ABSTRACT Turbulence structures in flow over three types of wall roughness: sand-grain, cube roughness and a realistic, multi-scale turbine-blade roughness, are compared to structures observed in flow over a smooth wall in open channel flow at , using direct numerical simulations. Two-point velocity correlations, length scales, inclination angles, and velocity spectra are analysed, and the applicability of Townsend's outer layer similarity hypothesis [Townsend. The structure of turbulent shear flow. Cambridge: Cambridge University Press; 1976] to these parameters was examined. Results from linear stochastic estimation suggest that, near the wall, the quasi-streamwise vortices observed in smooth-wall flow are present in the large-scale recessed regions of multi-scale roughness, whereas they are replaced by a pair of ‘head-up, head-down’ horseshoe structures in sandgrain and cube roughness, similar to those observed by Talapatra and Katz [Coherent structures in the inner part of a rough-wall channel flow resolved using holographic PIV. J Fluid Mech. 2012;711:161–170]. The configuration of conditional eddies near the wall suggests that the kinematical process of vortices differ for each kind of rough surface. Eddies over multiscale roughness are conjectured to obey a growth mechanism similar to those over smooth walls, while around the cube roughness the head-down horse-shoe vortices of Talapatra and Katz [Coherent structures in the inner part of a rough-wall channel flow resolved using holographic PIV. J Fluid Mech. 2012;711:161–170] may undergo solid-body rotation on top of a cube roughness on account of the strong shear layer, shortening the longitudinal extent of near-wall structure and promoting turbulence production during this process. These results illustrate the sensitivity of turbulence structure to the roughness texture, particularly within the roughness sublayer.

中文翻译：

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Reτ 下明渠流中真实和合成壁粗糙度的湍流结构

摘要 三种壁面粗糙度（沙粒、立方体粗糙度和真实的多尺度涡轮叶片粗糙度）上的湍流结构与在 t 处的明渠流中的光滑壁面流动中观察到的结构进行了比较，使用直接数值计算模拟。分析了两点速度相关性、长度尺度、倾角和速度谱，以及 Townsend 外层相似性假设 [Townsend. 湍流剪切流的结构。剑桥：剑桥大学出版社；1976] 对这些参数进行了检查。线性随机估计的结果表明，在壁面附近，在光滑壁面流动中观察到的准流向涡存在于多尺度粗糙度的大尺度凹陷区域，而它们被一对“平视”所取代。 , 沙粒和立方体粗糙度中的低头马蹄形结构，类似于 Talapatra 和 Katz 观察到的结构 [使用全息 PIV 解析粗糙壁通道流内部的相干结构。J 流体机械。2012;711:161-170]。壁面附近条件涡旋的配置表明，对于每种粗糙表面，涡旋的运动学过程是不同的。据推测，多尺度粗糙度上的涡流遵循类似于光滑壁上的生长机制，而在立方体粗糙度周围，塔拉帕特拉和卡茨的低头马蹄形涡旋 [粗糙壁通道流内部的相干结构已解析使用全息 PIV。J 流体机械。2012;711:161-170] 由于强剪切层，可能会在立方体粗糙度的顶部发生固体旋转，在此过程中缩短近壁结构的纵向范围并促进湍流的产生。这些结果说明了湍流结构对粗糙度纹理的敏感性，特别是在粗糙度亚层内。