Influence of a wall on the three-dimensional dynamics of a vortex pair
Influence of a wall on the three-dimensional dynamics of a vortex pair
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DOI：
10.1017/jfm.2017.114
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发表时间：
2017-04-25
影响因子：
3.7
通讯作者：
Williamson, C. H. K.
中科院分区：
文献类型：
--
作者：
Asselin, Daniel J.;Williamson, C. H. K.
在本文中，我们对墙壁或地平面影响下的扰动涡流感兴趣。此类流与地面效应中的飞机尾流、船体连接流、靠近地平面的湍流结构的基础研究以及涡流发生器流等相关。特别是，我们研究了下降涡对的涡动力学，当它与水平地平面相互作用时，它对于长波不稳定是不稳定的（Crow，AIAA J，第 8 卷（12），1970，第 2172-2179 页）。壁上的流动分离产生相反符号的次级涡流，这反过来又引起“反弹”效应，从而使主涡流远离壁上升。即使涡流中的微小扰动也会导致流动的显着拓扑变化，最终产生一系列涡环，这些涡环以三维“反弹”效应从壁上升起。与经典的克劳不稳定性相比，由此产生的涡动力学几乎无法识别。如果涡流在水平地平面上的临界高度以下产生，则长波不稳定性会被壁抑制。然后我们观察涡壁相互作用的两种模式。对于较小的初始高度，初级涡流靠得很近，使次级涡流彼此相互作用，形成垂直定向的涡环，即我们所说的“垂直环模式”。在“水平环模式”中，对于较大的初始高度，Crow 不稳定性在壁相互作用之前进一步发展；到达墙壁后，波峰位置相距较远，波谷位置靠得更近。槽彼此和壁的接近增加了涡度抵消，导致强大的轴向压力梯度和轴向流。最终，我们发现了一系列小的水平涡环，它们从墙上“反弹”。两种模式在每个不稳定波长中都包含两个小涡环，这与克罗不稳定涡环不同，每个波长仅形成其中一个涡环。这里观察到的现象并不限于上述扰动涡旋对。例如，当涡环倾斜地撞击墙壁时，会发现非常相似的现象。
In this paper, we are interested in perturbed vortices under the influence of a wall or ground plane. Such flows have relevance to aircraft wakes in ground effect, to ship hull junction flows, to fundamental studies of turbulent structures close to a ground plane and to vortex generator flows, among others. In particular, we study the vortex dynamics of a descending vortex pair, which is unstable to a long-wave instability (Crow, AIAA J, vol. 8 (12), 1970, pp. 2172-2179), as it interacts with a horizontal ground plane. Flow separation on the wall generates opposite-sign secondary vortices which in turn induce the 'rebound' effect, whereby the primary vortices rise up away from the wall. Even small perturbations in the vortices can cause significant topological changes in the flow, ultimately generating an array of vortex rings which rise up from the wall in a three-dimensional 'rebound' effect. The resulting vortex dynamics is almost unrecognizable when compared with the classical Crow instability. If the vortices are generated below a critical height over a horizontal ground plane, the long-wave instability is inhibited by the wall. We then observe two modes of vortex wall interaction. For small initial heights, the primary vortices are close together, enabling the secondary vortices to interact with each other, forming vertically oriented vortex rings in what we call a 'vertical rings mode'. In the 'horizontal rings mode', for larger initial heights, the Crow instability develops further before wall interaction; the peak locations are farther apart and the troughs closer together upon reaching the wall. The proximity of the troughs to each other and the wall increases vorticity cancellation, leading to a strong axial pressure gradient and axial flow. Ultimately, we find a series of small horizontal vortex rings which 'rebound' from the wall. Both modes comprise two small vortex rings in each instability wavelength, distinct from Crow instability vortex rings, only one of which is formed per wavelength. I he phenomena observed here are not limited to the above perturbed vortex pairs. For example, remarkably similar phenomena are found where vortex rings impinge obliquely with a wall.