Structure Analysis and Control of Necklace Vortices Generated between Two Circular Cylinders Forming a Separating Cross

采用染料注入法和示踪粒子法显示了雷诺数在500~2000范围内形成分离十字的两个圆柱体周围的三维流场和项链旋涡结构，并详细测量了雷诺数为1000~8000范围内两个圆柱体的表面压力分布，以阐明均匀流中项链旋涡的结构以及项链旋涡行为对进出圆柱体的流体作用力的影响。此外，还研究了气缸间隙对项链涡系结构的影响及其与气缸表面压力分布的关系，发现气缸之间的气隙距离G不同，会产生不同的气链涡系图案。当间隙距离比G/d在0~0.1时，上游圆柱表面的附面层在下游圆柱前方以一个小的项链涡的形式分离卷起，在y/d=0的对称面上形成4个涡的涡系。当G/d=0.2~1.0时，两柱间缝隙内形成一对主链涡系，共形成8个涡系。在G/d=1.0-1.5的情况下，在低雷诺数下，主链旋涡和柱体后的孪生旋涡一起开始振动。在G/d=1.5-2.0的情况下，上游圆柱后的项链涡与低雷诺数下的卡门涡的行为相似。

The three dimensional flow field area and the necklace vortices structure around two cylinders forming a separating cross were visualized using a dye injection method and a tracer particle method under the Reynolds number between 500 and 2000, and the surface pressure distribution on two circular cylinders was measured under the Reynolds number between 1000 and 8000 in detail to clarify the necklace vortex structures in uniform flow and the effects of the necklace vortex behavior on the fluid force into and on the separation flow from the cylinders. In addition, the relationship between the necklace vortex structure being made change by the gap distance and the pressure distribution on the surface of the cylinders was investigated.It is found that the various patterns of the necklace vortices system are produced, depending on the gap distance "G" between two cylinders. In the case of the gap distance ratio "G/d" between 0 and 0.1, the boundary layer on the surface of the upstream cylinder separates and rolls up as a small necklace vortex in the front of the downstream cylinder, and the vortex system with 4 vortices is produced in the symmetrical plane of y/d=0. In the case of G/d=0.2-1.0, a pair of main necklace vortices grows in the gap space between two cylinders, and the 8 vortices system as total is produced. In the case of G/d=1.0-1.5, the pair of main necklace vortices starts vibrating as well as twin vortices behind a cylinder in the low Reynolds number. In the case of G/d=1.5-2.0, the behavior of necklace vortices behind the upstream cylinder is similar to that of Karman vortices under the low Reynolds number.