Skin friction measurements and predictive correlations for systematically-varied roughness

Skin-friction, roughness functions and predictive correlations are presented for random roughness that have a Guassian distribution of roughness scales. The root-mean-square (rms) roughness height and the skewness of the probability distribution function are parametrically changed to investigate the role of these parameters in the creation of frictional drag. Results are presented for all roughness regimes, from hydraulically-smooth to fully-rough. Most surfaces follow a roughness function similar to the one developed by Nikuradase for uniform sand-grain. Predictive engineering correlations for the equivalent sandgrain roughness height indicate that the rms roughess height and skewness are important scales, however, different correlations are needed for surface roughness with positive, negative and zero skewness.