Vortex wakes of a flapping foil
Vortex wakes of a flapping foil
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DOI：
10.1017/s0022112009007964
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发表时间：
2009-08-25
影响因子：
3.7
通讯作者：
Bohr, Tomas
中科院分区：
文献类型：
--
作者：
Schnipper, Teis;Andersen, Anders;Bohr, Tomas
我们提出了一项关于对称箔在垂直流动的肥皂膜中进行俯仰振荡的实验研究。通过改变振荡的频率和振幅，我们可以看到每个振荡周期多达 46 个涡流的各种尾流，包括冯卡门涡街、倒转冯卡门涡街、2P 尾流、2P+2S 尾流以及从 4P 到 8P 的新颖尾流。我们在由基于宽度的斯特劳哈尔数和无量纲振幅跨越的相图中绘制出尾流类型。我们详细跟踪圆形前缘附近涡流形成的时间演变以及尖锐后缘处的脱落过程。这使我们能够识别 2P 尾流中涡流的起源，了解由于前缘和后缘处涡流脱落的时间而导致相图中存在两个不同的 2P 区域，并提出一个简单的涡度生成模型。我们使用该模型来描述随着振荡频率的增加从 2P 尾流到 2S 尾流的转变，以及从通常与阻力相关的冯卡门尾流到通常与推力产生相关的反冯卡门尾流的转变。
We present an experimental study of a symmetric foil performing pitching oscillations in a vertically flowing soap film. By varying the frequency and amplitude of the oscillation we visualize a variety of wakes with up to 46 vortices per oscillation period, including von Karman vortex street, inverted von Karman vortex street, 2P wake, 2P+2S wake and novel wakes ranging from 4P to 8P. We map out the wake types in a phase diagram spanned by the width-based Strouhal number and the dimensionless amplitude. We follow the time evolution of the vortex formation near the round leading edge and the shedding process at the sharp trailing edge in detail. This allows us to identify the origins of the vortices in the 2P wake, to understand that two distinct 2P regions are present in the phase diagram due to the timing of the vortex shedding at the leading edge and the trailing edge and to propose a simple model for the vorticity generation. We use the model to describe the transition from 2P wake to 2S wake with increasing oscillation frequency and the transition from the von Karman wake, typically associated with drag, to the inverted von Karman wake, typically associated with thrust generation.