Skin-friction, roughness functions and predictive correlations are presented for random roughness that has a Gaussian power spectral density distribution of surface elevations. The root-mean-square ( rms ) roughness height and the skewness of the probability density function are parametrically varied to investigate the role of these parameters in generating the friction at the wall. Results are presented for all roughness regimes, from hydraulically-smooth to fully-rough. Negative skewness (pits) had a much smaller influence on drag than positive skewness (peaks). Predictive engineering correlations for the equivalent sandgrain roughness height indicate that the rms roughess height and skewness are important scaling parameters. However, the scaling does not appear to be universal as different correlations are needed for surface roughness with positive, negative and zero skewness. Most surfaces collapse to a single roughness function in the transitionally-rough regime similar to the one developed by Nikuradase (1933) for uniform sand-grain roughness. The exceptions are the wavy surface (low effective slope) and the surface with high positive skewness.

中文翻译：

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系统变化粗糙度的皮肤摩擦测量：探讨粗糙度幅度和偏度的作用

对于具有表面高程的高斯功率谱密度分布的随机粗糙度，呈现了皮肤摩擦、粗糙度函数和预测相关性。均方根 (rms) 粗糙度高度和概率密度函数的偏度通过参数变化来研究这些参数在产生壁面摩擦中的作用。结果显示了所有粗糙度状态，从液压平滑到完全粗糙。负偏度（凹坑）对阻力的影响比正偏度（峰值）小得多。等效砂粒粗糙度高度的预测工程相关性表明均方根粗糙度高度和偏度是重要的缩放参数。然而，缩放似乎并不通用，因为具有正偏度、负偏度和零偏度的表面粗糙度需要不同的相关性。大多数表面在过渡粗糙状态下坍塌为单一的粗糙度函数，类似于 Nikuradase (1933) 为均匀砂粒粗糙度开发的那种。例外是波浪表面（低有效斜率）和具有高正偏度的表面。