Study on Interaction of Motions of Water and Immersed Low-Refractive Transparent Bodies

在用光学测量方法进行流体运动的实验研究中，不能看到浸没固体后面的流动。如果物体是透明的，并且折射率等于流体的折射率，则来自物体背面的光直接穿过它进入光学传感器，例如光电二极管、CCD摄像机等。水的折射率为1.33，纯石英玻璃的折射率为1.48，这是普通玻璃中最低的。一些特殊玻璃，如剧毒的F_2Be玻璃，难以制成厚度超过1mm的F_Li、F_Na、F_2Mg、F_2Ca和F_3Al等两相或三相玻璃，其折射率低于或接近水的折射率。然而，它们不能应用于实际应用。加入一些化学物质通常会增加水的折射率。然而，太多的化学品大大改变了水的其他特性，如密度，粘度等。本研究发现了一些添加剂化学品和透明的低折射率材料的优选组合。其中最好的是40%NaI或KI水溶液与低密度硅橡胶的组合，两者的折射率均为1.40。它们被应用于测量水和浸没的运动物体的运动的相互作用，再加上高性能的PTV算法，已经开发的作者。

In experimental studies of motion of fluids by means of optical measurement, flow behind an immersed solid body can not be visualized. If the body is transparent and the refractive index is equal to that of the fluid, light from the back of the body goes straight through it into optical sensors such as a photodiode, a CCD videocamera, etc.Refractive index of water is 1.33 and that of pure silica glass is 1.48, which is lowest among those of usual glasses. Some special glasses such as F2Be-glass, which is strongly poisonous, two- or three-phase glasses of FLi, FNa, F2Mg, F2Ca and F3A1, which are difficult to make thicker than 1mm, have refractive indices lower or close to that of water. They, however, cannot be applied to practical use.Addition of some chemicals usually increases the refractive index of water. However, too much chemicals considerably changes other characteristics of water such as density, viscosity, etc.The present study found some preferable combination of additive chemicals and transparent low-refractive materials. The best among them, so far, is the combination of solution of 40%-NaI or KI of water and a low density silicon rubber ; both refractive indices are 1.40.A technology to automatically track moving immersed bodies is also developed. They are applied to measure interaction of motions of water and immersed moving bodies, coupled with high-performance PTV algorithm which have already been developed by the authors.