Skin-friction behavior in the transitionally-rough regime

Results of an experimental investigation of the skin friction for a range of rough surfaces in fully-developed, turbulent channel flow are presented. The fifteen surfaces were generated by grit blasting with various types and sizes of media. The Reynolds number (Rem) for the experiments based on the bulk mean velocity and the channel height spanned from 10,000-300,000. The shape of the roughness function (ΔU+) for all of these surfaces shows self-similarity with an effective roughness height (keff+) indicating that a scale does exist to collapse results throughout the entire roughness function. The roughness function displays inflectional behavior in the transitionally-rough regime in a similar manner to the results of Nikuradse for uniform sand. The equivalent sand grain roughness height can be predicted using krms and the skewness (Sk) of the probability density function of the roughness amplitude. A long-wavelength filtering procedure based on the Taylor microscale (λ) of the roughness is presented, and it is recommended that roughness profiles be filtered at a scale of ∼ 100λ.