The proliferation of turbulence in subcritical wall-bounded shear flows involves spatially localised coherent structures. Turbulent spots correspond to finite-time nonlinear responses to pointwise disturbances and are regarded as seeds of turbulence during transition. The rapid spatial decay of the turbulent fluctuations away from a spot is accompanied by large-scale flows with a robust structuration. The far field velocity field of these spots is investigated numerically using spectral methods in large domains in four different flow scenarios (plane Couette, plane Poiseuille, Couette-Poiseuille and a sinusoidal shear flow). At odds with former expectations, the planar components of the velocity field decay algebraically. These decay exponents depend only on the symmetries of the system, which here depend on the presence of an applied gradient}, and not on the Reynolds number. This suggests an effective two-dimensional multipolar expansion for the far field, dominated by a quadrupolar flow component or, for asymmetric flow fields, by a dipolar flow component.

中文翻译：

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远场湍流斑

湍流在次临界壁边界剪切流中的扩散涉及空间局部的相干结构。湍流点对应于对点状扰动的有限时间非线性响应，并被视为过渡过程中湍流的种子。远离点的湍流涨落的快速空间衰减伴随着具有稳健结构的大规模流动。这些斑点的远场速度场是通过频谱方法在四个不同流动场景（平面库埃特，平面泊瓦伊，库埃特-泊伊埃和正弦剪切流）的大范围内进行数值研究的。与以前的预期不同，速度场的平面分量代数衰减。这些衰减指数仅取决于系统的对称性，在此取决于所应用梯度的存在}，而不是雷诺数。这表明对于远场有效的二维多极膨胀，主要由四极流分量主导，对于非对称流场，则由偶极流分量主导。