An analysis of drag reduction using spanwise forcing on rough walls

Sina Nozarian, Mahdi Abkar, Pourya Forooghi*

*Corresponding author af dette arbejde

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Abstract

Spanwise opposed wall-jet forcing has been shown to reduce the skin-friction drag of wall-bounded
turbulent flows by suppressing the near-wall turbulent motion (Yao et al., 2018). In the present
work, the response of this drag reduction mechanism to the presence of surface roughness is
studied. To this end, direct numerical
simulations of flow in smooth and rough plane channels at a matched friction Reynolds number (Re𝜏
= 180) are
carried out. The roughness is generated by distributing discrete roughness elements at two
different values of frontal solidity, namely 0.021 and 0.045. The roughness elements height is 𝑘⁺
= 18 (+indicates viscous scaling). By varying the forcing amplitude 𝐴⁺, it is shown that, when
other forcing parameters are fixed, maximum drag reduction in a rough channel is achieved at a
considerably larger 𝐴⁺ than in a smooth channel. Notably, the strength of the wall-jet at which
the maximum drag reduction is achieved, is comparable for the smooth
and both rough cases. Additionally, it is observed that the maximum drag reduction values attained
in rough channels are smaller compared to those observed in smooth channels, for both lower and
higher frontal solidity cases — 2.4% and 2%, respectively. The reduced drag reduction potential
originates from two observations; firstly, drag on roughness elements, which is dominated by
pressure drag, is not reduced by the method in question. Secondly, the drag on channel floor is
reduced to a lesser extent when roughness elements are present. The latter can be attributed to the
observation that, while in all cases the forcing suppresses streamwise random turbulent
fluctuations and shear stress, this suppression is less pronounced when roughness elements
are present.
OriginalsprogEngelsk
TidsskriftInternational Journal of Heat and Fluid Flow
Vol/bind106
Antal sider15
ISSN0142-727X
StatusE-pub ahead of print - 2024

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