低旋流数旋进射流流场结构演变的POD分析

利用粒子图像测速技术（PIV）对雷诺数Re = 4.5×104的低旋流数旋进射流流场进行了实验测量，并利用本征正交分解（POD）方法对测得的流场进行分解，提取流场中含能大尺度结构。针对3种不同旋流数（S = 0、 0.26和0.41），对比分析了POD分解得到的空间模态以及用POD模态重构后的脉动速度场的变化规律。POD分析得到的结果表明:旋进导致流体交替地从腔体一侧沿着壁面流出，从另一侧流入；旋进刚发生时，上游剪切层内的旋涡结构尚未完全破坏，它们会一直向下游发展直至旋进起始点附近后，开始随着主流一起偏转，而下游剪切层内的大尺度结构被完全破坏；随着旋流数的增加，旋进以及射流的自身振荡被加强，从而导致流场结构更加复杂、大尺度旋涡结构被破坏。

POD analysis of the dynamic structures of a low swirl number precessing jet

The flow field of a low swirl number precessing jet at Reynolds number Re = 4.5×104 is measured using particle image velocimetry （PIV） and the dynamics of the large-scale flow structures are examined further using the proper orthogonal decomposition（POD） analysis. The spatial modes obtained by POD and the fluctuating velocity field obtained by POD reconstruction at three swirl numbers, i.e., S = 0, 0.26 and 0.41, are compared and analyzed. The POD results show that the precession induces an alternating flow, switching between outflow from one side of the chamber along the chamber wall and inflow from another side. When the precession occurs, the vortex structures in the upstream shear layers have not broken down completely. They will develop downstream until approaching the starting point of the precession and then deflect with the mainstream. However, the large-scale structures in the downstream shear layers are completely destroyed. As the swirl number increases, the region affected by the precession moves upstream, and the orderly vortex structures in the shear layers break down.