1.School of Science, Nanjing University of Science and Technology, Nanjing 210094, China 2.Institute of Fluid Physics, China Academy of Engineering Physics, Mianyang 621999, China
Abstract:The instability of metal interface is an important problem in the process of implosion physical compression, which is significantly different from the traditional fluid interface instability. Due to the limitation of related theory and experimental diagnosis technology, this problem is studied still insufficiently. In order to understand in depth the perturbation growth behavior of metal interface instability, the technique for high explosive driven Rayleigh-Taylor instability experiment on the oxygen-free high conductivity (OFHC) copper is developed. The perturbation growth on OFHC copper interface with varying initial perturbation amplitude at a specific time is recorded by radiography. According to the data processing on the X-ray images, the perturbation growth behaviors of the interface at different times are obtained. The experimental results show that the larger the initial perturbation amplitude, the faster the perturbation grows, but the perturbation wavelength of the interface remains almost unchanged at the explosive loading. The perturbation on the front interface will have an effect on the back free interface, and cause some corresponding disturbance to occur on the surface, namely, on the back free interface, the position corresponding to the perturbation trough of the front interface first moves and gradually evolves into a spike, while the position corresponding to perturbation crest evolves into a bubble. The strain rate of instability perturbation growth reaches ~105/s, and the perturbation amplitude of the interface increases to about 700% of the initial value at 5.26 μs. The corresponding numerical simulation results show that the normal SCG model underestimates the strength of copper and cannot well describe the stabilizing effect of material strength at this high strain rate, thereby leading to the fact that the simulation results are higher than the experimental results. Keywords:Rayleigh-Taylor instability/ explosive loading/ perturbation growth/ radiography
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3.实验结果与分析根据图1所示的实验装置和测试系统布局, 开展了3发爆轰加载下高纯铜界面RT不稳定性实验, 采用低能450 keV的脉冲X光机对界面的扰动增长信息进行照相诊断. 3发实验清晰地给出了初始扰动波长为5 mm的高纯铜样品在不稳定性扰动发展早期T = 0, 1.98, 3.50和5.26 μs四个不同时刻界面扰动发展的X光图像(见图3), 其中X光机照相的时间零点为从爆轰产物到达样品前界面的时刻, 图像的上方为爆轰产物, 高纯铜样品的运动方向向下. 从图3可以较为清晰地看到在爆轰产物的加载下高纯铜样品前界面扰动幅值随着时间而逐渐地增长, 然而界面的扰动波长却基本不变. 不同初始幅值的界面扰动在相同时刻的扰动增长特征也基本一致, 界面初始扰动对应的波峰位置在爆轰产物的加载下形成了明显的“尖钉”, 而波谷位置则形成了所谓的“气泡”. 从图3中的X光图像上还可以看到在扰动增长的早期, 样品内部没有出现肉眼可见的空洞或裂纹, 说明本实验采用的爆轰产物加载方式下高纯铜样品没有出现层裂现象. 然而随着界面扰动的进一步发展, 以及在后期出现的大变形现象, 尖钉会不可避免地发生断裂或破碎. 为了准确地获得高纯铜界面扰动增长的相关信息, 需要对实验获得的X光图像进行数据处理, 以提取出界面扰动发展的图形边界. 首先给出原始X光图像的灰度图, 然后对实验图像进行高斯滤波处理以消除噪声. 图像的边缘处对应的是灰度梯度(一阶导数)的最大值, 同时也是其灰度二阶导数的零点, 因此可以得到两条相邻的边缘线-双边缘, 两条线间在灰度图上相差一个像素点(对应于0.05 mm), 最后选取其中的一条边缘线作为界面扰动边界曲线, 如图4所示. 图 3 不同时刻界面扰动增长的X光图像 Figure3. Radiographs of the perturbation growth at the different times.
图 4 数据处理后样品的边界图像 Figure4. Specimen edge images of after data processing.
根据上述图像处理方法, 得到了在爆轰产物加载下三个不同时刻高纯铜界面扰动的实验数据, 其中界面扰动的波长为5 mm, 与初始波长相比基本不变; 而界面扰动幅值则发生了明显的变化, 相关结果列于表1 (扰动幅值的测量误差主要依赖于统计分布, 其不超过0.1 mm).
实验 编号
初始波 长/mm
初始幅 值/mm
当前幅 值/mm
照相 时刻/μs
1
5.0
0.3
0.86 ± 0.05
1.98
0.5
1.17 ± 0.05
2
5.0
0.3
1.65 ± 0.05
3.50
0.5
2.67 ± 0.05
3
5.0
0.3
1.98 ± 0.05
5.26
0.5
3.42 ± 0.05
表1不同时刻高纯铜样品界面扰动特征参数 Table1.Interface perturbation characters of the high purity copper at different times.