Study on Three-Dimensional Vortex Strcture in a Fully Developed Region of a Turbulent Free Jet

在水洞实验中研究了共流圆形射流完全发育区域内涡的结构和动力学。利用激光诱导荧光技术对射流的横截面进行了可视化，并对横截面图像进行了数字图像处理，重建了涡结构的三维图像。将荧光染料与雷诺数为1700的射流混合，用薄激光片照射垂直于射流轴的截面。利用自行开发的快速、节省内存的计算机程序，对横断面图像进行图像叠加，重建三维图像。三维涡图表明，射流结构以全三维大尺度涡为特征。我们发现，完全发育的喷流的整个区域充满了无数的流向涡旋。其中一些形成了顶部向后和向外倾斜的u型弯曲涡。为了研究可视化结构中的速度场，热膜测量和流动显示同时进行。来自X热膜探头的信号与横截面图像的记录同步采样。同时测量得到了可视化旋涡结构引起的速度场信息。重点研究了射流远缘区域的孤立构造。结果表明，该结构具有三维涡流管的特点，涡流管循环大，芯径为喷管直径的数量级。

The structure and dynamics of vortices in a fully developed region of a coflowing circular jet is investigated in a water tunnel experiment. The cross sections of the jet are visualized by laser-induced fluorescence and a digital image processing is applied to the sectional images to reconstruct a three-dimensional pictures of vortex structures. The fluorescence dye is premixed to a jet fluid which Reynolds number is 1700, and a cross section normal to jet axis is illuminated by a thin laser sheet. Three-dimensional pictures are reconstructed by image stacking of cross sectional images using a fast and memory saving computer program developed by us.Three-dimensional pictures of vortices indicate that the structure of jet is characterized by fully three-dimensional large-scale vortices. The whole region of the fully developed jet is found to be filled with numerous streamwise vortices. And some of them are found to form U-shape bend vortices of which top is inclined to backward and outward.To investigate the velocity field in the visualized structure, the hot-film measurement and flow visualization are carried out simultaneously. The signals from a X hot-film probe are sampled synchronously with the recording of cross sectional images. This simultaneous measurement gives the information of velocity field induced by the visualized vortex structure. Attention is focused on isolated structures in far edge region of jet. the results indicate that the structures are characterized by three-dimensional vortex tubewhich circulation is large and core diameter is order of nozzle diameter.