Sponsor
W.Z. and M.W. acknowledge NSFC (grant nos 12102168, and 11988102), Shenzhen Science & Technology Program (grant no. KQTD20180411143441009), Department of Science and Technology of Guangdong Province (grant nos 2019B21203001, 2020B1212030001), for financial support. X.I.A.Y. acknowledges NSF grant no. 500000021104 and Penn State University for financial support. Numerical simulations have been supported by the Center for Computational Science and Engineering of Southern University of Science and Technology.
Published In
Journal of Fluid Mechanics
Document Type
Article
Publication Date
8-30-2023
Subjects
Fluid mechanics, Turbulence -- Mathematical models
Abstract
The mean flow behaviour of a turbulent boundary layer over rough walls is expected to exhibit symmetries that govern the flow dynamics. In particular, when roughness elements are arranged in a spanwise symmetric manner, the mean flow above them should also exhibit spanwise symmetry. This symmetrical consideration has garnered substantial empirical support. We conduct direct numerical simulations (DNS) of flow over aligned cube arrays to test such symmetry considerations further. We vary the surface coverage density from 0.25 % to 6.25 %, and employ an averaging time of about 100 large-eddy turnover times, which is longer than the typical averaging time in prior DNS studies of rough-wall boundary layers. The results suggest the presence of spanwise asymmetry in the mean flow. Specifically, we observe the development of a prominent secondary vortex on one side of the cubical roughness, accompanied by a relatively smaller secondary vortex on the other side. This asymmetry becomes most pronounced when the surface coverage density is approximately 0.59 %, and diminishes as the coverage density approaches either a low or a high value. We also establish that this mean flow asymmetry is robust across variations in the domain size, the initial condition, and the placement of the cubes in the spanwise direction.
Rights
Creative Commons 
This is an Open Access article, distributed under the terms of the Creative Commons Attribution licence (https://creativecommons.org/licenses/by/4.0/), which permits unrestricted re-use, distribution, and reproduction in any medium, provided the original work is properly cited. Copyright © The Author(s), 2023. Published by Cambridge University Press
DOI
10.1017/jfm.2023.590
Persistent Identifier
https://archives.pdx.edu/ds/psu/43567
Citation Details
Zhang W, Yang XIA, Zhu X, Wan M, Chen S. Asymmetric secondary flows above geometrically symmetric surface roughness. Journal of Fluid Mechanics. 2023;970:A15. doi:10.1017/jfm.2023.590