A note on “critical roughness height” and “transitional roughness”

摘要: An unrigorous but plausible analysis suggests that the concept of a critical roughness height, below which does not affect turbulent wall flow, is erroneous. The Oseen approximation implies effect on additive constant in logarithmic law should vary as square Reynolds number (specifically height “wall units”). This an important point determining whether surfaces used experiments at high unit can be regarded hydraulically smooth. Attention also called to qualitative difference between Nikuradse’s measurements friction factor pipe flow with uniform-size sand-grain “transitional” range and data correlation Moody chart 1944; latter was derived from tests miscellaneous real-life rough 1930s. Nearly all textbooks elementary fluid dynamics present, practically none discuss, this difference. m...