The natural and forced formation of spot-like ‘vortex dislocations’ in the transition of a wake
The natural and forced formation of spot-like ‘vortex dislocations’ in the transition of a wake
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DOI：
10.1017/s0022112092002763
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发表时间：
1992-10
影响因子：
3.7
通讯作者：
C. Williamson
中科院分区：
文献类型：
--
作者：
C. Williamson
本文研究了海崖体后流场的三维转捩，着重研究了尾流中大尺度结构的演变。以前已经发现，有两个基本模式的三维涡脱落在一个圆柱体的尾流（每一个模式是依赖于雷诺数的范围），与展向长度尺度相同的顺序作为主流的波长的涡街。然而.本研究表明，尾流转捩还涉及大尺度点状“涡旋位错”的出现，其向下游生长到10-20个主要波长的量级。不同频率的展向涡脱落单元之间产生涡位错。这些位错的存在解释了最初由Roshko（1954）和后来由Bloor（1964）发现的大的间歇性速度不规则性，以表征转变。尾流转捩中这些旋涡位错的存在是尾流向下游运动时产生湍流的主要原因。为了详细研究它们的演化，位错已经（被动地）被迫发生在一个局部的展向位置与使用一个小的环扰动。研究发现，“双面”位错是稳定的对称同相配置，它们诱导准周期的速度谱和（拍）位错频率振荡在近尾流。这些位错的内在机制是它们沿展向迅速扩散，包括旋涡的螺旋扭曲和轴向核心流。这被认为是旋涡在自然转捩中产生大尺度畸变的基本机制。随着尾流向下游行进，低位错频率处的能量缓慢衰减（与其他频率的快速衰减相反），使下游尾流由大位错结构主导。发现周期性强迫位错与自然相变中的间歇位错有明显的相似性。在不同类型的流动中位错的进一步相似性表明，涡旋或相位错可能是所有剪切流中过渡的一般特征。
The three-dimensional transition of the flow behind a bluff body is studied, with an emphasis placed on the evolution of large-scale structures in the wake. It has previously been found that there are two fundamental modes of three-dimensional vortex shedding in the wake of a circular cylinder (each mode being dependent on the range of Reynolds number), with a spanwise lengthscale of the same order as the primary streamwise wavelength of the vortex street. However. it is shown in the present study that the wake transition also involves the appearance of large-scale spot-like ‘vortex dislocations’, that grow downstream to a size of the order of 10–20 primary wavelengths. Vortex dislocations are generated between spanwise vortex-shedding cells of different frequency. The presence of these dislocations explains the large intermittent velocity irregularities that were originally found by Roshko (1954) and later by Bloor (1964) to characterize transition. The presence of these vortex dislocations in wake transition is largely responsible for the break-up to turbulence of the wake as it travels downstream. In order to study their evolution in detail, dislocations have been (passively) forced to occur at a local spanwise position with the use of a small ring disturbance. It is found that ‘two-sided’ dislocations are stable in a symmetric in-phase configuration, and that they induce quasi-periodic velocity spectra and (beat) dislocation-frequency oscillations in the near wake. Intrinsic to these dislocations is a mechanism by which they spread rapidly in the spanwise direction, involving helical twisting of the vortices and axial core flows. This is felt to be a fundamental mechanism by which vortices develop large-scale distortions in natural transition. As the wake travels downstream, the energy at the low dislocation frequency decays slowly (in contrast to the rapid decay of other frequencies), leaving the downstream wake dominated by the large dislocation structures. Distinct similarities are found between the periodic forced dislocations and the intermittent dislocations that occur in natural transition. Further similarities of dislocations in different types of flow suggest that vortex or phase dislocations could conceivably be a generic feature of transition in all shear flows.