6

Conclusion

The present experimental study of small aspect ratio cylinder wakes has confirmed and revealed new phenomena occuring on the vortex shedding. First, a classical hot wire anemometry study showed that even small aspect ratio (r=1/d=5) cylinder wakes still appear via Hopf bifurcations. The parameters of the bifurcation agree fairly well with the existing linear or non linear data. Following Wesfreid and co-workers, it was also proved that the spatial shape of the global mode of the wake obeys universal phase transi tion laws. This last study was realized by the use of an ultrasound anemometer that permits to avoid extended mapping of the flow. Transversal and longitudinal velocity profile measurements have been successfully performed. At 3.5 diameters downstream the cylinder, we observe the existence of a special location from where waves are emitted in the downstream and upstream directions. This may be the signature of the so-called absolute instability of the wake. The ultrasound measurements revealed also the presence of a symmetric vortex shedding. The presence of this varicose mode has then been checked by flow visualisation on a very small aspect ratio cylinder (1/d=0.5). An interpretation in terms of coupled oscillators was finally presented where the aspect ratio of the cylinders drives the coupling between the shear layers which develop on each side of the cylinders.

圆柱尾迹的时空结构

摘要

将一个宽高比较小的液压缸运行后放置在一个统一的流动实验研究平台中，之后在具有贝纳德冯Karmim漩涡脱落实验中采用一个经典的使用热丝风速计的相同行为，使用超声波风速测定技术应用研究液压缸的运行事的横向速度的包络空间的临界行为。结果表明，这种外壳代表的液压缸的整体空间形态运行事，遵循通用缩放lawswhich协议与第二阶相变。在第二组的实验中，对液压缸运行时的纵向速度波动进行了研究。它也被发现在几种特定直径的液压缸上，有一个特殊的点，起到了造波机的作用,源Z自+优尔=文@论(文]网[www.youerw.com。最后，对于具有非常小宽高比的液压缸，对其采用对称旋涡脱落实验，将实验结果进行报告并使用耦合振子方程系统建模。

介绍

本研究的目标是分析的一个气缸的的经典的贝纳德沃卡曼唤醒的时间和空间特征。虽然这唤醒无疑是最流行的流流体力学，很多问题及其时空结构的继续开放。最近，它已被证明几位作者（斯里尼瓦森等（1986），Provansal等（1987），Dusek等（1994）和舒姆等（1994»贝纳尔冯·卡门涡街棚由气缸，通过霍普夫分岔临界雷诺数Re在出现忽视三维功能的唤醒时，此分叉的正常形态被称为Landau方程