损耗的能量逐渐增多，弹丸存速能力下降。Abstract: Based on high-speed photography technology, the oblique water-entry experiments of high-speed projectile under multiple conditions are carried out. During the experiment, five experiments were conducted for each condition, and the same phenomenon appeared in the experiment. A self-programing is utilized to capture the image’s pixels and extract the experimental data for the experimental photographs. By analyzing the formation, development, and collapse processes of the oblique water-entry cavity of high-speed projectile, the evolution characteristics of projectile cavitation during tail-slapping are concluded. In addition, by comparing and analyzing the variations of the cavity size, and the velocity and acceleration of projectiles with different initial velocities of the water-entry of the projectile, the influence of the initial velocities of the water-entry of the projectile on cavitation evolution characteristics and water-entry motion traits is summarized. The results show that after the tail-slapping of the projectile, part of the projectile tail penetrates through the original cavity and gets wet, and a new tail-slapping cavity is generated backward from the projectile tail. The tail-slapping cavity fits closely with the original cavity. At the end of the tail-slapping, the location of the tail-slapping cavity under the water is basically unchanged. The tail-slapping cavity is pulled away from the surface of the original cavity of the projectile and collapses, while the original cavity at the same depth accelerates and collapses under the influence of the jet generated by the tail-slapping. With the increase of the initial velocities of the water-entry of the projectile, the size of the tail-slapping cavity and the length of the original gradually increase, and so does the maximum wet area of the tail. With the increase of the number of tail-slapping, the velocity attenuation amplitude and the energy loss of the projectile in each tail-slapping increase, and the capacity of the speed storage of the projectile decreases.
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图 4 弹丸倾斜入水空泡形态演化
Figure 4. Cavity evolutions of the oblique water-entry of the projectile
图 7 第2次尾拍空泡的演化细节
Figure 7. Details of the evolution of the second tail-slapping cavity
图 11 不同入水初速的弹丸入水情况
Figure 11. Projectile water-entry processes at different initial velocities
图 12 不同入水初速下无量纲速度及无量纲加速度
Figure 12. Dimensionless velocities and accelerations at different initial velocities
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