Turbulent boundary-layer flow over regular multiscale roughness

In this experimental study, multiscale rough surfaces with regular (cuboid) elements are used to examine the effects of roughness-scale hierarchy on turbulent boundary layers. Three iterations have been used with a first iteration of large-scale cuboids onto which subsequent smaller cuboids are unif...

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Bibliographic Details
Published in:Journal of fluid mechanics 2021-04, Vol.917, Article A1
Main Authors: Medjnoun, T., Rodriguez-Lopez, E., Ferreira, M.A., Griffiths, T., Meyers, J., Ganapathisubramani, B.
Format: Article
Language:English
Subjects:
Aerodynamics
Boundary layer flow
Boundary layers
Drag
Fluid dynamics
Fluid flow
Fractals
Investigations
JFM Papers
Laboratories
Particle image velocimetry
Plant cover
Reynolds number
Roughness
Shear stress
Simulation
Statistical methods
Thickness
Topography
Turbulent boundary layer
Turbulent flow
Urban areas
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Summary:In this experimental study, multiscale rough surfaces with regular (cuboid) elements are used to examine the effects of roughness-scale hierarchy on turbulent boundary layers. Three iterations have been used with a first iteration of large-scale cuboids onto which subsequent smaller cuboids are uniformly added, with their size decreasing with a power-law as the number increases. The drag is directly measured through a floating-element drag balance, while particle image velocimetry allowed the assessment of the flow field. The drag measurements revealed the smallest roughness iteration can contribute to nearly 7 $\%$ of the overall drag of a full surface, while the intermediate iterations are responsible for over $12\,\%$ (at the highest Reynolds number tested). It is shown that the aerodynamic roughness length scale between subsequent iterations varies linearly, and can be described with a geometrical parameter proportional to the frontal solidity. Mean and turbulent statistics are evaluated using the drag information, and highlighted substantial changes within the canopy region as well as in the outer flow, with modifications to the inertial sublayer (ISL) and the wake region. These changes are shown to be caused by the presence of large-scale secondary motions in the cross-plane, which itself is believed to be a consequence of the largest multiscale roughness phase (spacing between largest cuboids), shown to be of the same order of magnitude as the boundary-layer thickness. Implications on the classical similarity laws are additionally discussed.
ISSN:0022-1120
1469-7645
DOI:10.1017/jfm.2021.228