The Relationship Between Frictional Resistance and Roughness for Surfaces Smoothed by Sanding

An experimental investigation has been carried out to document and relate the frictional resistance and roughness texture of painted surfaces smoothed by sanding. Hydrodynamic tests were carried out in a towing tank using a flat plate test fixture towed at a Reynolds number $ReL$ range of $2.8×106−5.5×106$ based on the plate length and freestream velocity. Results indicate an increase in frictional resistance coefficient $CF$ of up to 7.3% for an unsanded, as-sprayed paint surface compared to a sanded, polished surface. Significant increases in $CF$ were also noted on surfaces sanded with sandpaper as fine as 600-grit as compared to the polished surface. The results show that, for the present surfaces, the centerline average height $Ra$ is sufficient to explain a large majority of the variance in the roughness function $ΔU+$ in this Reynolds number range.