Recovery of wall-shear stress to equilibrium flow conditions after a rough-to-smooth step change in turbulent boundary layers

Li, M., De Silva, C.M., Rouhi, A. ORCID: 0000-0002-7837-418X, Baidya, R., Chung, D., Marusic, I. and Hutchins, N., 2019. Recovery of wall-shear stress to equilibrium flow conditions after a rough-to-smooth step change in turbulent boundary layers. Journal of Fluid Mechanics, 872, pp. 472-491. ISSN 0022-1120

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Abstract

This paper examines the recovery of the wall-shear stress of a turbulent boundary layer that has undergone a sudden transition from a rough to a smooth surface. Early work of Antonia and Luxton (J. Fluid Mech., vol. 53, 1972, pp. 737–757) questioned the reliability of standard smooth-wall methods for measuring wall-shear stress in such conditions, and subsequent studies show significant disagreement depending on the approach used to determine the wall-shear stress downstream. Here we address this by utilising a collection of experimental databases at Reτ≈4100 that have access to both ‘direct’ and ‘indirect’ measures of the wall-shear stress to understand the recovery to equilibrium conditions of the new surface. Our results reveal that the viscous region ( z+≲4 ) recovers almost immediately to an equilibrium state with the new wall conditions; however, the buffer region and beyond takes several boundary layer thicknesses before recovering to equilibrium conditions, which is longer than previously thought. A unique direct numerical simulation database of a wall-bounded flow with a rough-to-smooth wall transition is employed to confirm these findings. In doing so, we present evidence that any estimate of the wall-shear stress from the mean velocity profile in the buffer region or further away from the wall tends to underestimate its magnitude in the near vicinity of the rough-to-smooth transition, and this is likely to be partly responsible for the large scatter of recovery lengths to equilibrium conditions reported in the literature. Our results also reveal that smaller energetic scales in the near-wall region recover to an equilibrium state associated with the new wall conditions within one boundary layer thickness downstream of the transition, while larger energetic scales exhibit an over-energised state for several boundary layer thicknesses downstream of the transition. Based on these observations, an alternative approach to estimating the wall-shear stress from the premultiplied energy spectrum is proposed.

Item Type: Journal article
Publication Title: Journal of Fluid Mechanics
Creators: Li, M., De Silva, C.M., Rouhi, A., Baidya, R., Chung, D., Marusic, I. and Hutchins, N.
Publisher: Cambridge University Press
Date: 10 August 2019
Volume: 872
ISSN: 0022-1120
Identifiers:
NumberType
10.1017/jfm.2019.351DOI
1378995Other
Divisions: Schools > School of Science and Technology
Record created by: Linda Sullivan
Date Added: 28 Oct 2020 12:25
Last Modified: 28 Oct 2020 12:25
URI: http://irep.ntu.ac.uk/id/eprint/41420

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