The meandering behaviour of large-scale structures in turbulent boundary layers
The meandering behaviour of large-scale structures in turbulent boundary layers
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DOI：
10.1017/jfm.2019.131
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发表时间：
2019-02
影响因子：
3.7
通讯作者：
K. Kevin;J. Monty;N. Hutchins
中科院分区：
文献类型：
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作者：
K. Kevin;J. Monty;N. Hutchins
本文量化了规范光滑壁边界层中大规模湍流结构的瞬时形式，证明它们遵循与 Flores & Jiménez (Phys. Fluids, vol. 22, 2010, 071704) 和 Hwang & Cossu (Phys. Fluids, vol. 23, 2011, 2011) 提出的自持条纹不稳定性模型一致的形式。 061702）。我们这项研究的动机源于之前对大尺度条纹的观察，这些条纹在空间上锁定在展向异质边界层内的位置。在这里，使用类似的工具，我们证明规范层中随机出现的大规模结构表现出类似的行为。从统计上看，我们表明，大规模相干结构的特征表现出随着距墙壁距离的增加而增加的蜿蜒行为。在边界层的上边缘，这些结构与主流方向严重错位，与强偏航涡流包/涡团相关的诱发速度产生显着的展向速度分量，导致展向速度波动的明显倾斜相干性。这种明显的蜿蜒行为还会在大约 $6\unicode[STIX]{x1D6FF}$ 波长处产生主要的流向周期性。我们进一步统计表明，与这些非常大的波浪运动相邻形成的准流向滚转模式通常是单侧的（翼展方向不对称），与传统条件平均表示所建议的反向旋转形式形成鲜明对比。总而言之，我们根据本研究中收集的证据勾勒出典型大型结构的代表性图景。
This paper quantifies the instantaneous form of large-scale turbulent structures in canonical smooth-wall boundary layers, demonstrating that they adhere to a form that is consistent with the self-sustaining streak instability model suggested by Flores & Jiménez (Phys. Fluids, vol. 22, 2010, 071704) and Hwang & Cossu (Phys. Fluids, vol. 23, 2011, 061702). Our motivation for this study stems from previous observations of large-scale streaks that have been spatially locked in position within spanwise-heterogeneous boundary layers. Here, using similar tools, we demonstrate that the randomly occurring large-scale structures in canonical layers show similar behaviour. Statistically, we show that the signature of large-scale coherent structures exhibits increasing meandering behaviour with distance from the wall. At the upper edge of the boundary layer, where these structures are severely misaligned from the main-flow direction, the induced velocities associated with the strongly yawed vortex packets/clusters yield a significant spanwise-velocity component leading to an apparent oblique coherence of spanwise-velocity fluctuations. This pronounced meandering behaviour also gives rise to a dominant streamwise periodicity at a wavelength of approximately $6\unicode[STIX]{x1D6FF}$ . We further statistically show that the quasi-streamwise roll-modes formed adjacent to these very large wavy motions are often one-sided (spanwise asymmetric), in stark contrast to the counter-rotating form suggested by conventional conditionally averaged representations. To summarise, we sketch a representative picture of the typical large-scale structures based on the evidence gathered in this study.