The formation of vortex rings in a strongly forced round jet
The formation of vortex rings in a strongly forced round jet
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DOI：
10.1007/s00348-011-1110-6
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发表时间：
2012-03
影响因子：
2.4
通讯作者：
E. Aydemir;N. Worth;James R. Dawson
中科院分区：
文献类型：
--
作者：
E. Aydemir;N. Worth;James R. Dawson
采用高速粒子图像测速技术研究了圆形射流在高振幅声强迫作用下发展区域涡环的周期性形成。在由系统声学响应决定的几个强迫频率下，实现了平均射流出口速度的20%至120%的谐波速度振荡。对涡环形成过程和环流的时间分辨历史作为强迫条件的函数进行了评价。总的来说，高振幅的强迫使射流的剪切层分裂成一系列大尺度的涡环，它们具有许多启动射流的特征。射流破碎的特征，如卷起位置和涡旋大小都与振幅和频率有关。在任何给定的急流强迫条件下，基于标准化强迫周期的极限时间尺度oft/T≈0.33都能限制涡环的环流增长。在正弦强迫射流中，这个时间尺度对应于一个运动约束，其中涡旋环的平移速度超过施加夹断的剪切层速度。这种运动学约束是由射流加速度在= 0和t= 0.33T之间的符号变化引起的。然而，观察到一些涡旋环在t= 0.33之前被掐断，这表明它们已经达到了最大循环。通过引用段塞流模型近似，并根据实验获得的时间和长度尺度定义段塞流参数，基于段塞流和环能量的分析模型表明，正弦强迫射流的地层数为l /D≈4，与Gharib等人的结果一致（J流体力学360:121-140,1998）。
The periodic formation of vortex rings in the developing region of a round jet subjected to high-amplitude acoustic forcing is investigated with High-Speed Particle Image Velocimetry. Harmonic velocity oscillations ranging from 20 to 120% of the mean exit velocity of the jet was achieved at several forcing frequencies determined by the acoustic response of the system. The time-resolved history of the formation process and circulation of the vortex rings are evaluated as a function of the forcing conditions. Overall, high-amplitude forcing causes the shear layers of the jet to breakup into a train of large-scale vortex rings, which share many of the features of starting jets. Features of the jet breakup such as the roll-up location and vortex size were found to be both amplitude and frequency dependent. A limiting time-scale oft/T≈ 0.33 based on the normalized forcing period was found to restrict the growth of a vortex ring in terms of its circulation for any given arrangement of jet forcing conditions. In sinusoidally forced jets, this time-scale corresponds to a kinematic constraint where the translational velocity of the vortex ring exceeds the shear layer velocity that imposes pinch-off. This kinematic constraint results from the change in sign in the jet acceleration betweent= 0 andt= 0.33T. However, some vortex rings were observed to pinch-off beforet= 0.33Tsuggesting that they had acquired their maximum circulation. By invoking the slug model approximations and defining the slug parameters based on the experimentally obtained time- and length-scales, an analytical model based on the slug and ring energies revealed that the formation number for a sinusoidally forced jet isL/D≈ 4 in agreement with the results of Gharib et al. (J Fluid Mech 360:121–140, 1998).