In this commentary, we revisit Raupach’s flow-sheltering paradigm that asserts reduced wall-shear stress behind a surface roughness element (MR Raupach in Boundary-Layer Meteorol, 60(4):375–395, 1992). Direct numerical simulations of a turbulent boundary layer over a wall-mounted rectangular roughness are conducted we consider roughness with three different aspect ratios and flows at two Reynolds numbers. A large computational domain is used to study the behaviours of the wall-shear stress in both the near-wake and the far-wake regions. Aside from a low wall-shear stress region in the near-wake as one would expect from the flow-sheltering paradigm, a high-stress region is found in the far-wake. The presence of such a high-stress region challenges the well-established flow sheltering paradigm and is also counter-intuitive. Detailed analysis of the vortical structures shows that the high wall-shear stress region is a consequence of the horse-shoe-vortex-induced downwash motion in the far-wake.

中文翻译：

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重新审视 Raupach 的流动遮蔽范式

在这篇评论中，我们重新审视了 Raupach 的防流范式，该范式断言表面粗糙度元素背后的壁面剪切应力降低（MR Raupach in Boundary-Layer Meteorol, 60(4):375–395, 1992）。我们对壁挂矩形粗糙度上的湍流边界层进行了直接数值模拟，我们考虑了具有三个不同纵横比的粗糙度和两个雷诺数下的流动。大型计算域用于研究近尾流和远尾区域的壁剪应力行为。除了近尾流中的低壁剪切应力区域外，正如人们对流动屏蔽范式所期望的那样，在远尾流中发现了一个高应力区域。这种高应力区域的存在挑战了完善的流动庇护范式，并且也是违反直觉的。