Abstract Previous work by the authors (Flack and Schultz, 2010) has identified the root-mean-square roughness height, krms, and the skewness, Sk, of the surface elevation distribution as important parameters in scaling the skin-friction drag on rough surfaces. In this study, three surfaces are tested in turbulent boundary layer flow at a friction Reynolds number, Reτ = 1600–2200. All the surfaces have similar root-mean-square roughness height, while the skewness is varied. Measurements are presented using both two-component LDV and PIV. The results show the anticipated trend of increasing skin-friction drag with increasing skewness. The largest increase in drag occurs going from negative skewness to zero skewness with a more modest increase going from zero to positive skewness. Some differences in the mean velocity and Reynolds stress profiles are observed for the three surfaces. However, these differences are confined to a region close to the rough surface, and the mean velocity and Reynolds stress profiles collapse away from the wall when scaled in outer variables. The turbulence structure as documented through two-point spatial correlations of velocity is also observed to be very similar over the three surfaces. These results support Townsend’s (1976) concept of outer-layer similarity that the wall boundary condition exerts no direct influence on the turbulence structure away from the wall except in setting the velocity and length scales for the outer layer.

中文翻译：

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表面粗糙度偏度的系统变化对湍流和阻力的影响

摘要 作者之前的工作（Flack 和 Schultz，2010 年）已将表面高程分布的均方根粗糙度高度 krms 和偏度 Sk 确定为缩放粗糙表面上的皮肤摩擦阻力的重要参数. 在这项研究中，在摩擦雷诺数 Reτ = 1600–2200 的湍流边界层流中测试了三个表面。所有表面都具有相似的均方根粗糙度高度，而偏度则不同。使用双分量 LDV 和 PIV 进行测量。结果显示了随着偏度增加，皮肤摩擦阻力增加的预期趋势。从负偏度到零偏度，阻力的最大增加发生在从零到正偏度的更温和的增加。观察到三个表面的平均速度和雷诺应力分布的一些差异。然而，这些差异仅限于靠近粗糙表面的区域，并且平均速度和雷诺应力分布在外部变量中缩放时会从壁上坍塌。通过速度的两点空间相关性记录的湍流结构也被观察到在三个表面上非常相似。这些结果支持 Townsend (1976) 的外层相似性概念，即壁边界条件对远离壁的湍流结构没有直接影响，除了设置外层的速度和长度尺度。当在外部变量中缩放时，平均速度和雷诺应力分布从壁上坍塌。通过速度的两点空间相关性记录的湍流结构也被观察到在三个表面上非常相似。这些结果支持 Townsend (1976) 的外层相似性概念，即壁边界条件对远离壁的湍流结构没有直接影响，除了设置外层的速度和长度尺度。当在外部变量中缩放时，平均速度和雷诺应力分布从壁上坍塌。通过速度的两点空间相关性记录的湍流结构也被观察到在三个表面上非常相似。这些结果支持 Townsend (1976) 的外层相似性概念，即壁边界条件对远离壁的湍流结构没有直接影响，除了设置外层的速度和长度尺度。