Three-dimensional Measurement of Fluid Flow by Stereoscopic Tracking Velocimetry

摘要

Convective motion in fluid dynamics and heat transfer is the most important phenomenon to be understood since it can greatly influence the performances of fluid and heat transfer systems in various manners. With the advances of modern technologies, new diagnostics for mapping three-dimensional (3-D) convective flow is very necessary for fundamentals of flow physics. Especially, modern computational modeling has been greatly advanced to demand 3-D convective-flow diagnostics in order to verify and tune the methodologies and approaches. Conventional velocimetry is either pointwise or two-dimensional. If available, 3-D gross-field velocimetry can allow us unprecedented physical insight as well as the needed data for validation of numerical codes and understanding of fundamental flow physics. In an effort to meet the need of 3-D flow diagnostics, we have developed stereoscopic tracking velocimetry (STV). STV is based on the simultaneous stereoscopic monitoring of numerous particles dispersed in a carrier fluid. It can thus provide time-sequence velocity maps of an entire flow field. Here we briefly present the methodology of STV and its experimental measurement results of 3-D flow fields including the traditional flow involving a free jet and the directional solidification for material processing. STV provides a good potential for monitoring 3-D flow fields for all 3-C velocity vectors. The system is simple and the monitoring can be made near real-time. Through the preliminary investigations including the Marangoni and natural convection flows that frequently arise in extraterrestrial material processing, we have demonstrated the STV application potential. For the STV data processing, the utilization of neural networks has proven to be very effective. The BP neural networks pose an excellent pattern recognition capability for superimposed particle image identification while the Hopfield neural networks offer a desired global-optimization scheme in attaining potentially valid particle tracks to follow the flow motion. The accuracy of the STV measurement appears to be better than 3% when a CCD camera of 782x582 pixels is employed. It is believed that the accuracy can be further enhanced with use of high-resolution cameras. The investigations thus far reported here have been limited in scope at this initial stage of the technology development. With the power of the developed STV for 3-D velocity diagnostics, various phenomena involving 3-D flow will further be investigated in the future.