Roughness effects in turbulent pipe flow

Abstract

Mean flow measurements are presented for fully developed turbulent pipe flow over a Reynolds number range of $57\,{\times}\,10^3$ to $21\,{\times}\,10^6$ where the flow exhibits hydraulically smooth, transitionally rough, and fully rough behaviours. The surface of the pipe was prepared with a honing tool, typical of many engineering applications, achieving a ratio of characteristic roughness height to pipe diameter of 1 : 17000. Results for the friction factor show that in the transitionally rough regime this surface follows a Nikuradse (1933)-type inflectional relationship rather than the monotonic Colebrook (1939) relationship used in the Moody diagram. This result supports previous suggestions that the Moody diagram in the transitional regime must be used with caution. Outer scaling of the mean velocity data shows excellent collapse and strong evidence for Townsend's outer layer similarity hypothesis for rough-walled flows. Finally, the pipe exhibited smooth behaviour for scaled roughness height $k_s^+ \,{\le}\, 3.5$, which supports the suggestion by Zagarola & Smits (1998) that their pipe was hydraulically smooth for $Re_D\,\,{\leq}\, 24\,{\times}\,10^6$.

Direct numerical simulation of turbulent channel flow over a surrogate for Nikuradse-type roughness

“…Experiments on honed roughness carried out by Shockling et al (2006) and Schultz & Flack (2007), and grit-blasted surfaces studied by Flack et al (2016) also showed similar trends to those of Nikuradse (1933) over the Reynolds number range, consistent with the present results. However, this tends to contradict the observation of Bradshaw (2000), who suggested that realistic roughness should exhibit more Colebrook-type than Nikuradse-type behaviour due to the large range of length scales present compared with Nikuradse's sand grains.…”

“…In particular, the honed pipe roughness of Shockling et al (2006) did not exhibit as pronounced an inflectional behaviour as shown by the data of Nikuradse (1933) or Ligrani & Moffat (1986), and reached the fully rough regime at comparatively lower k …”

“…Outside this regime, the problem becomes more difficult. Nikuradse (1933) remains the most complete study of the transitionally rough regime, while subsequent work (Ligrani & Moffat 1986;Shockling, Allen & Smits 2006;Schultz & Flack 2007) has produced evidence for non-universal behaviour. Other recent roughness studies covering a wide Reynolds number range include Flack et al (2016), where the skin-friction behaviour of surfaces blasted with different grades of grit was studied, and Barros, Schultz & Flack (2017), where skin-friction studies were conducted on mathematically generated irregular roughness.…”

Similarity of the streamwise velocity component in very-rough-wall channel flow

“…It has long been accepted that inner scaling fails for higher-order statistics near the wall even for the case of smooth walls, though there are many examples demonstrating that the mean velocity is self-similar; see, for example, the reviews by Raupach, Antonia & Rajagopalan (1991) and Jiménez (2004), as well as the more recent results of Shockling, Allen & Smits (2006) and . Townsend (1961) explained this discrepancy by introducing the concept of 'inactive' motion (see also Bradshaw 1967;Morrison, Subramanian & Bradshaw 1992), in which the 'top-down' effect (Hunt & Morrison 2000) of the large-scale, non-shear-stress-bearing motion near the wall is assumed to be a low-frequency modulation of the shear-stressbearing, active motion by the low-wavenumber wall-parallel velocity components.…”

Assessing biofilm development in drinking water distribution systems by Machine Learning methods

“…Friction along the solid-liquid interface tends to increase with increasing surface roughness and interface instabilities [100]. It is the primary cause of energy losses and thus, of the flow capacity reduction within pipelines [17].…”