The onset of roughness effects in the transitionally rough regime

Understanding the relationship between a surface's topography and its hydraulic resistance is an important, yet illusive, goal in fluids engineering. Particularly poorly understood are the flow conditions at which a given surface will begin to show the effects of roughness in the form of increased wall shear stress above that of the hydraulically smooth wall. This phenomenon is the focus of the present study. The results from a small scale fully-developed turbulent channel flow facility are presented for a hydraulically smooth wall and three rough surfaces (a sandpaper surface and two types of ship bottom paints). Experiments were conducted over a Reynolds number (ReH) range of 3,200-64,000 based on the channel height and the bulk mean velocity. The onset of roughness effects occurs for the sandpaper surface at krms+ ∼ 1 or ks+ ∼ 5, and this surface reaches fully rough conditions at krms+ ∼ 12 or ks+ ∼ 60. Both these values of k s + and the shape of the roughness function in the transitionally rough regime agree rather well with the results of Nikuradse (1933) for uniform sand. The frictional resistance of the two painted surfaces agrees within experimental uncertainty despite a factor of two difference in k rms. The roughness functions for the painted surfaces do not exhibit either Nikuradse or Colebrook-type behavior. Further research is planned in which systematically varied roughness topographies will be tested. From this work, it is hoped that the roughness scales that most significantly contribute to the onset of roughness effects can be identified. Also planned are tests in a larger scale turbulent channel flow facility to map out the behavior of a range of roughness types from the hydraulically smooth to fully rough flow regime.