Scaling of the roughness effects in turbulent flows over systematically-varied irregular rough surfaces

To clarify the effects of surface topology on the scaling of roughness effects, we conduct experiments on turbulent rough-walled channel flows from transitionally to fully rough regimes. We considered three-dimensional irregular rough surfaces with different effective slope ES, skewness factor Sk, and fixed roughness height scales. The ES values are varied from 0.09 to 0.72 for positively and negatively skewed surfaces with Sk = +/- 0.4. It is revealed that the transitional behavior to the fully rough regime is not influenced by Sk, but rather by ES. The transition to the fully rough regime for surfaces with ES >= 0.18 is insensitive to the ES values, whereas wavy surfaces with ES = 0.09 lead to a moderate transition. The equivalent sand-grain roughness ks steeply increases with increasing ES values from 0.09 to 0.36, whereas a further increase in the ES value does not significantly influence ks. It is also found that positively skewed surfaces with Sk = +0.4 yield larger ks values compared to surfaces with Sk = -0.4; this Sk effect is more pronounced for wavy surfaces with small ES values.