Commercial Sandpaper and Nikuradse Sandgrain Roughness: Similarities and Differences

Sandpaper is commonly employed as a wall roughness in experiments and even simulations. This is largely due its widespread availability, range of available roughness heights, and strict manufacturing standards. Its name often links it to the classic pipe flow experiments of Nikuradse (1933) which used close-packed sand with a narrow grainsize distribution. Nikuradse’s experiments led to the adoption of the equivalent sand roughness height, k_s, as a hydraulic length scale to compare the drag associated with a given surface roughness. Recent results reveal that while commercial sandpaper shares a number of similarities with Nikuradse sand, it also displays a number of notable differences. Similarities include the display of hydraulically-smooth behavior at small but finite roughness Reynolds number and a dip in the skin-friction coefficient in the transitionally-rough regime. Differences include a k_s that is significantly larger than the median grit size for commercial sandpaper. This talk will discuss both the similarities and differences as well as the reasons for each.