Experimental study of a turbulent boundary layer with a rough-to-smooth change in surface conditions at high Reynolds numbers. (25th September 2021)

• Record Type: Journal Article
• Authors: Li, Mogeng; de Silva, Charitha M.; Chung, Daniel; Pullin, Dale I.; Marusic, Ivan; Hutchins, Nicholas
• Is Part Of: Journal of fluid mechanics. Volume 923(2021)
• Publication Date: 2021-09-25
• Subjects: turbulent boundary layers; Fluid mechanics -- Periodicals; 532.005
• DOI: 10.1017/jfm.2021.577
• Languages: English
• ISSNs: 0022-1120

Abstract

Abstract: Abstract : This study presents an experimental dataset documenting the evolution of a turbulent boundary layer downstream of a rough-to-smooth surface transition. To investigate the effect of upstream flow conditions, two groups of experiments are conducted. For the Group-Re cases, a nominally constant viscous-scaled equivalent sand grain roughness $k_{s0}^{+}\approx 160$ is maintained on the rough surface, while the friction Reynolds number $Re_{\tau 0}$ ranges from 7100 to 21 000. For the Group-ks cases, $Re_{\tau 0}\approx 14\, 000$ is maintained while $k_{s0}^{+}$ ranges from 111 to 228. The wall-shear stress on the downstream smooth surface is measured directly using oil-film interferometry to redress previously reported uncertainties in the skin-friction coefficient recovery trends. In the early development following the roughness transition, the flow in the internal layer is not in equilibrium with the wall-shear stress. This conflicts with the common practise of modelling the mean velocity profile as two log laws below and above the internal layer height, as first proposed by Elliott ( Trans. Am. Geophys. Union, vol. 39, 1958, pp. 1048–1054). As a solution to this, the current data are used to model the recovering mean velocity semi-empirically by blending the corresponding rough-wall and smooth-wall profiles. The over-energised large-scale motions leave a strong footprint in the near-wall region of the energy spectrum, the frequency and magnitude of which exhibit dependence on $Re_{\tau 0}$ and $k_{s0}^{+}$, respectively. The energy distribution in near-wall small scales is mostly unaffected by the presence of the outer flow with rough-wall characteristics, which can be used as a surrogate measure to extract the local friction velocity.