Hydraulic Characterization of Sandpaper Roughness

Sandpaper roughness is often used as a boundary condition for rough wall flows. Friction curves for 220, 320 and 500-grit sandpaper will be presented from the hydraulically smooth to the fully rough flow regimes. The skin friction was determined from pressure drop measurements in a high Reynolds number channel flow facility (300 < Re_τ < 7000). The roughness function for sandpaper will be compared to the close-packed sandgrain surfaces used by Nikuradse, the reference surfaces for the equivalent sandgrain roughness height, k_s. Of interest is the shape of the transitionally rough regime and the roughness Reynolds numbers, k_s⁺, where the flow is no longer hydraulically smooth and becomes fully-rough. Mean flow and Reynolds stresses, obtained using a 2-D LDV system, for the 320-grit sandpaper at a Reynolds number range of 1000 < Re_τ < 6000 will also be presented.