Experimental/numerical study of directional solidification using stereoscopic tracking velocimetry.

摘要

Measurement of three-dimensional (3-D) three-component velocity fields is of great importance in both terrestrial and extraterrestrial experiments for understanding materials processing and fluid physics. The experiments in these fields most likely inhibit the application of conventional planar probes for observing 3-D phenomena. Here, we present the investigation results of stereoscopic tracking velocimetry (STV) for measuring 3-D velocity fields, which include diagnostic technology development, experimental velocity measurement, and comparison with analytical and numerical computation. STV is advantageous in system simplicity for building compact hardware and in software efficiency for continual near-real-time monitoring. STV is based on stereoscopic observation of particles seeded in a flow by CCD sensors. In this approach, parts of the individual particle images that provide data points are likely to be lost or cause errors when their images overlap and crisscross each other, especially under a high particle density. In order to maximize the valid recovery of data points, neural networks are implemented for these two important processes. For the step of particle tracking, the Hopfield neural network is employed to find appropriate particle tracks based on global optimization. Our investigation indicates that the neural networks are very efficient and useful for stereoscopically tracking particles. For testing in material processing applications, a simple directional solidification apparatus is built for experimenting with a metal analog of Succinonitrile (SCN). Its 3-D velocity field at the liquid phase is then measured to be compared with those from numerical computation. As an additional effort, the numerical modeling has been investigated to study the complex flow phenomena involving free boundary problem, surface tension force, and phase change in solidification. The cross-examination of the experiments and numerical simulation has defined the appropriate steps to properly run the numerical codes, that is, to attain proper numerical solutions. The confidence on the numerical solution leads to the further investigation on the Prandtl number effects on the solidification process and the accompanying velocity field. Our theoretical, numerical, and experimental investigations have proven STV to be a viable candidate for reliably measuring 3-D flow velocities.