Effect of surface finish on the onset of roughness effects in turbulent channel flow

The ability to reliably predict the onset of roughness effects in wall-bounded turbulent flows is an important yet elusive aim. This is especially relevant for the test engineer or experimental researcher who must specify the surface finish for a wind or water tunnel model. In this study, the effect of the surface finish resulting from sanding was investigated systematically using a turbulent channel flow facility. The experiments were conducted over a Reynolds number range of 6,000 -- 64,000 based on the channel height and the bulk mean velocity. Painted surfaces sanded with a range of grit sizes were tested. The results indicate that all of these surfaces depart from hydraulically smooth behavior at roughness Reynolds numbers ($k_{rms}^+ )$ of between 0.5 and 0.7. The implications of these results with regard to model surface finish required in order to avoid roughness effects will be discussed.