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

通过水洞实验研究了同向圆射流充分发展区的涡结构和动力学特性。利用激光诱导荧光技术对射流截面进行可视化，并对截面图像进行数字图像处理，重建出三维涡结构图像。将荧光染料预混到雷诺数为1700的射流中，并通过薄激光片照射垂直于射流轴的横截面。利用自行开发的快速、节省内存的计算机程序，对射流截面图像进行图像叠加，重建出三维图像，三维旋涡图像表明射流结构具有全三维大尺度旋涡特征。充分发展的射流的整个区域被发现充满了许多流向涡。为了研究可视化结构中的速度场，同时进行了热膜测量和流动显示。来自X热膜探头的信号与横截面图像的记录同步采样。这种同步测量给出了可视化涡结构诱导的速度场信息。重点研究了射流远边缘区域的孤立结构。结果表明，该结构具有环量大、涡核直径为喷嘴直径数量级的三维涡管结构特征。

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.